Postnatal Development of the Human Cerebral Cortex, Volume 4: the Cortex of the Six-Month Infant [1 ed.] 9780674187689

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THE POSTNATAL DEVELOPMENT OF THE HUMAN CEREBRAL CORTEX Volume IV: The Cortex of the Six-Month Infant

THE POSTNATAL DEVELOPMENT OF THE HUMAN CEREBRAL CORTEX BY

J. LeROY CONEL Research Associate in Pathology, The Children's Medical Center, Boston Professor of Anatomy, Emeritus, Boston University School of Medicine

VOLUME IV

THE CORTEX OF THE SIX-MONTH INFANT

H A R V A R D U N I V E R S I T Y PRESS CAMBRIDGE, MASSACHUSETTS 1951

COPYRIGHT,

1951

T H E PRESIDENT AND FELLOWS OF HARVARD COLLEGE

DISTRIBUTED IN GREAT BRITAIN BY GEOFFREY CUMBERLEGE OXFORD UNIVERSITY PRESS LONDON

PRINTED IN T H E UNITED STATES OF AMERICA

ACKNOWLEDGMENTS of the condition of the human cerebral cortex in the infant at the age of six months has been conducted in the Department of Pathology of The Children's Medical Center, and in the Department of Anatomy of Boston University School of Medicine. The investigation has been supported in part by research grants from the Charles A. Brant Fund of the Committee on Scientific Research of the American Medical Association, The John and Mary R. Markle Foundation, and the National Institute of Mental Health, of the National Institutes of Health, Public Health Service. I am very grateful for the continued interest and encouragement of Dr. S. Burt Wolbach, Director, T H E INVESTIGATION

Division of Nutritional Research, The Children's Medical Center, and Shattuck Professor of Pathological Anatomy, Emeritus, Harvard Medical School. I am pleased to express my gratitude to Dr. Sidney Färber, Pathologist-in-Chief and Chairman, Division of Laboratories and Research, The Children's Medical Center, for the many ways in which he generously supports this investigation. Mr. Salvatore Lunetta and Miss Elizabeth Thompson have provided the technical assistance without which the investigation could not proceed. Miss Etta Piotti has made the drawings. J. L. C. BOSTON, MASSACHUSETTS

CONTENTS INTRODUCTION

3

LOBUS FRONTALIS

4

Gyrus centralis anterior, area FAy Region of the lower extremity Region of the upper trunk, shoulder, arm, and forearm Region of the hand Region of the head Posterior region of the frontal gyri Gyrus frontalis superior, area FB Gyrus frontalis medius, area FB Gyrus frontalis inferior, area FCBm Middle region of the frontal gyri Gyrus frontalis superior, area FC Gyrus frontalis medius, area FD Gyrus frontalis inferior, area F D r Anterior region of the frontal gyri Gyrus frontalis superior, area FDm Gyrus frontalis medius, area FDA Gyrus frontalis inferior, area FDp Orbital gyri, area F F Polus frontalis, area FE LOBUS PARIETALIS

Gyrus centralis posterior Area postcentralis oralis granulosa, PB Area postcentralis intermedia, PC Lobulus parietalis superior, area PE Lobulus parietalis inferior Gyrus supramarginalis, area PF Gyrus angularis, area PG Occipito-temporal zone, area PH LOBUS OCCIPITALIS

Area peristriata, OA Area parastriata, OB Area striata, OC LOBUS TEMPORALIS

Gyrus temporalis superior, area TA Gyri temporales transversi Area supra-temporalis simplex, TB Area supra-temporalis granulosa, TC Gyrus temporalis medius et inferior, area TE Gyrus fusiformis, area T F Polus temporalis, area TG

4 4 n 17 22 27 27 31 35 39 39 43 47 51 51 54 56 59 62 65

65 65 76 86 89 90 92 95 98

98 100 103 110

no 113 113 116 120 123 126

viii

CONTENTS

LOBUS I N S U L A E

130

Area precentralis insulae, IA Area postcentralis insulae, IB RHINENCEPHALON

130 133 136

Gyrus cinguli, area L A Gyrus cinguli, area L C Gyrus cinguli, area L D Gyrus cinguli, area L E Gyrus hippocampi Area uncinata, HA Area presubicularis granulosa, H D Area pyramidalis, H E Area pyramidalis presubiculi, H E l a Area pyramidalis subiculi, H E l b Hippocampus (cornu ammonis) Area pyramidalis ammonica, HE 2 Gyrus dentatus Area dentata, H F

136 140 142 144 146 146 150 152 152 153 154 154 155 155

COMMENTS

158

TABLES

179

LITERATURE

190

EXPLANATION

OF

PLATES

191

Volume I V The Cortex of the Six-Month Infant

INTRODUCTION

used for the study of the cortex in the infant at the age of six months were removed at autopsies in the Department of Pathology of The Children's Hospital. The following data are taken from the protocol for each autopsy: T H E BRAINS

No. 32-46. A well-developed, well-nourished white, male infant, born two weeks prematurely. Age 6 months. No illness previous to the terminal one. Death occurred three days after the first indication of difficulty in respiration. Diagnosis: tracheal obstruction of infectious character, asphyxia. Brain weighs 746 grams. Autopsy 2 hours post mortem. No. 32-100. A well-developed, well-nourished white, male infant. Age 6 y2 months. Died five days after first indication of illness. Diagnosis: Bronchopneumonia. Brain weighs 786 grams. Autopsy 5 hours post mortem. No. 37-49. A fairly well-developed, but poorly nourished white, female infant. Age 6 months. Admitted to hospital during third week of life, and represented clearly a case of severe malnutrition. Did not gain much under long course of treatment. Diagnosis: Chronic pneumonia, malnutrition. Brain weighs 585 grams, and presents no unusual gross features. No. 38-35. A well-developed, fairly well-nourished white, male infant. Age 6 months. One week prior to admission to hospital abdomen was noted to be large. Died two days after admission. Diagnosis: Retroperitoneal embryoma, acute tracheitis and

bronchopneumonia. Brain weighs 734 grams, and presents no unusual gross features. Autopsy iol/2 hours post mortem. No. 38-44. A well-developed, fairly well-nourished Negro, male infant. Age 6 months. Was admitted to hospital at age of four months because of chronic cough of six weeks' duration. Continued to fail under treatment. Diagnosis: Influenzal pneumonia and septicemia. Autopsy 12 hours post mortem. Brain weighs 870 grams. No. 40-139. White, male; age 6 months. Full-term, normal delivery. Died on the 42 nd day after admission to hospital. Diagnosis: Chronic pancreatic fibrosis and pancreatitis. Brain shows no unusual gross features; was not weighed. No. 41-91. A well-developed, very well-nourished white, male infant. Age 6 V2 months. Perfectly well 50 hours before admission to hospital, when suddenly began to vomit. Died the second day after admission. Diagnosis: Intussusception, toxic reactions. Autopsy 15J4 hours post mortem. Brain shows no unusual features; weighs 842 grams. The normal weight of the six-month brain is 660 grams. The brains listed above were fixed in the same manner as those of the newborn, one-month, and three-month infants. The same procedures were followed in preparing the cresyl violet, Cajal, GolgiCox, and Weigert sections of the six-month cortex as were used for the younger brains.

LOBUS F R O N T A L I S GYRUS CENTRALIS ANTERIOR AREA PRECENTRALIS GIGANTO-PYRAMIDALIS, FAy

THE GIANT pyramidal cells of Betz constitute the chief characteristic of area F A y . The Betz cells are distributed in the cortex of the three-month infant in the same manner and location as in the cortex of the adult as described by Economo and Koskinas. These cells are located in the paracentral lobule surrounding the superior end of the central sulcus, the crown and posterior wall of gyrus centralis anterior, the floor of the central sulcus, and the deepest portion (approximately one-fourth) of the anterior wall of gyrus centralis posterior. Economo designated the cortex in the deepest part of the anterior wall of gyrus centralis posterior as area PA. In the crown of gyrus centralis anterior, area F A y is widest at the superior end of the gyrus and narrowest at the lower end. The decrease in width of the area is gradual. In the lowermost end of the gyrus area F A y does not extend into the crown of gyrus centralis anterior, but is confined entirely to the posterior wall of the gyrus and the floor of sulcus centralis. The approximate boundaries of the functional regions of area F A y are regarded as being the same in the six-month cortex as they are in the cortex of the three-month infant. REGION OF THE LOWER EXTREMITY

That portion of area F A y which is located in the paracentral lobule around the superior end of sulcus centralis and in the crown and posterior wall of approximately the superior one-fourth of gyrus centralis anterior is regarded as the functional region of the lower extremity. Separate descriptions are given below for the cortex in the crown of gyrus centralis anterior and in the paracentral lobule. C R O W N OF THE SUPERIOR O N E - F O U R T H OF G Y R U S CENTRALIS ANTERIOR

The cortex in this location measures 2.520 mm. in width, and is wider than that in the same location in the cortex of the three-month infant. Layers II and VI are slightly narrower, but all the other layers are wider than the corresponding layers in the three-

month cortex. The cortex in this location is illustrated in Figures 5, 6, 7, and 8 which show its appearance in the cresyl violet, Cajal, and Golgi-Cox sections. CELL-BODIES

Layer I. This layer measures 0.286 mm. in width, and contains 51 cells to the unit. Horizontal cells of Cajal are seen occasionally. Layer II. An outer granular layer is clearly visible. The layer measures 0.086 mm. in width and contains 251 small nerve cells to the unit. The cells range in size from 5/4/u to 9/8/i. The external boundary of layer II is fairly sharply defined, but the internal limit is indistinct because the small cells of layer II are intermingled with the larger cells of lamina I l i a . Layer III. This layer measures 0.771 mm. in width. Small neurons are scattered throughout the entire width of the layer, but the larger pyramidal cells become gradually larger from the outer to the inner boundaries of the layer. The neurons number 63 to the unit in lamina I l i a and range in size from ιο/6μ to 23/13/1. In lamina IHb the neurons number 53 to the unit and measure 10/6-26/13/*. Lamina IIIc contains 52 neurons to the unit and they range in size from ιο/6μ to 33/13/t. Layer IV. An inner granular layer is clearly visible but is less distinct than layer II. The layer measures 0.218 mm. in width. The small cells of layer IV number 108 to the unit and measure 5/4-13/τομ. Both the outer and inner boundaries of layer IV are indistinct because the cells of this layer are intermingled with the cells of lamina IIIc and lamina Va, respectively. Layer V. This layer measures 0.613 mm. in width. The neurons of layer V are designated as small, large, and extra-large. The layer is subdivided into two laminae, Va and Vb. The small neurons measure 10/6-16/ιομ, and number 45 to the unit in Va and 40 to the unit in Vb. The large nerve cells measure 20/12-29/15/1, and number 20 to the unit in lamina Va and 16 in lamina Vb. The extra-large cells in this

LOBUS FRONTALIS area (FAy) are the giant pyramidal cells of Betz. In this region of area FAy these cells number 7 to the unit, and range in size from 36/13/* to 69/23/*. They occur chiefly in lamina Vb, but some may be present in lamina Va also. They are principally collected in groups of 3 or 4 cells, but also occur singly. Layer VI. The width of this layer is 0.546 mm. The neurons number 65 to the unit in lamina Via and 40 per unit in lamina VIb. The pyramidal cells range in size from 10/6/* to 26/15//., and the spindle cells measure 16/6-29/13/*. LOBULUS PARACENTRALIS

The cortex in area FAy in the paracentral lobule does not differ in appearance from that in the crown of the superior end of gyrus centralis anterior. The larger Betz cells are somewhat more numerous in the paracentral lobule. The cortex measures 2.531 mm. in width. CELL-BODIES

Layer I. The marginal layer measures 0.263 m m · i n width and contains 52 neurons to the unit. Horizontal cells of Cajal have been observed occasionally in the cresyl violet sections. Layer II. The outer granular layer measures 0.120 mm. in width and contains 273 neurons to the unit. The cells range in size from 5/4/i to 9/8/*. Layer III. This layer measures 0.800 mm. in width. In lamina I l i a the neurons number 56 to the unit and measure 10/6-23/12/*. The neurons number 50 to the unit and range in size from το/6μ to 26/15/* in lamina IHb. The nerve cells in lamina IIIc number 50 to the unit and measure 10/6-33/13/*. Layer IV. The inner granular layer measures 0.232 mm. in width and contains n o nerve cells to the unit. The cells range in size from 5/4/* to 13/10/*. Layer V. This layer measures 0.596 mm. in width. The small neurons measure 10/6-16/1 o/*, and number 45 to the unit in lamina Va and 40 per unit in lamina Vb. The large neurons range in size from 20/10/* to 30/12/*, and number 19 to the unit in Va and 16 in Vb. The giant pyramidal cells of Betz number 7 to the unit and range in size from 36/13/* to 62/20/*. They are located principally in lamina Vb, but occur also in lamina Va. Layer VI. The total width of this layer is 0.521 mm.

5

The neurons number 70 to the unit in lamina Via and 45 in lamina VIb. The pyramidal cells measure 10/6-26/13/* and the spindle cells range in size from 16/6/* to 29/13/A. In the sections stained with cresyl violet the giant pyramidal cells of Betz contain more chromophil substance than any other cells in this area. Most of the giant cells are stained more darkly than any other cells, but some are as light in color as the most lightly stained large ordinary pyramidal cells in layers V and VI. Nissl bodies are much more numerous than in the Betz cells in this area in the three-month cortex. These bodies are small, and most of them are flakelike, but some of those around the periphery of the cell-body and in the apical dendrite are elongated. The outlines of the Nissl bodies are not sharply defined, but on the contrary the periphery of these bodies presents a fuzzy appearance. Amorphous clumps of chromophil substance are smaller and less numerous than in the Betz cells in this region in the cortex of the three-month infant. The clumps are stained very darkly, and are chiefly located at the periphery of the cell. Often they are seen in the base of the cell, particularly at the place of origin of the basal dendrites. The apical and basal dendrites of the Betz cells are stained for a greater distance and contain much more chromophil substance than those of corresponding cells in this area in the three-month cortex. Some of the apical dendrites are visible as far as layer IV, but are very faintly stained toward their terminations. The chromophil substance is greatest in quantity at the proximal end of each apical dendrite, and gradually diminishes distally. The substance is chiefly in the form of small Nissl bodies, but a few small clumps are also present. Most of the Nissl bodies are elongated, but some are in the form of flakes. Some of the clumps which are located most proximally are also elongated. A small, darkly stained clump is located at the place of origin of each of the most proximal three or four collaterals. No collaterals are stained. The basal dendrites are stained for a much shorter distance from the cell-body than the apical dendrite. Only a few scattered Nissl bodies and clumps of chromophil substance are present in each basal dendrite. A few granules are also present in the basal

6

THE CORTEX OF THE SIX-MONTH INFANT

dendrites. The chromophil substance is chiefly collected along the periphery of each dendrite. Branches of the basal dendrites are stained more frequently and for a greater distance than in the three-month cortex. A small, dark clump is present at the place where each branch emerges. The membrane of the nucleus is darkly stained and irregularly thickened. A few small beads of chromophil substance are scattered along the external surface. In almost every Betz cell a dark clump of chromophil substance rests upon the nuclear membrane immediately subjacent to the base of the apical dendrite. This may be in the shape of a broad disc or a cone. The base of the cone rests upon the nuclear membrane and the apex is directed toward the apical dendrite. In some cells dark, elongated strands of chromophil substance appear to be streaming upward from the cone toward the apical dendrite. A stained axon is seen on a Betz cell only occasionally. The axon hillock is present, but it is very small and does not arch into the cytoplasm. The large ordinary and special pyramidal cells in layers V and VI and the large spindle cells in VI are stained more lightly than the Betz cells. Some of the large ordinary, special pyramidal, and spindle cells are stained more darkly than others, and the dark ones are of about the same color as the lightest of the Betz cells. The large pyramidal and spindle cells have much less cytoplasm and chromophil substance in proportion to the size of the nucleus than the Betz cells, but they have more chromophil substance than the large pyramidal and spindle cells in layers V and VI in the three-month cortex. The chromophil substance in the large pyramidal and spindle cells is in the form of fine granules, small Nissl flakes, and small darkly stained clumps. The clumps are more numerous in these cells than they are in the Betz cells. The agglutinations of chromophil substance are smaller and less numerous in the large pyramidal and fusiform cells in layers V and VI, and Nissl flakes are correspondingly more numerous than in these cells in the cortex of the three-month infant. The nuclear membrane is especially thick immediately under the base of the apical dendrite, and in many cells a small cone-shaped clump of chromophil substance is present in this location. The apical dendrites of the large pyramidal cells are stained for only a short distance,

and only rarely is a basal dendrite stained. Both dendrites of the spindle cells are stained for a short distance from the cell-body. Granules and small Nissl flakes invade the apical dendrites for a very short distance. The axons are not stained, and there is no indication of an axon hillock in any of these cells. The large pyramidal cells in lamina IIIc are all stained about the same color, though one is occasionally seen which is darker than the others. The color in these cells is of about the same density as that in the large pyramidal and spindle cells in layers V and VI, excepting that the darkest cells in the latter layers are somewhat darker than the most heavily stained cells in lamina IIIc. The chromophil substance in the large cells in IIIc is less in quantity than in the large pyramidal and spindle cells in layers V and VI, but it is slightly greater in quantity than that in the large pyramidal cells in lamina IIIc in the cortex of the three-month infant. In the large pyramidal cells in lamina IIIc very small, flakelike Nissl bodies are present in about the same quantity as in these cells in the three-month cortex. Amorphous, darkly stained clumps of chromophil substance are present, but they are much less numerous and smaller than those in the corresponding cells in the cortex of the three-month infant. These clumps are scattered throughout the cytoplasm, and in many cells two or three are clinging to the outer surface of the membrane of the nucleus. In most of the cells a small cone-shaped clump rests on the nuclear membrane immediately under the base of the apical dendrite. Fine granules and a few Nissl flakes invade the apical dendrite for a much shorter distance than in the large pyramidal and spindle cells in layers V and VI. The basal dendrites of the largest cells only are stained, and they are very short and slender. Axons are not stained. The membrane of the nucleus is wide and darkly stained. The width is fairly uniform excepting in places where the small agglutinations of chromophil substance are located. The karyoplasm is lighter in color than the cytoplasm, and contains fine granules and small, dark clumps of chromophil substance. The nucleolus is more darkly stained than any other part of the cell. A few small, heavily stained spheres are clinging to the periphery of each nucleolus. The larger cells in lamina I l l b contain less cytoplasm than is present in the large cells in IIIc. In the

LOBUS FRONTALIS large cells of I l l b agglutinations of chromophil substance are more numerous and granules are less numerous than in the large cells of lamina IIIc. The chromophil substance decreases further in quantity in the large cells in I l i a , and is chiefly in the form of small agglutinations, but a few granules are present. Proceeding externally from lamina IIIc the nuclear membranes in the neurons become thicker and are made more irregular in contour by the presence of an increasing number of small clumps of chromophil substance. A small clump of chromophil substance on the nuclear membrane immediately under the origin of the apical dendrite is seen more frequently as the cells decrease in size toward I l i a . The clumps of chromophil substance in the larger neurons of laminae I l i a , I l l b , and IIIc are not as large as the clumps in the corresponding cells in this area in the threemonth cortex. The small cells in layers III, V, and VI have about the same amount of chromophil substance, and it is in the form of small clumps and a few granules. The large horizontal cells of Cajal in layer I are less numerous than in this area in the cortex of the three-month infant. Some of these cells appear to be normal, while others seem to be degenerating. The nerve cells in layers II and IV present about the same appearance in this area as in the threemonth cortex, except that the chromophil substance is greater in quantity and the clumps are smaller in size in the six-month cortex. The cells in layer IV have more chromophil substance than those in layer II. Neurofibrils are present in all the giant pyramidal cells in this area, both in the crown of gyrus centralis anterior and in lobulus paracentralis. The neurofibrils are more numerous in the larger Betz cells than in the smaller ones. Both large and small Betz cells contain longitudinal rows of granules as well as neurofibrils. The neurofibrils extend into the apical and basal dendrites, and all the dendrites are stained more darkly than the cell-body. Neurofibrils extend into the proximal collaterals of the apical dendrites. The neurofibrils are more distinct, more numerous, and extend into the apical dendrites for a greater distance than in Betz cells of corresponding size in this area in the cortex of the three-month infant. The cytoplasm in all other cells in this area is

granular, but the granules are arranged in longitudinal strands in the cell-bodies and dendrites of the extra-large cells in layer V, in the large pyramidal and spindle cells in layer VI, and in the largest pyramidal cells in lamina IIIc. A few longitudinal strands of granules are present in the large pyramidal cells in lamina I l l b . In all cells in I l i a and in the small cells in all layers the granules are scattered irregularly throughout the cell-bodies and dendrites. FIBERS. In the Golgi-Cox preparations the processes of the giant pyramidal cells of Betz are in general larger and coarser than those of the Betz cells in this area of the cortex of the three-month infant. The apical dendrites of the Betz cells in the six-month cortex are coarser, but no longer, than those of the Betz cells in the three-month cortex. The collateral branches of the apical dendrites are somewhat coarser and longer but not any more numerous in the six-month cortex. The location of the collateral branches on the apical dendrites and the distribution of the collaterals in the cortex is similar in the two ages. In both the three-month and the six-month cortex the apical dendrites end by arborizations among the tangential fibers in layer I, but these terminal branches are coarser and more numerous in the sixmonth cortex. The basal dendrites of the Betz cells in the six-month cortex are coarser and somewhat longer than those of corresponding cells in this area in the three-month cortex. No increase either in the number of the basal dendrites or in the number of their branches is apparent in the six-month cortex, and the distribution of these dendrites is quite similar in the six-month and three-month brains. They extend horizontally, obliquely, and vertically downward in lamina Vb and layer VI. Some of the longest vertical basal dendrites extend throughout the entire width of layer VI. Many of the giant pyramidal cells have one basal dendrite which is much larger and longer than the others. The basal dendrites are distributed chiefly within the inner band of Baillarger. Occasionally an inverted Betz cell is observed. All of the dendrites of these cells are distributed in lamina Vb and layer VI. The apical dendrites of the inverted cells extend vertically downward through the entire width of layer VI, and end as a single stalk, not by arborizing as in layer I. The collateral branches

8

THE CORTEX OF THE SIX-MONTH INFANT

of the apical dendrites are directed obliquely downward in layer VI, except that those nearest the cellbodies extend horizontally in lamina Vb. Pedunculated bulbs are more numerous on the apical and basal dendrites and their branches than in the three-month cortex. The bulbs are more numerous on the apical dendrites than on any of the basal dendrites. They are most numerous on the apical dendrites as they traverse layer IV and lamina IIIc, and are least numerous on the terminal branches in layer I. The bulbs on the terminal branches in layer I, however, are more numerous than those on these branches of the apical dendrites in the three-month cortex. The axons of the giant pyramidal cells have the same appearance as those in the three-month cortex. The nodules and small surface irregularities indicating myelin are not conspicuously larger or more numerous than in the three-month cortex. No pedunculated bulbs are present on any axons. The collateral branches of the axons are not any more numerous than in the three-month cortex, usually numbering about three for each axon, and they emerge from the axon as it passes through lamina Via and the upper part of VIb. The collaterals are approximately the same size and length as those in the three-month cortex, but they have more secondary branches than the collaterals in the three-month cortex. As many as five secondary branches have been counted on a primary collateral, and some of the secondary collaterals may have one or two tertiary branches. All the collaterals emerge from the parent stem at right angles. All of them are distributed in layers VI and V, and all seem to end freely; not one has been observed ending on a dendrite of another cell or on any fiber. The apical dendrites of the large ordinary and special pyramidal cells in layer V are more compact in structure, are longer, and have more terminal branches than the corresponding cells in this area of the cortex of the three-month infant. The terminal branches of a large ordinary or special pyramidal cell end in layer I, and they are not as numerous as the terminal branches of a giant pyramidal cell of Betz. The apical dendrites of some cells divide into two apical dendrites of equal size, each of which ascends to layer I. The subdivision of the apical dendrite may occur in layers V, IV, or I I I . The collateral

branches of the apical dendrites are present in practically the same number and arrangement as in the three-month cortex. The apical dendrites of the smaller cells of this category have a few varicosities scattered along their distal ends, and varicosities are present toward the distal ends of the collateral branches. The basal dendrites of the large ordinary and special pyramidal cells and their branches are not any more numerous, but are somewhat coarser and longer, than those of corresponding cells in this area of the cortex of the three-month infant. The basal dendrites are distributed in layer V. A few varicosities are present toward the distal ends of the branches of the basal dendrites. Pedunculated bulbs are not as numerous on the apical dendrites of the large ordinary and special pyramidal cells as on the apical dendrites of the giant pyramidal cells of Betz. This difference in quantity of the bulbs is especially noticeable when an apical dendrite of a large ordinary or special pyramidal cell lies alongside the apical dendrite of a Betz cell. The apical dendrites of the large ordinary and special pyramidal cells have more pedunculated bulbs than the basal dendrites or the collaterals of the apical dendrites. The pedunculated bulbs appear to be somewhat more numerous on the apical dendrites of these cells in the cortex of the six-month infant than those on the apical dendrites of corresponding cells in this area of the cortex in the three-month infant. Pedunculated bulbs are clearly more numerous on the collaterals of the apical dendrites and on the basal dendrites of these cells in the six-month cortex than in the three-month cortex. The axons of most of the large ordinary and special pyramidal cells are stained for only about 2θμ to 30^ from the cell-bodies. Only rarely is an axon seen which descends as far as the mid-region of layer VI. The axons can easily be distinguished from dendrites by their smooth contour and by the absence of spines, thorns, and pedunculated bulbs. Surface irregularities and nodules are thickly scattered along the longer axons from the upper part of lamina Via to their distal ends. The number of axons stained is insufficient to permit any comparisons with axons of these cells in this region of the cortex of the threemonth infant.

LOBUS FRONTALIS The large ordinary and special pyramidal cells and the large spindle cells in layer V I are more advanced in development than the corresponding cells in this area in the cortex of the three-month infant. The dendrites are coarser and longer, and have more pedunculated bulbs than those of the cells in the threemonth cortex. Neither the dendrites nor their branches are any more numerous in the six-month cortex. Inverted pyramidal cells, both large and small, are more numerous in layer V I than in layer V, but are seen only occasionally. The apical dendrites of the larger pyramidal cells in all laminae of layer III terminate among the tangential fibers in layer I. These dendrites are coarser than those of corresponding cells in this area of the cortex in the three-month infant. The collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the three-month cortex. The terminal branches, however, are not only coarser but also more numerous in the six-month than in the three-month cortex. The basal dendrites of the pyramidal cells in all laminae of layer III are coarser and longer, but are no more numerous, than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in lamina IIIc than on the pyramidal cells in lamina I l l b , and are less numerous on the cells in I l i a than on those of cells in I l l b . The bulbs are slightly more numerous on the apical and basal dendrites, and are definitely more numerous on the terminal branches of the apical dendrites, than in the three-month cortex. The apical dendrites of the pyramidal cells in layer II end within the layer or in layer I. These dendrites are coarser than those of corresponding cells in this area of the cortex in the three-month infant. The basal dendrites are coarser and longer than those of corresponding cells in the three-month cortex. Pedunculated bulbs are not as numerous as those on the pyramidal cells in lamina I l i a in the six-month cortex, but they are slightly more numerous than those on the cells in layer II in the cortex of the threemonth infant. The axons of cells in layers II and III are stained for only a very short distance from the cell-bodies;

therefore, comparisons can not be made with the axons of corresponding cells in the three-month cortex. The tangential fibers in layer I are slightly larger and more numerous in the crown of gyrus centralis anterior than in the crown of the paracentral lobule. The fibers are collected chiefly in the outer one-half of layer I. The fibers in the inner one-half of the layer are not only fewer but much smaller than those in the outer one-half. In the crown of gyrus centralis anterior the tangential fibers in the outer one-half of layer I number about 14 to 16 to the unit. The fibers are of three sizes, large, small, and intermediate. In the Golgi preparations the largest fibers measure about ι.&μ in diameter between varicosities. Varicocities are present on all of the tangential fibers in the Golgi preparations. Short spines and thorns on the varicosities cause the fibers to have a tufted appearance. No collaterals have been observed on any of the tangential fibers, in either the Golgi or the Cajal preparations. Horizontal exogenous fibers are present in layer II. They are of about the same size as the tangential fibers in the inner one-half of layer I, and number 4 to 6 to the unit in the Cajal preparations. In I l i a the horizontal exogenous fibers number 10 to 12 to the unit. They are small, intermediate, and large in size. The horizontal fibers number 12 to 14 to the unit in lamina I l l b . In the inner part of lamina IIIc and layer IV (the outer band of Baillarger) the horizontal exogenous fibers number 14 to 16 to the unit. The fibers are small, intermediate, and large in size. The smallest fibers are very fine, barely visible with No. 40 objective and 15 X ocular. In Va the horizontal fibers number 12 to 14 to the unit. They are small, intermediate, and large in size. The largest fibers measure ι.&μ in diameter in the Golgi preparations. In the inner band of Baillarger (laminae Vb and V i a ) the horizontal exogenous fibers number 14 to χ 6 to the unit. The fibers are small, intermediate, and large in size, but the small and intermediate fibers are more darkly stained than they are in the more external strata. The largest fibers in the inner band of Baillarger measure 1 ,&μ in diameter. The horizontal exogenous fibers in lamina VIb number 12 to 14 to the unit. They are small, inter-

10

T H E C O R T E X OF THE SIX-MONTH I N F A N T

mediate, and large in size. T h e large fibers measure ι ,8μ in diameter. T h e subcortical association fibers, which are the horizontal fibers lying immediately internal to layer V I , number 34 to 36 to the unit. These fibers are small, intermediate, and large in size. T h e largest fibers measure 1.8/1 in diameter. In all layers of the cortex the horizontal exogenous fibers are of three sizes, small, intermediate, and large. T h e fibers of all three of these categories are smallest in the inner one-half of layer I and in layer II, and they gradually increase in size from layer I I toward layer V I . T h e horizontal fibers of all three categories of size are largest in lamina V I b and in the layer of subcortical association fibers. In the Golgi preparations all of the horizontal exogenous fibers and the subcortical association fibers bear varicosities, spines, and thorns. N o collaterals have been observed on any of these fibers.

in the Weigert sections, the first being the darkest in color: ( 1 ) 4 1 - 9 1 , (2) 37-82, (3) 38-35, (4) 37-49· The sections of 37-49 are but slightly darker than those of the sections in this area of the most heavily myelinated three-month brain, namely, 38-128. T h e sections of the other three six-month brains show a slight and gradual increase in the density of stain, those of brain 4 1 - 9 1 being the darkest. T h e following description is based upon the sections of brains 4 1 - 9 1 and 37-82.

Vertical exogenous fibers are more numerous in the core of gyrus centralis anterior than elsewhere. In this location they number 40 to 45 to the unit, and are small, intermediate, and large in size. In the Golgi preparations the largest fibers measure 1.8/1, in diameter. In the core of the gyrus the vertical exogenous fibers are chiefly collected in small bundles, but many fibers are scattered between bundles. T h e vertical fibers continue in this arrangement as they radiate toward the cortex. As the fibers ascend in the cortex they become subdivided into smaller and smaller bundles, and finally those which reach the highest level are isolated fibers. Most of the radiating vertical exogenous fibers end in lamina V I b , many end in the neurophil in laminae V b and V i a , and a few continue to layer I V . Only occasionally is a vertical exogenous fiber seen in layers III, II, or I. In the lower part of lamina V I b , the radiating fibers number 16 to 18, in the lower part of the inner band of Baillarger they number 8 to 10, and in the lower part of layer I V these fibers number but 4 to 6 to the unit. All the vertical fibers in Golgi preparations bear varicosities, spines, and thorns. N o collaterals are present.

Stained tangential fibers are uniformly distributed along the entire length and breadth of layer I in the crowns of the gyri as well as in the walls of sulci. T h e myelinated tangential fibers in the crowns of the gyri are faintly stained and number 10 to 12 to the unit of width (50/1), but in the walls of sulci they are more darkly stained and more numerous, averaging 12 to 14 to the unit. T h e stained tangential fibers are small, intermediate, and large in size, and all bear varicosities. In layer I I stained horizontal fibers number 2 or 3 to the unit, and they are lighter in color than the tangential fibers in layer I. A short, lightly stained horizontal fiber is occasionally seen in laminae I l i a and I l l b . In lamina I I I c and layer I V (the outer band of Baillarger) stained horizontal fibers number 5 or 6 to the unit, and they are light in color. In lamina V a there are 3 or 4 lightly stained horizontal fibers to the unit. In laminae V b and V i a (the inner band of Baillarger) the stained horizontal fibers number 8 to 10 per unit and are larger and more darkly stained than those in lamina Va. T h e stained horizontal fibers in lamina V I b are about the same size and color as those in lamina V i a and they number 10 to 12 to the unit. T h e stained horizontal fibers are small, intermediate, and large in size, and all bear varicosities. Stained subcortical association fibers number 16 to 18 per unit of width. T h e fibers are stained slightly more darkly than those in layer V I . T h e stained subcortical association fibers are small, intermediate, and large in size. T h e large fibers are more numerous than in layer V I . All the stained subcortical association fibers bear varicosities.

MYELiNATiON. T h e four brains used for study of the condition of myelination in this area of the cortex fall into the following order as to the density of the stain

T h e stained vertical exogenous fibers in the core of the gyrus are small, intermediate, and large in size and all bear varicosities. T h e y number 30 to 35 per unit of width. T h e vertical exogenous fibers are

LOBUS FRONTALIS stained more darkly than those in this area in the three-month cortex. The largest stained fibers measure 1.95/x between varicosities. As the vertical fibers ascend in the cortex they gradually become smaller and lighter in color. On most of them the stain ends in the middle of layer V I , on many it ends in layer V, and on still fewer fibers the stain ends in layer IV. A stained vertical fiber is occasionally seen in layer I I I ; one of them extends from layer V to layer I. These are not continuous with vertical fibers in layers V or V I , and are interpreted as myelinated axons of cells in this area of the cortex. Stained vertical fibers are more numerous in layers IV, V, and V I than in this area in the three-month cortex. REGION OF THE UPPER TRUNK, SHOULDER, ARM, AND FOREARM The cortex in this region of F A y measures 2.578 mm. in width. It is illustrated in Figures 9, 10, 11, and 12. CELL-BODIES

Layer I. The molecular layer measures 0.275 m m · width and contains 43 neurons to the unit. Horizontal cells of Cajal are few in number. Layer II. The outer granular layer is distinctly visible macroscopically. It measures 0.108 mm. in width, and contains 268 nerve cells to the unit. The cells range in size from 5/4/i to 9,/8/n. The external border of the layer is fairly sharp, but the internal boundary is not distinct because the small cells of layer II mingle with the larger pyramidal cells belonging to lamina I l i a . Layer III. This layer measures 0.756 mm. in width. In lamina I l i a the neurons number 62 to the unit and measure 10/6-2ο/ιομ. Lamina I H b contains 52 nerve cells to the unit, and they range in size from ιο/βμ to 26/12/Lt. The neurons number 52 to the unit in lamina IIIc and measure 10/6-29/τβμ. Layer IV. The inner granular layer is less distinct than the outer one. The small cells of this layer intermingle with the cells of laminae IIIc and Va. Layer IV measures 0.238 mm. in width, and contains 108 neurons to the unit. The nerve cells range in size from 5/4/* to 13/ιομ. Layer V. This layer measures 0.598 mm. in width. Small and large neurons are scattered throughout the

11

layer. The giant pyramidal cells of Betz are located chiefly in lamina Vb, but some are present in lamina Va also. Lamina V a contains 50 small and 21 large neurons to the unit. In lamina V b there are 45 small, 18 large, and 9 giant neurons per unit. The small cells measure 10/6-16/10^, the large cells measure 23/ 12-29/13/i, and the giant pyramidal cells of Betz range in size from 33/12/A to 59/20/*. The Betz cells are larger in the posterior wall of gyrus centralis anterior than elsewhere. In the crown of the gyrus the Betz cells become smaller and are usually long and slender in shape. In the crown of the gyrus they become merely the extra-large pyramidal cells in lamina V b without any perceptible change, and it is impossible to determine where the Betz cells end and the ordinary extra-large pyramidal cells begin. In the anterior lip of the gyrus the extra-large cells begin to become broader and shorter. In the anterior wall of the gyrus they become yet shorter and broader, and they are smaller than the giant pyramidal cells in the posterior wall of the gyrus. The extra-large pyramidal cells continue across the floor of the precentral sulcus and into the posterior wall of the superior frontal gyrus, area F B . There is no perceptible difference in size or appearance between the extra-large pyramidal cells in the anterior wall of gyrus centralis anterior, the so-called strip area, and those in the posterior part of the superior frontal gyrus, area F B . Layer VI. The polymorphic layer measures 0.603 mm. in width. Lamina V i a contains 60 neurons to the unit and lamina V I b has 40 cells per unit. The pyramidal cells measure 10/6-26/15/1 and the spindle cells measure 16/6-2 9/13/t. In general, the giant pyramidal cells of Betz are more darkly stained than any other cells in this region, although some of the giant cells are quite light in color. The chromophil substance is further advanced in differentiation into Nissl bodies than in the Betz cells of this area in the three-month cortex. The Nissl bodies are larger and more numerous, granules are less numerous, and clumps of chromophil substance are smaller and less numerous than in the cortex of the three-month infant. In the six-month brains the Nissl bodies are larger and more numerous, and granules and clumps are less in quantity in the Betz cells in this region than in the region of the lower extremity. The Nissl bodies are chiefly in the form

12

THE CORTEX OF THE SIX-MONTH INFANT

of flakes. Elongated strands of chromophil substance are more numerous in these cells than in the region of the lower extremity, and the chromophil substance invades the apical and basal dendrites for a greater distance. Nissl bodies, elongated and directed longitudinally, are more numerous in the apical dendrites than they are in the three-month brain. The nuclear membrane is thick and darkly stained. It is fairly uniform in width throughout the entire extent, except that in most of the Betz cells it is slightly thicker immediately under the base of the apical dendrite than elsewhere. In many cells the nuclear membrane is similarly thickened immediately under the origin of the larger basal dendrites. A cone-shaped clump of chromophil substance is resting on the nuclear membrane just under the base of the apical dendrite in many of the Betz cells. The karyoplasm is lighter in color than the cytoplasm, and contains scattered small clumps and granules of chromophil substance. The nucleolus is stained very darkly, and usually has two or more small, darkly stained spheres clinging to the periphery. The large ordinary and special pyramidal cells in layers V and VI and the large spindle cells in VI have much the same appearance as corresponding cells in the area of the lower extremity in the six-month cortex. The chromophil substance is principally in the form of amorphous clumps, but granules and a few Nissl flakes are present. The clumps are smaller and the Nissl flakes are more numerous than those in the corresponding cells in this area in the cortex of the three-month infant. In almost all of these cells a small clump of darkly stained chromophil substance rests upon the outer surface of the nuclear membrane immediately under the origin of the apical dendrite. This clump is cone-shaped in many of the cells. The chromophil substance extends for a greater distance into the apical dendrite than in the three-month cortex. The basal dendrites are stained for a very short distance from the cell-body, and a clump of darkly stained chromophil substance is usually present at the origin of each dendrite. There is slightly more chromophil substance in the basal dendrites of these cells in the six-month cortex than in that of the threemonth infant. The large pyramidal cells in lamina IIIc are smaller than the large ordinary and special pyramidal

cells in layers V and VI. The cells in IIIc are more darkly stained than any other cells in layer III, but they are not as dark as the most heavily stained large ordinary pyramidal cells in layers V and VI. Clumps of chromophil substance are more numerous than in the large ordinary pyramidal cells in layers V and VI. The clumps are collected chiefly at the origin of the basal dendrites, and in many of the cells a coneshaped clump rests upon the outer surface of the nuclear membrane immediately under the base of the apical dendrite. Granules and a few small Nissl flakes are present in these cells. The chromophil substance extends into the apical dendrites for a shorter distance than in the large ordinary pyramidal cells in layers V and VI. The basal dendrites are stained very lightly and for a very short distance from the cellbody. They contain a small number of granules of chromophil substance at their origins only. The chromophil substance decreases in quantity with the decrease in size of cells in laminae I l l b and Ilia. It is in the form of fine granules and small clumps. With decrease in cell size the nuclear membrane becomes thicker, and the small beads of chromophil substance on its surface increase in number. The small nerve cells in layers III, V, and VI are all similar to one another in appearance and are lightly stained. The small cells in layer II are more lightly stained than any other cells and contain the least amount of chromophil substance. The large horizontal cells of Cajal in layer I are seen less frequently than in this area of the cortex in the three-month infant. The nucleus is eccentric in almost every one of these cells. Some of the cells are normal in appearance, but others are more or less shrunken and appear to be degenerating. All of the giant pyramidal cells of Betz are well filled with neurofibrils. The neurofibrils extend far into the apical and basal dendrites and into the axons. The dendrites and axons are stained more darkly than the cell-bodies. The collaterals of the apical dendrites also contain neurofibrils. The largest of the ordinary and special pyramidal cells in layers V and VI contain some neurofibrils, and the latter extend far into the axons and dendrites. These cells also contain granules arranged in longitudinal rows. The axons and dendrites are stained more darkly than the cell-bodies.

LOBUS FRONTALIS

13

The large cells in lamina IIIc closely resemble the large ordinary pyramidal cells in layer V. Some neurofibrils are present in the cell-body and in the processes. Longitudinal rows of granules are also present. The axons and dendrites are stained more darkly than the cytoplasm in the cell-body. The cytoplasm in the large cells in lamina I l l b is entirely granular, but the cells and processes contain some longitudinal strands of granules. The cytoplasm in the larger cells in lamina I l i a is entirely granular in the cell-bodies and processes, and no longitudinal rows of granules are present. In all small cells, including those in layers I I and IV, the cytoplasm is granular and the granules are scattered irregularly throughout the cell-bodies and processes.

I than in the cortex of the three-month infant. The bulbs are most numerous on the apical dendrite of each Betz cell as it traverses layer IV and lamina IIIc (the inner band of Baillarger), and they gradually decrease in number in lamina I l l b , then decrease rapidly in lamina I l i a , and are least numerous on the terminal branches in layer I. The bulbs are present on the collaterals of the apical dendrites and on the basal dendrites in approximately equal quantity, but are much less numerous than those on the apical dendrite as it passes through layer IV and lamina IIIc. Pedunculated bulbs are much more numerous on the collaterals of the apical dendrites and on the basal dendrites than on the terminal branches of the apical dendrites in layer I.

The large cells in layers I I I , V, and VI are more advanced as to differentiation of neurofibrils than corresponding cells in this region of area F A y in the cortex of the three-month infant. In the cortex of the six-month infant the differentiation of neurofibrils is more advanced in the cells in this region of area F A y than in corresponding cells in the region of the lower extremity.

The axons of the giant pyramidal cells of Betz are the largest of all axons in this area. They extend vertically downward in a more or less tortuous course, and enter the white substance subjacent to the cortex, where they soon become lost in the multitude of exogenous fibers. The axons are smooth in contour until they reach the level of lamina Via, where they acquire a fairly heavy coating of myelin, indicated by a fairly abrupt enlargement of the axon and by the presence of surface irregularities. In the lower part of layer V I and in the subjacent white substance the axons bear varicosities which are more or less regularly spaced, and thus resemble the exogenous fibers. Each axon gives off two or three collaterals while it is passing through lamina Via. The collaterals extend horizontally in lamina Via, and some of them give off one or two secondary collaterals. No tertiary branches have been observed on any secondary collaterals. The axons of the Betz cells in this region of the six-month cortex are larger and have more myelin than the Betz cells in the same region in the cortex of the threemonth infant. The primary collaterals of the axons are of the same number (2 or 3 per axon) in the two ages, but those in the six-month cortex are coarser, longer, and have more branches than those in the three-month brain. In the six-month cortex the axons of the Betz cells in the region of the trunk, shoulder, and arm are not as coarse, as long, or as extensively branched as those of the Betz cells in the region of the lower extremity.

FIBERS. The apical dendrites of the Betz cells are of about the same size as those of the Betz cells of corresponding size in the region of the lower extremity. In general, the apical dendrites of the giant pyramidal cells are coarser than those of corresponding cells in the area of the trunk, shoulder, and arm in the threemonth cortex. Terminal branches of the apical dendrites in layer I are more numerous than in the cortex of the three-month infant. The collateral branches of the apical dendrites are coarser, but not any longer or more numerous, than those of corresponding cells in the three-month cortex. The basal dendrites of the Betz cells are coarser, but not any longer or more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are present on the dendrites of the Betz cells in approximately the same quantity as in the region of the lower extremity. The bulbs are more numerous than on the dendrites of corresponding cells in the region of the trunk, shoulder, and arm in the cortex of the three-month infant. The pedunculated bulbs are conspicuously more numerous on the terminal branches of the apical dendrites in layer

The longest apical dendrites of the large ordinary and special pyramidal cells in layer V end in layer I

14

THE CORTEX OF THE SIX-MONTH INFANT

by branching dichotomously two or three times. The apical dendrites of most of these cells end in lamina M b or IIIc, where they terminate as a single stalk or by two or three branches. The apical dendrites of the ordinary pyramidal cells are larger than those of the special pyramidal cells, and those of both these kinds of cells are coarser and more compact in structure than those of corresponding cells in this area of the three-month cortex. The terminal branches of the apical dendrites of the large ordinary and special pyramidal cells are more numerous than those of corresponding cells in the cortex of the three-month infant. The basal dendrites of these cells are coarser, longer, and more compact in structure than those of corresponding cells in the three-month cortex. Pedunculated bulbs are most numerous on the apical dendrites as they traverse layers IV and IIIc. The bulbs gradually decrease in number in I l l b and IIIc, and only a few scattered ones are present on the terminal branches in layer I. The pedunculated bulbs are less numerous on the basal dendrites and on the collaterals of the apical dendrites than they are on the apical dendrites. The bulbs are much less in quantity on the apical dendrites of these cells than on the apical dendrites of the giant pyramidal cells. The bulbs are more numerous than those on the apical and basal dendrites of the large ordinary and special pyramidal cells in layer V in this area in the three-month cortex. The axons of the large ordinary and some of the special pyramidal cells in layer V extend vertically downward, pursuing a more or less tortuous course through layer VI, enter the layer of subcortical association fibers, then turn and extend parallel to these fibers. The axons of these cells are coarser and the branches are larger than those of corresponding cells in the cortex of the three-month infant. Each axon acquires a coat of myelin a short distance from the cell-body, and has two or three collaterals. The collaterals mingle with and extend parallel to the horizontal exogenous fibers in laminae Vb and Via. The axons of some large special pyramidal cells extend downward for a short distance within layer V, then form a loop and ascend. Some of these ascending axons have been traced to the mid-region of layer III. The axons of these cells give off several collaterals; as many as five have been counted on some of them. The collaterals extend horizontally, mingling with the

horizontal exogenous fibers. Small globules and surface irregularities indicate a slight amount of myelin on the axon and collaterals. The apical dendrites of the large ordinary and special pyramidal cells in layer VI are neither as large nor as long as the apical dendrites of the cells of this category in layer V. Pedunculated bulbs are not as numerous as they are on the dendrites of these cells in layer V. The apical dendrites ascend to various levels, some ending in layer V, others in layer IV, and a few in the lower part of layer III. The axons of these cells in layer VI are smaller in caliber than those of the large ordinary and special pyramidal cells in layer V. The axons extend downward into the subcortical layer of exogenous fibers, and soon become lost among these. Two or three delicate branches are seen on some of the axons. A small amount of myelin is present on the axons as they pass through layer VI, and the myelin increases in quantity in the lower part of the layer. The large spindle cells in layer VI are in the same state of development as the large pyramidal cells. Their axons descend into the layer of subcortical association fibers, then turn to extend among and parallel to these fibers. The dendrites of the large pyramidal and spindle cells in layer VI are coarser and have more pedunculated bulbs than those of corresponding cells in this area of the cortex of the three-month infant. The axons are coarser than those of these cells in the three-month cortex. The apical dendrites of the large pyramidal cells in lamina IIIc ascend to layer I, ending by branching dichotomously two or three times. The apical dendrites are larger than those of corresponding cells in this area in the three-month cortex. The collaterals of the apical dendrites, the basal dendrites, and their branches are coarser and longer, but no more numerous, than those in the three-month cortex. Pedunculated bulbs on the dendrites and their branches are more numerous than on the dendrites of corresponding cells in the cortex of the three-month infant. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V. The longest axons of large pyramidal cells in lamina IIIc extend vertically downward and can be traced to lamina Vb or Via, where they end abruptly

LOBUS FRONTALIS as though they have been cut. At about $ομ to 60μ from the cell-body small nodules and other surface irregularities appear, and the axon increases slightly in diameter. The nodules gradually increase in quantity and the axon gradually increases in diameter as it proceeds downward. As the axon traverses layer V enlarged nodules or varicosities appear and these are more or less regularly spaced along the axon to its termination. The nodules, surface irregularities, and increase in diameter are interpreted as myelin. One or two very short, very delicate branches emerge from the axon as it passes through layer IV. These collaterals are much smaller and shorter than those of the axons of the large ordinary and special pyramidal cells in layer V. Proceeding externally from lamina IIIc, the pyramidal cells in laminae I l l b and I l i a gradually become smaller, and there is a gradual decrease in the caliber of their apical and basal dendrites. There is also a gradual decrease in the number of pedunculated bulbs on the dendrites and their branches. The dendrites of the pyramidal cells in laminae I l i a and I l l b are coarser than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the cortex of the three-month infant. Pedunculated bulbs are present in greater quantity on the dendrites of these cells in the six-month cortex than in the three-month brain. Pedunculated bulbs are present on the terminal branches of the apical dendrites of the pyramidal cells in I l i a , but they are rather widely distributed. The axons of the large pyramidal cells in lamina I l l b are much more slender than the axons of the large pyramidal cells in lamina IIIc. The longest axons of cells in I l l b can be traced into the midregion of layer IV. The axons of the larger pyramidal cells in lamina I l i a are shorter and more slender than those of the large pyramidal cells in lamina I l l b . They end at various levels in lamina I l l b . Small pyramidal cells are present in all laminae of all six layers of the cortex. Their dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the three-month cortex. These small pyramidal cells are more advanced in development in the deeper laminae than in the more super-

IS

ficial laminae. These cells in layer IV are more advanced than those in layer II. Pedunculated bulbs are more numerous on the dendrites of the small pyramidal cells in layer IV than on the small pyramidal cells in layer II. The bulbs are very small and sparsely scattered on the terminal branches of the apical dendrites of the small pyramidal cells in layer II. Golgi type II cells are present in all parts of all six layers, but they are most numerous in layers II and IV. All the Golgi type II cells have larger and longer dendrites and axons than the corresponding cells in the area of the cortex in the three-month infant. The axons are more elaborately branched than in the three-month cortex, and the mesh of fine fibers formed by the axons is more dense in all layers than in the three-month brain. This mesh is more dense in layer IV than elsewhere. The mesh of fibers in layer IV does not receive any fibers from the ascending vertical exogenous fibers, but consists entirely of axons of Golgi type II cells. The mesh diminishes in quantity in lamina Va, then increases in laminae Vb and V i a (the inner band of Baillarger), then decreases markedly in lamina VIb. Proceeding externally the mesh of axons of Golgi type II cells is less in quantity in lamina IIIc than in layer IV, is much less in density in I l l b than in IIIc, and is present as only a few fibers in lamina I l i a . The axons of Golgi type II cells in layer II are not sufficiently long or numerous to form a mesh in this layer. Horizontal cells of Cajal are seen but rarely in layer I. The tangential fibers in layer I are more numerous, larger, and more darkly stained in the floor of the central sulcus and in the posterior wall and posterior one-half of the crown of gyrus centralis anterior than in the anterior one-half of the crown and anterior wall of the gyrus. In other words, the tangential fibers are most numerous, larger, and more darkly stained in the region of the giant pyramidal cells. Furthermore, the tangential fibers are larger and more numerous in the posterior wall of gyrus centralis anterior than in the posterior one-half of the crown of the gyrus or in the floor of sulcus centralis. The giant pyramidal cells are larger and more numerous in the posterior wall of the gyrus than in the crown of the gyrus or in the floor of the sulcus. The tangential

16

THE CORTEX OF THE SIX-MONTH INFANT

fibers are almost entirely confined to the outer onehalf of layer I, only a very few small fibers being located in the inner one-half. In the external one-half of layer I in the posterior wall of gyrus centralis anterior the tangential fibers number 16 to 18 to the unit. The fibers are small, intermediate, and large in size. The largest fibers in the Golgi preparations measure τ.&μ in diameter. All the tangential fibers bear varicosities, spines, and thorns, and present a tufted appearance. No collaterals have been observed on any of the tangential fibers. The horizontal exogenous fibers in layer II number 4 to 6 to the unit. They are the same size as the tangential fibers in the internal one-half of the molecular layer, and are much smaller than those in the external one-half. Internal to layer II the horizontal exogenous fibers gradually increase in number. Small, intermediate, and large horizontal exogenous fibers are present in all the layers, but internal to layer II there is a gradual increase in the size of the fibers in each category in the successive layers. In lamina I l i a the fibers number 8 to ι ο to the unit, and are but slightly larger than those in layer II. In lamina I l l b the horizontal fibers number 12 to 14 to the unit. Those in lamina IIIc and in layer IV (the outer band of Baillarger) number 14 to 16 to the unit. In lamina Va the horizontal fibers number 10 to 12 to the unit. The inner band of Baillarger in laminae Vb and V i a contains 14 to 16 horizontal exogenous fibers to the unit. In lamina VIb the horizontal fibers number 12 to 14 to the unit. The horizontal subcortical association fibers number 28 to 30 to the unit. In all layers the horizontal exogenous fibers are small, intermediate, and large in size. The largest horizontal fibers in laminae V i a and VIb measure τ.8μ in diameter. The largest subcortical association fibers measure τ.&μ in the Golgi preparations. All the horizontal exogenous fibers bear varicosities, spines, and thorns, and present a tufted appearance. No collaterals have been observed on any of these fibers. The vertical exogenous fibers are the most numerous in the center of the core of gyrus centralis anterior, where they number 40 to 45 to the unit. They are small, intermediate, and large in size. The largest fibers measure 2.ομ in diameter in the Golgi sections. All the vertical exogenous fibers bear varicosities, spines, and thorns, and by these structures they can

be distinguished from endogenous axons. The vertical exogenous fibers are chiefly collected in bundles as they radiate toward and into the cortex, but a few fibers are scattered between the bundles. Most of the radiating fibers end in layer VI. In the lower part of this layer the radiating fibers number 12 to 14 to the unit. Some of the radiating vertical fibers end in the neuropil in the inner band of Baillarger, numbering here 6 to 8 to the unit. A few vertical fibers end in layer IV, numbering 2 to 4 to the unit, and still fewer ascend into the middle of layer III. Isolated vertical fibers are occasionally present in layer II or I, and these are seen more frequently than in this area in the three-month cortex. M Y E L i N A T i O N . With reference to the density of stain in the Weigert sections in this area, the brains fall into the following order, beginning with the one having the most myelin: (1) 37-82, (2) 38-35, (3) 3 7 49. The sections of brain 37-49 are definitely more darkly stained than those of brain 38-128, which is the three-month brain with the most myelin in this region. The sections of brain 37-82 in the region of the trunk, shoulder, and arm are more darkly stained than the sections of this brain in the region of the lower extremity. The following description is based upon the sections of brain 3 7-82.

Only an occasional, isolated, and faintly stained tangential fiber is seen in the anterior one-half of the crown of gyrus centralis anterior. The stained fibers become more numerous at about the middle of the crown of the gyrus, where they number only 2 or 3 to the unit, and they are small and faintly stained. The stained tangential fibers in layer I gradually increase in number, size, and density of stain toward the posterior lip, and reach a maximum in these three respects in the posterior wall of the gyrus. In the posterior wall of the gyrus the stained fibers are small, intermediate, and large in size, and number 6 to 8 to the unit of width. All of these fibers bear varicosities. No stained horizontal fibers are present in layer II or in laminae I l i a and I l l b in the crown of gyrus centralis anterior. In the posterior wall of the gyrus a short, small, lightly stained horizontal fiber is occasionally present in layer II and in lamina I l i a . About 2 or 3 short, small, lightly stained horizontal fibers

LOBUS FRONTALIS per unit are present in lamina I l l b . In lamina IIIc and layer IV (the outer band of Baillarger) stained horizontal fibers number 6 to 8 to the unit, and they are slightly larger and more darkly stained than those in lamina IIIc. In lamina Va the stained horizontal fibers are slightly larger and more darkly stained than those in layer IV, but they are less numerous, numbering only 4 or 5 to the unit. The stained horizontal fibers in laminae Vb and V i a (the inner band of Baillarger) are larger, more darkly stained, and more numerous than those in lamina Va. They number 8 to 10 to the unit of width. In lamina VIb the stained horizontal fibers are larger and more darkly stained than those in the inner band of Baillarger. They number 10 to 12 to the unit. The stained horizontal fibers in the cortex are small, intermediate, and large in size, and all of them bear varicosities. The stained subcortical association fibers are more darkly stained than the horizontal fibers in layer VI. The stained fibers are small, intermediate, and large in size, but the fibers are generally larger than those in lamina VIb. The stained subcortical association fibers number 16 to 18 to the unit. All the stained subcortical fibers bear varicosities. The vertical exogenous fibers in the core of the gyrus are stained more darkly than those in this area in the three-month cortex. The stain in the Weigert sections is darker on the vertical fibers underlying the cortex in the posterior wall of gyrus centralis anterior than elsewhere. It is slightly lighter on the vertical fibers in the middle of the core, and yet slightly lighter on those fibers which are subjacent to the cortex in the anterior wall of the gyrus. There is less difference between the color of the vertical fibers under the posterior wall and that of the fibers under the anterior wall than in the three-month cortex. The stain on the vertical fibers gradually becomes lighter in color as the fibers radiate toward the cortex, and it continues to become lighter as the fibers ascend within the cortex. The stain ends on most of the vertical fibers in the middle of layer VI, on some in layer V, and on a few in layer IV. The vertical exogenous fibers are small, intermediate, and large in size and all bear varicosities. The largest fibers measure 2.ζμ between varicosities, and they are the darkest in color. In the core of the gyrus immediately below .the place where they begin to radiate toward the cortex in the pos-

17

terior wall of the gyrus the stained vertical fibers number 30 to 35 to the unit. REGION OF THE HAND

The cortex in this region of area F A y has the same general appearance microscopically as in the threemonth infant. It measures 2.256 mm. in width in the posterior wall of gyrus centralis anterior and is illustrated in Figures 13, 14, 15, and 16. CELL-BODIES

Layer I. This layer measures 0.243 m m · i n width, and contains 50 neurons to the unit. Horizontal cells of Cajal are seen occasionally. Layer II. The external granular layer is distinctly visible macroscopically. The external boundary of the layer is fairly sharply defined, but the inner boundary is indistinct because the small cells of layer II intermingle with the cells of lamina I l i a . Layer II measures 0.093 m m · i n width and contains 251 neurons to the unit. The nerve cells range in size from 5/4/i to ς>/8μ. Layer III. This layer measures 0.672 mm. in width. In lamina I l i a the neurons number 58 to the unit and measure 10/6-23/12μ; in lamina I l l b they number 52 to the unit and measure 10/6-26/13^. In lamina IIIc the nerve cells number 52 to the unit and range in size from ιο/6μ to 29/1 ζμ. Layer IV. The inner granular layer is not visible macroscopically, but is clearly apparent under the microscope. The external and internal boundaries of the layer are indistinct because the large cells of laminae IIIc and Va invade the granular layer. Layer IV measures 0.243 m m · ' n width and contains 109 neurons to the unit. The nerve cells range in size from 5/4/x to 13/ιομ. Layer V. The inner pyramidal layer measures 0.456 mm. in width, and is divisible into an external light lamina and an internal darker lamina. Small and large neurons are scattered throughout the width of the entire layer. The giant pyramidal cells of Betz are located principally in lamina Vb, but also occur in lamina Va. The Betz cells are distributed in gyrus centralis anterior in the same manner and extent as in the three-month cortex. The Betz cells extend across the floor of sulcus centralis and occupy approximately the lower one-fourth of the anterior wall

18

THE CORTEX OF THE SIX-MONTH INFANT

of gyrus centralis posterior. In lamina V a the small neurons number 50 to the unit and the large neurons number 19 per unit. In lamina V b the small cells number 40 to the unit and the large cells average 15 per unit. The giant pyramidal cells of Betz number 8 to the unit. The small neurons measure ι ο / 6 - ι ό / ι ο μ , the large neurons measure 2 0/10-2 9/12 μ, and the Betz cells range in size from 33/12/u, to 56/15/*. Layer VI. This layer measures 0.549 mm. in width. The neurons number 70 to the unit in lamina V i a and 45 in lamina VIb. The pyramidal cells measure 10/6—2 6/12 μ, and the spindle cells range in size from ι6/6μ to 33/10/i. The chromophil substance is greater in quantity and more advanced in development in the giant pyramidal cells of Betz than in any other cells in this area. Most of the Betz cells are more darkly stained than any of the other cells, but some of them are quite light in color. The chromophil substance in the Betz cells is more advanced in differentiation into Nissl bodies than it is in these cells in this area in the three-month cortex. In the six-month cortex the chromophil substance in the giant cells in the region of the hand is further advanced in differentiation into Nissl bodies than it is in the region of the trunk and arm or in the region of the lower extremity. The Betz cells in the region of the hand have not only some Nissl flakes with fuzzy borders, but also many well-formed Nissl bodies with sharply defined outlines. Clumps of chromophil substance are present, but they are smaller and less numerous than in the region of the trunk or the lower extremity. The clumps may be scattered throughout the cytoplasm, but are usually clustered around the origins of the basal dendrites. In many of the Betz cells a coneshaped clump rests upon the outer surface of the nuclear membrane immediately under the origin of the apical dendrite. Small granules of chromophil substance are present in less quantity than in the Betz cells in the regions of the trunk and lower extremity. Chromophil substance extends into the apical dendrites in greater quantity and for a greater distance than in the Betz cells in the regions of either the trunk or the lower extremity. On some of the apical dendrites one or two collaterals near the cell-body are faintly stained for a short distance. Chromophil substance also extends into the basal dendrites in greater

quantity and for a greater distance than it does in the region of the trunk or the lower extremity. The chromophil substance in both the apical and basal dendrites is in the form of granules and of slender, darkly stained strands which are directed longitudinally. Such strands are present also in the cellbody. Chromophil substance invades the apical and basal dendrites for a greater distance than in the Betz cells in this area in the three-month cortex. The chromophil substance in the large ordinary and special pyramidal cells in layers V and V I and in the large spindle cells in V I is greater in quantity and is more advanced in development than in corresponding cells in this area in the three-month cortex. Nissl bodies are more numerous, and amorphous clumps of chromophil substance are smaller than in the threemonth cortex. Chromophil substance extends into the apical dendrites for a greater distance in the sixmonth cortex. The basal dendrites are stained for a very short distance, and contain very little chromophil substance. A collection of clumps of chromophil substance is usually located at the point where a basal dendrite arises from the cell-body. The large cells in lamina IIIc resemble the large ordinary pyramidal cells in layer V as to the condition of the chromophil substance. The large cells in lamina IIIc are more advanced in development than the corresponding cells in this area in the cortex of the three-month infant. The chromophil substance in these cells in the six-month cortex is greater in quantity, and extends into the apical dendrites for a greater distance, than in the three-month cortex. One or two short, faintly stained collaterals are present on some of the apical dendrites. A small clump of darkly stained chromophil substance is present at the place of origin of a collateral. The basal dendrites are short and faintly stained, but they are stained for a greater distance from the cell-body than in the three-month cortex. A small accumulation of clumps of chromophil substance is often present at the place where a basal dendrite arises from the cell-body. In some cells a cone-shaped clump of chromophil substance rests upon the nuclear membrane under the origin of the apical dendrite. As the cells decrease in size external to lamina IIIc the chromophil substance diminishes in quantity. The chromophil substance in the large cells in lami-

LOBUS FRONTALIS nae I l l b and I l i a is greater in quantity than in corresponding cells in the three-month cortex. Amorphous clumps of chromophil substance are smaller and more numerous in the cells in the sixmonth than in the three-month cortex. No elongated strands of chromophil substance like those in large cells in lamina IIIc are present in the large cells in either laminae I l l b or I l i a . The large cells in I l l b contain a few well-formed Nissl bodies. The small cells in layers III, V, and V I closely resemble one another as to the chromophil substance. These cells contain more chromophil substance than corresponding cells in this region in the cortex of the three-month infant. The substance is in the form of granules and clumps. Agglutinations are present on the outer surface of the nuclear membrane. The apical dendrites are extremely short, and the basal dendrites are not in evidence. Large horizontal cells of Cajal are present in layer I in about the same number and condition as in the region of the trunk and lower extremity. All the giant pyramidal cells of Betz contain neurofibrils. The neurofibrils are more numerous and more distinct than those in the Betz cells of corresponding size in the region of the trunk, shoulder, and arm. The neurofibrils extend into the apical dendrites as high as layer II, and into the basal dendrites for almost their entire length. Neurofibrils also extend into the axons for some distance. The apical and basal dendrites and the axons are stained more darkly than the cell-body. No longitudinal rows of granules are present in any of the Betz cells. A few neurofibrils are present in the bodies, dendrites, and axons of the large ordinary pyramidal cells in layers V and V I and in lamina IIIc. Longitudinal rows of granules and scattered granules are also present in these cells. The large pyramidal cells in lamina IIIc and in layers V and V I are in about the same state of development as to differentiation of neurofibrils as corresponding cells in the region of the trunk, shoulder, and arm. Differentiation of neurofibrils is more advanced in these cells than in corresponding cells in the region of the hand in the cortex of the three-month infant. The cytoplasm in all other cells in the region of the hand is granular in structure. In the largest cells in lamina I l l b a few longitudinal rows of granules are

19

present in the cell-bodies and in the dendrites and axons. In all other cells the granules are scattered irregularly throughout the cell-bodies and their processes. The apical and basal dendrites of the giant pyramidal cells of Betz are coarser and longer than those of any other cells in this area. The dendrites and their branches are coarser, and the basal dendrites and their branches are longer, than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the apical dendrites than on their collaterals or on the basal dendrites and their branches. The bulbs are present on the terminal branches of the apical dendrites. Pedunculated bulbs are present in greater quantity than on the dendrites of corresponding cells in the cortex of the three-month infant. In the sixmonth cortex the dendrites of the giant pyramidal cells in this area are in approximately the same state of development as to size and quantity of pedunculated bulbs as those of corresponding cells in the region of the upper trunk, shoulder, and arm. FIBERS.

The axons of the giant pyramidal cells of Betz are much coarser and have much more myelin than the axons of corresponding cells in this area of the cortex of the three-month infant. In the six-month cortex the axons of the Betz cells in this region are of about the same caliber as those of corresponding cells in area FA-y in the regions of the lower extremity and of the shoulder and arm, but the axons of the cells in the region of the hand are more heavily coated with myelin than the axons in the other two regions. The axons of the Betz cells are much larger than those of any other cells in the cortex in this area. They extend vertically downward, enter the white substance in the core of gyrus centralis anterior, and mingle with and extend parallel to the vertical exogenous fibers. The apical dendrites of the large ordinary and special pyramidal cells in layer V are coarser and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The basal dendrites and their branches, and the collaterals of the apical dendrites are coarser and longer and have more peduculated bulbs than those of corresponding cells in the cortex of the three-month infant. In the six-month cortex the dendrites of these cells in this

20

THE CORTEX OF THE SIX-MONTH INFANT

area are in about the same state of development as those of corresponding cells in the area of the trunk. The axons of the large ordinary and special pyramidal cells in layer V are much smaller than those of the giant pyramidal cells of Betz. As they descend through layer V I they acquire a thin coat of myelin at about the mid-region of this layer, and small amorphous clumps are scattered along the axons as they traverse lamina VIb and the subjacent white substance. The large pyramidal and spindle cells in layer V I are in practically the same state of development as the large ordinary and special pyramidal cells in layer V. The apical dendrites of the large pyramidal cells in all three laminae of layer I I I end in layer I. The terminal branches are more numerous than those of the apical dendrites of corresponding cells in this area in the three-month cortex. The apical dendrites are coarser than those in the three-month cortex. The basal dendrites and collaterals of the apical dendrites are larger and longer, but no more numerous, than those in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of the large cells in lamina IIIc than those on the large cells in lamina I l l b , and those on the dendrites of the large cells in I l l b are more numerous than those on the dendrites of the large cells in lamina I l i a . The bulbs are much less numerous on the dendrites of the large pyramidal cells in IIIc than those on the dendrites of the giant pyramidal cells of Betz, but are somewhat more numerous than those on the dendrites of the large ordinary and special pyramidal cells in layer V. Pedunculated bulbs are present in small quantity on the terminal branches of the apical dendrites of the large pyramidal cells in lamina IIIc, are still less in quantity on the terminal branches of cells in I l l b , and are sparsely scattered on these branches of the apical dendrites of the pyramidal cells in I l i a . The pedunculated bulbs are present on all the dendrites of all cells in layer I I I in greater quantity than on the dendrites of corresponding cells in this area in the three-month cortex. The axons of the large cells in IIIc are smaller and have less myelin than those of the giant pyramidal cells of Betz. The longest axons of large cells in laminae I l l b and IIIc can be traced to lamina VIb.

Myelin appears on the axons as near the cell-bodies as in the case of the axons of Betz cells. The myelin gradually increases in quantity as the axons descend, then increases abruptly in lamina Va, and remains about the same below this location. Two or three short, delicate branches emerge from some of the axons as they traverse the outer band of Baillarger. The axons of the large cells in laminae IIIc and I l l b are larger and have more myelin than those of corresponding cells in this area in the cortex of the threemonth infant. The axons of pyramidal cells in lamina I l i a are stained for only a very short distance from the cell-bodies and end within the lamina. The dendrites of the small pyramidal cells in layers VI, V, IV, III, and I I are coarser and longer than those of corresponding cells in this area in the cortex of the three-month infant. The apical dendrites of the pyramidal and granular cells in layer II end by arborizations in layer I. A few pedunculated bulbs are sparsely scattered along the apical and basal dendrites. The axons end in layer II or in lamina I l i a , and show no special increase in size or length beyond the three-month stage of development. Horizontal cells of Cajal are very rarely seen in layer I. The mesh of fibers formed by the axons of Golgi type II cells is more dense than that in this area in the cortex of the three-month infant. It is the most dense in layer IV, considerably less dense in laminae Vb and Via, and is least dense in lamina I l i a . The mesh receives no fibers from the ascending vertical exogenous fibers. No branches, either collateral or terminal, have been observed on any ascending vertical exogenous fiber. All of these fibers end as vertically directed stalks. The tangential fibers in layer I are more numerous in the posterior wall of gyrus centralis anterior than in the crown of the gyrus or in the floor of sulcus centralis. The giant pyramidal cells of Betz are largest and most numerous in the posterior wall of the gyrus. The tangential fibers are much more numerous in the external one-half of layer I than in the internal onehalf. In the posterior wall of gyrus centralis anterior the fibers number 14 to 16 to the unit. They are small, intermediate, and large in size. The largest fibers in the Golgi preparations measure ι.&μ in diameter. The smallest fibers are very fine and very lightly stained.

LOBUS FRONTALIS All the fibers bear varicosities, spines, and thorns. No branches have been observed on any of the tangential fibers. Horizontal exogenous fibers number 4 to 6 to the unit in layer II. They are principally small and faintly stained, but large and medium-sized fibers are also present. Internal to layer I I the horizontal exogenous fibers in each layer of the cortex are small, intermediate, and large in size, but the fibers in each of these categories gradually become larger and more darkly stained at the successively deeper levels, and are largest and darkest in layers V and VI, and in the layer of subcortical association fibers. In lamina I l i a the horizontal exogenous fibers number 8 to 10 to the unit. In lamina M b these fibers number 12 to 14. The outer band of Baillarger in lamina IIIc and layer IV contains 14 to 16 horizontal exogenous fibers to the unit. In lamina Va these fibers number 10 to 12 to the unit. In laminae Vb and Via, the inner band of Baillarger, the horizontal exogenous fibers number 14 to 16 to the unit. In Vlb the horizontal fibers number 12 to 14 to the unit. The horizontal subcortical association fibers number 26 to 28 to the unit. The largest horizontal exogenous fibers in lamina Vlb in the Golgi sections measure 1.8/1 in diameter. The largest subcortical association fibers measure 2.0/1. All the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. No branches have been observed on any of these fibers. The vertical exogenous fibers are most numerous in the core of gyrus centralis anterior, and here number 35 to 40 to the unit. The fibers are small, intermediate, and large in size. The largest fibers measure 2.3/1 in diameter in the Golgi preparations. As the vertical fibers radiate toward the cortex they are principally collected in bundles, but a few radiating fibers are scattered between the bundles. The radiating vertical fibers decrease in size and quantity as they ascend within the cortex. Many of them end in lamina Vlb where they number 8 to 10 to the unit. Some of the vertical radiating fibers end in the neuropil in the inner band of Baillarger in laminae Vb and Via. The fibers number 4 to 6 to the unit in the lower part of the band. A few vertical fibers ascend into layer IV, here numbering 1 to 3 to the unit. Occasional vertical fibers are seen in layers I I I , II, and I ; these are small

21

fibers, scattered at wide intervals. All vertical fibers bear varicosities, spines, and thorns. No collateral or terminal branches have been observed on any of the vertical exogenous fibers. With reference to the depth of stain in the Weigert sections in this region of area F A y the brains fall into the following sequence, beginning with the darkest: ( 1 ) 37-82, (2) 38-44, (3) 38-35, (4) 37~49· The stain in the sections of brain 37-82 is definitely darker than that in the sections of brain 38-44. The stain in the sections of brain 38-44 is but slightly darker than that in the sections of brain 38-35. The stain in the sections of brain 37-49 is distinctly lighter than that in the sections of brain 38-35. The stain on the vertical fibers in the core of gyrus centralis anterior in the sections of brain 37-49 is definitely darker and is more evenly distributed than in the sections of brain 33-197, which is the three-month brain having the most myelin in this region. The sections in the region of the hand in brain 37-82 are stained slightly more darkly than those in the region of the lower extremity in this brain, but are lighter in color than the sections in the region of the trunk, shoulder, and arm. MYELiNATioN.

Stained tangential fibers in layer I number 4 or 5 to the unit in the posterior wall of gyrus centralis anterior. They are very small and very lightly stained. Toward the posterior lip of the gyrus they decrease in number and size, and become lighter in color. Even though the stained fibers are small, the three categories of size can be recognized, viz., small, intermediate, and large. All the fibers bear varicosities. No stained horizontal fibers are present in layer I I or in lamina I l i a . Isolated very short, small, and lightly stained horizontal fibers are occasionally present in lamina M b . In lamina IIIc and layer IV (the outer band of Baillarger) stained horizontal fibers number 3 or 4 to the unit. In lamina Va stained horizontal fibers number 4 or 5 to the unit. They are larger and more darkly stained than those in layer IV. The stained horizontal fibers are yet larger, darker, and more numerous in the inner band of Baillarger. Here they number 8 to 10 to the unit. In lamina Vlb the stained horizontal fibers number 10 to 12 to the unit, and they are larger and more darkly stained than those in lamina Via. The stained hori-

22

THE CORTEX OF THE SIX-MONTH INFANT

zontal fibers are small, intermediate, and large in size, and all bear varicosities. Stained subcortical association fibers number 12 to 14 to the unit. They are small, intermediate, and large in size. The large fibers are mostly larger than those in lamina VIb. Some of the small fibers are extremely fine. All the stained fibers bear varicosities. The stained vertical fibers in the core of gyrus centralis anterior are larger, more numerous, and more darkly stained in the posterior one-half of the core than in the anterior one-half, but the difference in color is not as great as it is in this region in the threemonth cortex. The stain is darker in all parts of the core and in the subjacent centrum ovale than in threemonth brain 33-197. In the posterior wall of the gyrus the stained vertical fibers number 25 to 30 per unit of width. The stained vertical fibers are small, intermediate, and large in size and all bear varicosities. The largest stained fibers measure 2.5/i in diameter between varicosities. The smallest stained fibers are extremely fine. As the stained fibers radiate toward and into the cortex they gradually decrease in size and become lighter in color. The stain ends on most of the fibers in the upper part of layer VI. Some vertical fibers in layer V are stained, and the stain on a few fibers extends to layer IV. The stained fibers which reach layer IV are very small and very lightly stained. A few small, lightly stained vertical fibers are present here and there in layer III, especially in lamina IIIc. These vertical fibers are not continuous with vertical fibers lower in the cortex, and are probably myelinated axons of neurons in layer III. REGION OF THE HEAD The cortex in this region measures 2.281 mm. in width. It is illustrated in Figures 17, 18, 19, and 20. CELL-BODIES

Layer I. This layer measures 0.239 mm. in width and contains 54 neurons to the unit. Horizontal cells of Cajal are very few in number. Layer II. The external granular layer is distinctly visible macroscopically. It measures o . i n mm. in width and contains 273 nerve cells to the unit. The cells range in size from 4/4/Λ to 8/8/x. The outer margin of the layer is fairly sharp, but the inner

boundary is indistinct because the small cells of layer II intermingle with larger cells belonging to lamina Ilia. Layer III. The external pyramidal layer measures 0.682 mm. in width. In lamina I l i a the neurons number 61 to the unit and measure 10/6-20/10/x. Lamina IHb has 52 neurons per unit, the cells ranging in size from το/6μ to 26/12/x. The neurons number 51 to the unit in lamina IIIc, and measure 10/629/13^. Layer IV. The internal granular layer is much less distinct than the external one. The cells belonging to layer IV intermingle with the cells of laminae IIIc and Va to such an extent that the outer and inner margins of the granular layer are indistinct. Layer IV measures 0.212 mm. in width and contains 106 neurons to the unit. The nerve cells measure 5/412/iOju.

Layer V. The internal pyramidal layer measures 0.490 mm. in width. Small and large cells are scattered throughout the width of the layer. The extralarge or giant pyramidal cells of Betz are located chiefly in lamina Vb, but occur also in lamina Va. In lamina Va the small cells number 50 to the unit and the large cells average 21 per unit. In Vb the small cells number 40 to the unit, the large cells number 16, and the giant pyramidal cells number 8 per unit. The small cells measure 10/6-16/10^,, the large cells measure 20/10-29/12^, and the giant pyramidal cells range in size from 36/ιομ. to 56/12^. Layer VI. The polymorphic layer measures 0.547 mm. in width. In lamina V i a the neurons number 65 to the unit, and in lamina VIb they average 40 per unit. The pyramidal cells measure 10/6-2 6/12 μ and the spindle cells range in size from τ6/6μ to 26/ιομ. The giant pyramidal cells of Betz are stained more darkly than any other cells in this area, and they contain more chromophil substance than any other neurons. The chromophil substance is in the form of fine granules, clumps, and Nissl bodies. The granules and clumps are less numerous and Nissl bodies are more numerous than in the Betz cells in this area in the cortex of the three-month infant. The Nissl bodies are chiefly in the form of flakes, but some of them are well formed and have sharply defined borders. The latter are not as numerous as they are in

LOBUS FRONTALIS the Betz cells in the region of the hand in the sixmonth cortex. The chromophil substance extends into the apical dendrite for a greater distance than it does into the basal dendrites. In the apical dendrite many of the Nissl bodies are elongated in the vertical plane of the dendrite, but only a few such Nissl bodies are present in the cell-body. No collaterals of the apical dendrites are stained, but in many of these dendrites the place of origin of one or two of the most proximal collaterals is indicated by a darkly stained clump of chromophil substance. The basal dendrites contain very little chromophil substance and are stained for a short distance only. The clumps of chromophil substance are chiefly located at the origins of the basal dendrites, and in many of the Betz cells a cone-shaped clump rests upon the nuclear membrane immediately below the origin of the apical dendrite. The chromophil substance in the Betz cells in the area of the head is less advanced in differentiation than in the giant cells in the areas of the trunk and hand, but is further advanced than in the corresponding cells in the region of the lower extremity. The large ordinary and special pyramidal cells in layers V and VI and the large spindle cells in VI are stained almost as darkly as the Betz cells. The chromophil substance is chiefly in the form of granules and small clumps. A few small Nissl bodies are present in most of these cells, and they are more numerous than in the corresponding cells in this area in the three-month cortex. In many, but not all, of these cells a small clump of chromophil substance rests upon the nuclear membrane immediately below the base of the apical dendrite. The chromophil substance is less in quantity, and Nissl bodies are less numerous, in the large pyramidal cells in lamina IIIc than in the large ordinary pyramidal cells in layers V and VI. Nissl bodies are more numerous and clumps of chromophil substance are less numerous in all the cells in layer I I I than in corresponding cells in the three-month cortex. The small nerve cells in layers II, III, and IV appear to be in about the same condition of development of the chromophil substance as the corresponding cells in the three-month cortex. The small cells in layer I I are the least developed. The nuclear membrane in these cells is thick and darkly stained, and

23

the extranuclear chromophil substance consists of a few small beads which are clinging to the outer surface of the membrane. The large horizontal cells of Cajal in layer I are present in approximately the same quantity and condition as in the other regions of area FA-y. Neurofibrils, longitudinal rows of granules, and scattered granules are present in all the giant pyramidal cells of Betz. The neurofibrils are more distinctly formed than those in corresponding cells in the region of the lower extremity in area FAy. This is true especially of the neurofibrils in the apical and basal dendrites. Neurofibrils extend into the apical dendrites as high as layer II. A few neurofibrils are present in the axon and basal dendrites of all the Betz cells. The dendrites and axons are stained more darkly than the cell-bodies. Irregularly scattered granules predominate in the cell-bodies, dendrites, and axons of the large ordinary pyramidal cells in layers V and VI and in the large spindle cells in VI, but a few longitudinal rows of granules and still fewer neurofibrils are also present. The rows of granules and neurofibrils are more conspicuous in the dendrites and axons than in the cellbodies. The axons and dendrites are stained more darkly than the cell-bodies, but the contrast in color is less than that between the processes and cell-bodies of the Betz cells. The largest cells in lamina IIIc are in approximately the same state as to differentiation of neurofibrils as the large ordinary pyramidal cells in layers V and VI. In all other cells in layer I I I the cytoplasm is filled with granules which are scattered irregularly throughout the cell-bodies and processes, except that in some of the largest cells in lamina I l l b a few longitudinal rows of granules are present. The rows of granules are more conspicuous in the dendrites and axons than in the cell-bodies. The cytoplasm in all cells of layers I I and IV and in all the small cells in layers I I I , V, and V I is filled with granules which are scattered irregularly throughout the cell-bodies and processes. The large cells in layers I I I , V, and VI are more advanced as to differentiation of neurofibrils than corresponding cells in this region of area F A y in the cortex of the three-month infant. In the six-month cortex the differentiation of neurofibrils in the cells

24

THE CORTEX OF THE SIX-MONTH INFANT

in this region of area FAy is more advanced than in corresponding cells in the region of the lower extremity, but is less advanced than in the cells in the region of the hand or the trunk and shoulder. The apical dendrites of the giant pyramidal cells of Betz are larger than those of any other cells in this area. They are larger than those of corresponding cells in this area in the cortex of the threemonth infant. The terminal branches in layer I are more numerous than those in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites than in the cortex of the three-month infant. In the six-month cortex the pedunculated bulbs are less numerous on the dendrites of the giant pyramidal cells in this region than those on the corresponding dendrites of corresponding cells in the region of the hand. FIBERS.

The axons of the giant pyramidal cells descend in a vertical but wavy course through layer VI and enter the subjacent white matter. They mingle with and run parallel to the vertical exogenous fibers. The axons of the Betz cells are larger than those of any other cells in this region of area FAy. The axons are smooth in contour from the cell-body to the upper part of lamina Via, where small irregularities and granules appear on their surfaces. These structures on the surface gradually increase in size, becoming amorphous clumps, as the axon descends, and the diameter of the axon increases, all of which are interpreted as evidence of myelin. The quantity of myelin on the axons of the Betz cells is greater than that on the axons of corresponding cells in this region in the three-month cortex, but it is less than that on the axons of corresponding cells in the region of the hand in the six-month cortex. The apical dendrites of the large ordinary and special pyramidal cells in layers V and VI and of the large spindle cells in VI are somewhat coarser than those of corresponding cells in this region in the three-month cortex. The basal dendrites and the collaterals of the apical dendrites are coarser and longer than those in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the

dendrites of these cells than in the three-month cortex. The axons of the large ordinary and special pyramidal cells in layers V and VI and of the large spindle cells in layer VI are somewhat larger and their collaterals are larger and longer than those of corresponding cells in this region in the cortex of the three-month infant. Myelin is present on the axons of all these cells. The myelin is much less in quantity than that on the axons of the Betz cells in this region in the six-month cortex but is greater in quantity than that on the axons of the large ordinary and special pyramidal and spindle cells in layers V and VI in the three-month cortex. The apical dendrites of the large pyramidal cells in all parts of layer III end in layer I. The terminal branches are more numerous than those in this region in the three-month cortex. The apical dendrites of the pyramidal cells in layer III are coarser than those of corresponding cells in this region of FAy in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are larger and longer, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all the pyramidal cells in layer III than on the dendrites of corresponding cells in the threemonth cortex. The quantity of bulbs varies directly with the size of the cell. For each cell the bulbs are greatest in quantity on the apical dendrite and least on the terminal branches. The bulbs are few in number on the terminal branches of the cells in lamina Ilia. Pedunculated bulbs are more numerous on the dendrites of the large pyramidal cells in IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V in this region of the cortex. The axons of the large pyramidal cells in lamina IIIc are smaller than those of the giant pyramidal cells and about the same size as those of the large ordinary and special pyramidal cells in layer V. The axons of the large cells in lamina IIIc extend vertically downward and can be traced as far as the inner band of Baillarger. At about the lower border of layer IV the axons acquire a layer of myelin. More of the axons of large cells in IIIc have a coat of myelin than in the cortex of the three-month infant. The myelin is greater in quantity on the axons

LOBUS FRONTALIS of some of these cells than in the three-month cortex, but is about the same on the axons of other cells as that on the most heavily myelinated axons in the three-month cortex. The axons of large pyramidal cells in lamina I l l b are smaller than those of large cells in IIIc. At about the lower part of lamina I l l b the axons increase slightly in size, and small irregularities and beads appear on the surface, and these are interpreted as evidences of myelin. As the axons descend they increase slightly in size and the surface irregularities and beads become larger. The longest axons can be traced into layer V. The axons of the large pyramidal cells in lamina I l i a are smaller than those of the large pyramidal cells in lamina I l l b . They are stained for only a short distance from the cell-bodies. The axons of the large pyramidal cells in all three laminae are larger than those of corresponding cells in this region in the cortex of the three-month infant, but the branches are no larger in the six-month cortex. In descending, the axons of the pyramidal cells in layer I I I do not pursue a wavy course as do the axons of the giant pyramidal cells in layer V. The dendrites of the small pyramidal cells in all layers of the cortex are larger and longer, but no more numerous, than those of corresponding cells in this region of the three-month cortex. The small pyramidal cells in layer IV are more advanced in development in this respect than those in layer II. Pedunculated bulbs are present on the dendrites of all the small cells in much greater quantity than they are on the dendrites of corresponding cells in the three-month cortex. Pedunculated bulbs are less numerous on the dendrites of the small pyramidal cells in layer I I than on the dendrites of the small pyramidal cells in layer IV. Widely scattered bulbs are present on the dendrites of the granular cells in layer II. The axons of the small pyramidal cells in layers IV and I I are coarser and more compact in structure than those of corresponding cells in this region of the three-month cortex. Horizontal cells of Cajal are rarely seen in layer I in the Golgi-Cox preparations. The dendrites of Golgi type I I cells of all sizes in all layers of the cortex are larger, longer, and have more pedunculated bulbs than these cells of comparable size in the cortex of the three-month infant. A

25

mesh of fibers formed by the axons of these cells is present in all layers of the cortex except layer I. The mesh consists of very fine to large fibers. The largest fibers are larger than the largest fibers in the mesh in this region of the three-month cortex. The mesh is more dense in each layer than it is in the corresponding layer in the three-month cortex. This network of fibers is of maximum density in layer IV, is considerably less dense in lamina Va, then increases in density in laminae Vb and Via, and becomes less dense in lamina VIb. In laminae Vb and V i a (the inner band of Baillarger) the mesh is much less dense than it is in layer IV. In lamina IIIc the mesh is slightly less dense than in layer IV. It decreases sharply and greatly in density in lamina I l l b and is thinnest in lamina I l i a . The mesh is slightly more dense in layer I I than in lamina I l i a and consists almost entirely of very small fibers, the axons of the small Golgi type I I cells in this layer. The vertical exogenous fibers which radiate from the core of white substance and ascend into the cortex do not contribute any fibers to the mesh of fibers in layer IV. The tangential fibers in layer I are larger and more numerous in the posterior wall of gyrus centralis anterior than in the crown of the gyrus. The fibers are much more numerous and larger in the external onehalf of layer I than in the internal one-half. Here the fibers number 14 to 16 to the unit. The largest tangential fibers measure ι.ζμ in diameter in the Golgi preparations. All the tangential fibers in the Golgi sections bear varicosities, spines, and thorns. No collaterals have been observed on any tangential fibers. The horizontal exogenous fibers in layer I I are much smaller than the tangential fibers in the outer one-half of layer I, and somewhat smaller than those in the internal one-half. The fibers are small, intermediate, and large in size, and number 4 to 6 to the unit. In lamina I l i a the horizontal exogenous fibers number 8 to 10 to the unit. They are slightly larger and more darkly stained than the horizontal fibers in layer II. In lamina I l l b the horizontal fibers number 12 to 14 to the unit. They are small, intermediate, and large in size, and the fibers in each category of size are larger and more darkly stained than the horizontal fibers in lamina I l i a . In lamina IIIc and

26

THE CORTEX OF THE SIX-MONTH INFANT

layer IV (the outer band of Baillarger) the horizontal exogenous fibers number 14 to 16 to the unit. In lamina Va the horizontal exogenous fibers number 12 to 14 to the unit. The inner band of Baillarger in laminae Vb and Via contains 14 to 16 horizontal exogenous fibers to the unit. In lamina VIb these fibers number 12 to 14 to the unit. The largest fibers in the Golgi sections measure ι.$μ in diameter. The horizontal subcortical association fibers number 22 to 24 to the unit. The largest of these fibers measure ΐ·5μ in diameter. All horizontal exogenous fibers in all layers and all horizontal subcortical association fibers are varicose and tufted in the Golgi preparations. No collateral branches have been observed on any of these fibers. The vertical exogenous fibers are more numerous in the core of gyrus centralis anterior than elsewhere in this region. Here they number 35 to 40 to the unit. The fibers are small, intermediate, and large in size. The largest vertical fibers measure 1.5^ in diameter in the Golgi sections. As the vertical fibers radiate toward the cortex they are chiefly collected into bundles, but a few radiating fibers are scattered between the bundles. The great majority of the radiating vertical exogenous fibers end in lamina VIb, where they number 14 to 16 in the lower part of the layer. Most of the remaining ascending fibers end in the neuropil in the inner band of Baillarger in laminae Vb and Via, here numbering 6 to 8 to the unit. A few vertical fibers ascend singly to layer IV, where they number 2 to 4 to the unit. Occasionally single vertical fibers are present in layers III, II, or I. All of the vertical fibers bear varicosities, spines, and thorns. No branches, either collateral or terminal, have been observed on any of the vertical exogenous fibers. The strain in the Weigert sections in this region of brain 37-82 is very much lighter than that in the sections in the region of the lower extremity in area FAy. The stain in the sections in this region of brain 37-82 is but slightly darker than the stain in the region of the head in area F A y in brain 33-197, which is the three-month brain having the most myelin. Stained tangential fibers in layer I number 4 or 5 to the unit in the posterior wall of gyrus centralis

MYELiNATiON.

anterior. The fibers are very small and very lightly stained. No horizontal fibers are stained in layers I I or III. Stained horizontal fibers are present in the cortex in this area in the posterior wall of gyrus centralis anterior and in the floor of sulcus centralis only. In layer IV there are 2 or 3 small, faintly stained fibers per unit. In lamina Va the stained fibers are slightly larger and darker, and number 4 or 5 to the unit. In laminae Vb and Via (the inner band of Baillarger) the stained fibers are yet larger, darker, and more numerous, averaging 6 to 8 to the unit. In lamina VIb stained horizontal fibers number 8 to 10 to the unit, and they are larger and darker than those in lamina Via. The stained horizontal fibers are small, intermediate, and large in size, and all bear varicosities. The stained subcortical association fibers are larger, darker, and more numerous than the stained horizontal fibers in lamina VIb. They number 12 to 14 to the unit. They are small, intermediate, and large in size. The largest stained fibers measure 1.6 μ in diameter. The stained vertical exogenous fibers are largest, most numerous, and darkest in color in the posterior one-half of the core of gyrus centralis anterior. This is the side of the core which lies immediately under the Betz cells. Proceeding toward the anterior wall of the gyrus the stained vertical fibers gradually become smaller, less numerous, and lighter in color. These differences between the fibers in the posterior and anterior walls of the gyrus are less than in this region in the three-month cortex. The stain in the fibers of the centrum ovale subjacent to this region is darker than in the three-month cortex. The vertical fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure 2.ομ in diameter between varicosities. In the posterior part of the core of the gyrus the stained vertical fibers number 24 to 26 to the unit. As the vertical fibers radiate into the cortex they gradually become smaller and lighter in color. The stain ends on most of the ascending vertical fibers in the lower part of lamina VIb, and the stained fibers are few in number. Very rarely a stained fiber reaches the lower part of layer IV.

POSTERIOR GYRUS

LOBUS

FRONTALIS

REGION

OF THE

FRONTALIS

T H E C O R T E X in the crown of the posterior part of gyrus frontalis superior measures 2.210 mm. in width. It is illustrated in Figures 21, 22, 23, and 24.

CELL-BODIES

Layer I. The molecular layer measures 0.253 m m · width and contains 52 neurons to the unit. Horizontal cells of Cajal are seen occasionally. Layer II. This layer is clearly visible macroscopically. It measures 0.097 m m · in width and contains 274 cells to the unit. The cells range in size from 5/4/1 to 9/8μ. Layer III. The external pyramidal layer measures 0.682 mm. in width. The neurons number 61 to the unit in lamina I l i a and measure 10/6-23/12/1. In lamina IHb the nerve cells number 51 to the unit and measure 10/6-26/13/*, and in lamina IIIc they number 53 per unit and range in size from ιο/βμ to 29/16/i. Layer IV. The internal granular layer is less distinct than the external one. It measures 0.207 mm. in width and contains 103 cells to the unit. The cells are small, measuring 5/4-13/ιομ, and intermingle with the large cells of laminae IIIc and Va. Layer V. This layer measures 0.431 mm. in width. In lamina Va the small neurons number 50 to the unit and the large ones number 21 per unit. The small cells number 45 to the unit in lamina Vb, and the large cells average 21 per unit. The extra-large pyramidal cells are located chiefly in lamina Vb but some are present in Va also. The extra-large cells number 12 to the unit and range in size from 36/ιομ, to 53/13/*. The small neurons measure 10/6-16/10/1 and the large ones measure 20/10-29/12/1. Layer VI. The polymorphic layer measures 0.540 mm. in width. The neurons number 70 to the unit in lamina V i a and 40 per unit in lamina VIb. The pyramidal cells measure 10/6-26/13/1, and the spindle cells range in size from ιβ/6μ to 26/ιομ. The extra-large pyramidal cells in layer V are stained more darkly in the cresyl violet sections than any other cells in this area, but they are not as dark in color as the Betz cells in area F A y in the region of the lower extremity. The contrast in color be-

FRONTAL

SUPERIOR, AREA

27 GYRI FB

tween the extra-large pyramidal cells and the other cells in area F B is not as great as that between the Betz cells and the other cells in FAy. The chromophil substance is present in the extra-large pyramidal cells in greater quantity than in corresponding cells in the three-month cortex. Nissl bodies are more numerous in these cells, and the clumps of chromophil substance are smaller and less numerous than in the cortex of the three-month infant. Nissl bodies are much less numerous in these cells than in the Betz cells in the region of the lower extremity in area FAy. The apical dendrites are stained more darkly and for a greater distance than in the three-month brain, the stain extending in some of them to the lower part of layer IV. A few small Nissl bodies and some elongated strands of chromophil substance are present in the proximal ends of the apical dendrites. No collaterals are stained in the cresyl violet sections, but a slight projection and a small darkly stained clump of chromophil substance indicate the location of two or three of the most proximal collaterals. The basal dendrites are stained for a very short distance from the cell-body, and do not contain any Nissl bodies. In most of the extra-large pyramidal cells a small clump of chromophil substance rests upon the nuclear membrane just under the base of the apical dendrite. In some of the cells this clump is coneshaped, with the apex of the cone directed toward the apical dendrite. The membrane of the nucleus is thickened and has small clumps of chromophil substance adhering to the external surface. The large ordinary and special pyramidal cells in layers V and VI and the large spindle cells in V I are stained almost as darkly as the extra-large pyramidal cells in layer V. A few small Nissl bodies are present in these large pyramidal and spindle cells, but the chromophil substance is chiefly in the form of small clumps and fine granules. The clumps are more numerous in proportion to the size of the cell than in the extra-large pyramidal cells. The apical dendrites are stained for a short distance, but contain no clumps or tigroid bodies. The basal dendrites are rarely stained, and if they are, they contain no granules or Nissl bodies. In many of these cells a cone-

28

THE CORTEX OF THE SIX-MONTH INFANT

shaped clump of chromophil substance rests upon the nuclear membrane immediately under the base of the apical dendrite, or under the base of each dendrite of the spindle cells. The large pyramidal cells in lamina IIIc are stained about the same depth of color as the large pyramidal cells in layers V and V I , and they are darker than other cells in layer III. A few Nissl bodies are present in the cell-bodies. The apical dendrites are stained for a greater distance and contain more Nissl bodies and clumps of chromophil substance than those of the large ordinary pyramidal cells in layers V and V I . In almost all of the large pyramidal cells in lamina IIIc a darkly stained, cone-shaped clump of chromophil substance rests upon the nuclear membrane just below the base of the apical dendrite. The chromophil substance in the large and medium-sized cells in laminae I l l b and I l i a is more lightly stained than it is in the large cells in I I I c except in very large cells scattered here and there in the outer laminae. The substance is chiefly in the form of small clumps, and these are more numerous than in corresponding cells in this area in the three-month cortex. The clumps and granules of chromophil substance extend into the apical dendrites for a greater distance than in these cells in the three-month brain. In most of the cells in laminae I l l b and I l i a a small cone-shaped clump of chromophil substance is resting on the nuclear membrane under the base of the apical dendrite. The small cells in layers I V and I I have more chromophil substance than the corresponding cells in this area in the cortex of the three-month infant. The chromophil substance is in the form of small clumps, some of which extend into the proximal end of the apical dendrite. The apical dendrites are stained for a greater distance than in the threemonth cortex. The cells in layer IV contain more chromophil substance than those in layer II, and their apical dendrites are stained for a greater distance than those of cells in II. The large horizontal cells of Cajal in layer I are seen less frequently than in this area in the cortex of the three-month infant. Some of these cells have a normal appearance, and some bear evidences of degeneration. In the latter the nucleus lies at the periphery of the cell, and the cytoplasm is shrunken and lightly stained.

Granules scattered irregularly, granules arranged in longitudinal rows, and neurofibrils are present in the cell-bodies and processes of the extra-large pyramidal cells in layer V . The neurofibrils are less numerous and less definitely formed than those in the Betz cells in the region of the lower extremity in area F A y . Neurofibrils extend into the dendrites and axons, and are there more conspicuous than those in the cell-bodies. The axons and dendrites are stained more darkly than the cell-bodies, but the contrast in color is not as great as it is in the Betz cells in the region of the lower extremity in area F A y . N o neurofibrils are present in the large ordinary pyramidal cells in layers V and V I or in the large spindle cells in layer V I . The cytoplasm is filled with granules irregularly scattered throughout the cellbodies and their processes. A few longitudinal rows of granules are present, and these are more conspicuous in the apical and basal dendrites than in the cellbodies. There is no difference between the cell-bodies and their processes as to depth of stain. The cytoplasm in all other cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. There is no contrast between cell-bodies and processes as to color of the stain. Differentiation of neurofibrils in the cells in layers V and V I is more advanced than in corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the differentiation of neurofibrils in cells in this area is not as advanced as in corresponding cells in the region of the lower extremity in area F A y . The dendrites of the extra-large pyramidal cells in layer V are larger than those of any other nerve cells in this area. The apical dendrites are coarser, and have more terminal branches, than those of corresponding cells in this area of the three-month cortex. The terminal branches are more compact in structure in the six-month cortex. The terminal branches in layer I are not as numerous as those of the apical dendrites of the giant pyramidal cells in either the region of the lower extremity or the region of the head in area F A y . The basal dendrites and collateral branches of the apical dendrites are coarser, longer, and more compact in structure than in the FIBERS.

LOBUS FRONTALIS cortex of the three-month infant. The dendrites of these cells are not as coarse or as long as those of the giant pyramidal cells of Betz in the region of the head in area FAy. Pedunculated bulbs are more numerous on all the dendrites of these cells than those on the dendrites of corresponding cells in this area in the cortex of the three-month infant. The bulbs are not as numerous as those on the dendrites of the giant pyramidal cells of Betz either in the region of the lower extremity or the region of the head in area FAy. Pedunculated bulbs are the most numerous on the apical dendrite as it passes through layer IV and laminae IIIc and Illb, and are smallest and least in number on the terminal branches of the apical dendrites, where they are sparsely scattered. The axons of the extra-large pyramidal cells descend into the subjacent white matter, pursuing a slightly wavy course. They are larger than the axons of any other cells in this area, but they are smaller than the axons of the giant pyramidal cells of Betz in the region of the lower extremity in area FAy. At about the mid-region of layer VI the axons become slightly larger and acquire small, amorphous clumps which are interpreted as evidences of myelin. The myelin is slightly greater in quantity than that on the axons of corresponding cells in this area in the cortex of the three-month infant. In the sixmonth cortex the myelin on the axons of these cells is much less in quantity than that on the axons of the giant pyramidal cells of Betz in the region of the lower extremity in area FAy. From one to three branches have been observed on some of the axons. These branches are larger, longer, and have more secondary branches than the axons of corresponding cells in this area in the three-month cortex. As many as five secondary branches have been counted on some of the primary branches. The primary collateral branches emerge from the parent axon in laminae Vb or Via. They always emerge at right angles and pursue a horizontal course in laminae Vb or Via, mingling with the horizontal exogenous fibers. The secondary branches usually, but not always, emerge from the primary branches at right angles, and extend either horizontally or more or less vertically upward. The secondary branches are much more delicate than the primary collaterals.

29

The apical dendrites of the large ordinary and special pyramidal cells in layer V are larger than those of corresponding cells in this area in the threemonth cortex. The basal dendrites and the branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the cortex of the three-month infant. In the sixmonth brain the apical and basal dendrites and their branches are smaller than those of the extra-large pyramidal cells in this area. Pedunculated bulbs are more numerous on the dendrites of the large ordinary and special cells than those on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs are much less numerous on the dendrites of the large ordinary and special cells than those on the dendrites of the extra-large pyramidal cells in this area. The axons of the large ordinary and special cells in layer V are smaller than those of the extra-large pyramidal cells in this area in the six-month cortex, but they are larger than the axons of the large ordinary and special pyramidal cells in the cortex of the three-month infant. Myelin is present on the axons of these cells in greater quantity than in the three-month cortex, but in less quantity than that on the axons of the extra-large pyramidal cells in this area in the six-month cortex. The apical dendrites of the large pyramidal cells in lamina IIIc are coarser and more compact in structure than those of corresponding cells in this area in the cortex of the three-month infant. Terminal branches of the apical dendrites of these cells in layer I are more numerous than in the three-month cortex. In the six-month cortex the apical dendrites of the large pyramidal cells in lamina IIIc are smaller than those of the extra-large pyramidal cells, but they are larger than those of the large ordinary and special pyramidal cells in layer V. The basal dendrites and collateral branches of the apical dendrites of the large pyramidal cells in IIIc are coarser, longer, and more compact in structure, but no more numerous than those of corresponding cells in this area in the three-month cortex. The axons of the large cells in laminae I l l b and IIIc show no appreciable increase in size or in quantity of myelin since the three-month stage of develop-

30

THE CORTEX OF THE SIX-MONTH INFANT

ment. The axons of these cells are smaller and have less myelin than those of corresponding cells in the region of the lower extremity in area F A y in the sixmonth cortex. Pedunculated bulbs are present on the dendrites of pyramidal cells in all three laminae of layer I I I in greater quantity than on the dendrites of corresponding cells in this area in the cortex of the three-month brain. In the six-month cortex the bulbs are less numerous on the dendrites of the pyramidal cells in layer III than those on the dendrites of corresponding cells in layer III in the region of the lower extremity in area F A y . In the posterior part of the superior frontal gyrus in the six-month cortex pedunculated bulbs are present on the dendrites of the large pyramidal cells in lamina IIIc in less quantity than they are on the dendrites of the large ordinary and special pyramidal cells in layer V. The bulbs are less numerous on the dendrites of the pyramidal cells in lamina I l l b than those on the dendrites of pyramidal cells in lamina IIIc, and are still less in quantity on the dendrites of the pyramidal cells in lamina I l i a . Pedunculated bulbs are very sparsely scattered on the larger dendrites of the cells in lamina Ilia. The axons of Golgi type II cells form a mesh of fibers in all layers except layer II. The mesh is thinnest in lamina I l i a , increases in density in lamina I l l b , reaches the maximum density in lamina IIIc and layer IV (the outer band of Baillarger), decreases markedly in lamina Va, increases considerably in the inner band of Baillarger, then becomes thinner in lamina VIb. The mesh in VIb is more dense and contains a greater number of large fibers than that in lamina I l l b . The mesh of fibers formed by the axons of Golgi type II cells is more dense in all layers than that in the corresponding layers in the region of the lower extremity in area F A y . In area F B the mesh is more dense in all layers in the sixmonth cortex than in corresponding layers in the cortex of the three-month infant. No horizontal cells of Cajal have been observed in layer I in the Cajal or Golgi sections. The tangential fibers in layer I in the posterior region of gyrus frontalis superior are smaller, less numerous, and more lightly stained than the tangen-

tial fibers in area F A y in the region of the lower extremity. The fibers are small, intermediate, and large in size. They are collected chiefly in the external one-half of layer I, and in this location number 12 to 14 to the unit. The largest fibers measure ι.ζμ in diameter in the Golgi preparations. All the tangential fibers bear varicosities. No branches have been observed. The fibers in the internal one-half are very lightly stained, and are much smaller than those in the external one-half. Horizontal exogenous fibers in layer II are very small and are stained very lightly. They number 2 or 3 to the unit. Internal to layer II the horizontal exogenous fibers gradually become larger and more darkly stained in the successively deeper layers, being largest and darkest in layers V and V I and in the layer of subcortical association fibers. The horizontal exogenous fibers number 6 to 8 to the unit in lamina I l i a and 8 to 10 in lamina I l l b . In lamina IIIc and layer IV (the outer band of Baillarger) the horizontal exogenous fibers number 10 to 12 to the unit. The horizontal exogenous fibers in Va number 8 to 10 to the unit. In the inner band of Baillarger in laminae Vb and V i a the horizontal exogenous fibers number 10 to 12 to the unit. In lamina VIb the horizontal fibers average 12 to 14 to the unit. The subcortical association fibers number 20 to 22 to the unit. The largest horizontal fibers in lamina VIb measure 1.8μ in diameter in the Golgi preparations. The largest subcortical association fibers measure ι.8μ. in diameter. All the horizontal exogenous fibers and subcortical association fibers bear varicosities, spines, and thorns. No branches have been observed. The vertical exogenous fibers are most numerous in the core of gyrus frontalis superior, where they number 35 to 40 to the unit. The fibers are small, intermediate, and large in size. The largest vertical fibers measure ι.8μ in diameter. Most of the radiating vertical fibers end in layer VI, numbering 14 to 16 to the unit in lamina VIb and 6 to 8 in lamina Via. A few, widely scattered individual vertical fibers end in layer IV. Occasionally a very small, faintly stained vertical fiber is seen in layer III, and rarely a vertical fiber is seen in layer II or in layer I. All the vertical exogenous fibers bear varicosities,

LOBUS FRONTALIS spines, and thorns. Neither collateral nor terminal branches have been observed on any of the vertical exogenous fibers. MYELINATION. With reference to the depth of stain in the Weigert sections in this region, the brains fall into the following sequence, beginning with the darkest: ( i ) 38-35, (2) 38-44, (3) 37-82, (4) 37-49· There is a slight but distinct difference between the sections of 38-35 and those of 38-44, and between those of 38-44 and those of 37-82, but the sections of 37-49 are much lighter than those of 37-82. The stain in the sections of brain 37-49 is but slightly darker than that in the Weigert sections of brain 38-128, which is the three-month brain having the most myelin in this region of the cortex. The stain in the sections in this region of brain 38-35 is much lighter than that in the sections of brain 41-91 in area F A y in the region of the lower extremity, but it is much darker than the stain in the sections of brain 37-82 in area F A y in the region of the head. The following description is based upon the sections of brain 38-35.

Stained tangential fibers are present in layer I only in the lower part of the medial surface of the gyrus. The fibers are small and very lightly stained, and number only 3 or 4 to the unit. Stained horizontal fibers are present only in the cortex on the medial surface of the gyrus. No stained fibers are present in layers II or III. In layer IV there are 2 or 3 very small, lightly stained, and widely scattered horizontal fibers to the unit. In laminae Vb and V i a (the inner band of Baillarger) the stained fibers number 3 or 4 to the unit, and they are slightly larger and darker than those in layer IV. In layer V I the stained horizontal fibers are yet larger and darker, and they number 6 to 8 to the unit in lamina VIb. The stained subcortical association fibers number 8 to 10 to the unit. They are larger and more darkly stained than the stained horizontal fibers in lamina VIb. They are small, intermediate, and large in size, and all of them bear varicosities. The largest subcortical fibers measure ι.τ,μ between varicosities. The stained vertical exogenous fibers in the core of the gyrus are of rather uniform color in all parts

31

of the core. The stained fibers are small, intermediate, and large in size, and all bear varicosities. The small fibers are the lightest in color and the large fibers are the darkest. The largest stained vertical fibers measure ι.ζμ in diameter between varicosities. In the core of the gyrus just before they begin to radiate toward the cortex the stained vertical fibers number 25 to 30 per unit of width. As the vertical fibers ascend in the cortex they decrease in size and number and become lighter in color. The stain ends on most of the ascending fibers in the middle of layer VI. Some fibers are stained as far as layer V, and a few very small and lightly stained vertical fibers are present in the lower part of layer IV. GYRUS FRONTALIS MEDIUS, AREA FB

The frontal eye-field is believed to be located in the cortex in the posterior part of the middle frontal gyrus. The cortex in this region measures 2.279 m m · in width. It is illustrated in Figures 25, 26, 27, and 28. CELL-BODIES

Layer I. This layer measures 0.216 mm. in width and contains 53 neurons to the unit. Large horizontal cells of Cajal are seen occasionally in layer I. Layer II. The external granular layer is visible macroscopically. It measures 0.099 m m · width and contains 313 neurons to the unit. The cells measure 5/4-8/6,*. Layer III. This layer measures 0.764 mm. in width. In lamina I l i a the neurons number 55 to the unit and measure 10/6-20/12/i. The neurons number 48 to the unit in both laminae I l l b and Hie. They measure 10/6-29/13^ in I l l b and 10/6-33/1 $μ in IIIc. Layer IV. The inner granular layer is visible macroscopically. It measures 0.239 mm. in width and contains n o cells to the unit. The cells range in size from 5/4μ to 13/10/x. The layer is invaded by large cells of laminae IIIc and Va much less than it is in area F A y immediately posterior to the middle frontal gyrus. Layer V. This layer measures 0.390 mm. in width. The small neurons number 55 to the unit in lamina Va and 45 in lamina Vb. The large neurons number

32

THE CORTEX OF THE SIX-MONTH INFANT

21 per unit in Va and χ 7 in Vb. The small neurons measure 10/6-16/1 ομ and the large ones measure 20/10-29/15/*. The extra-large pyramidal cells are located chiefly in lamina Vb, but some are present in lamina Va. They number 11 to the unit and range in size from 33/12/1 to 46/13/u,. Layer VI. This layer measures 0.571 mm. in width. The neurons number 75 per unit in lamina V i a and 45 in lamina VIb. The pyramidal cells measure 10/6-26/13111, and the spindle cells range in size from 16/6μ to 29/ιομ,. In the cresyl violet sections the extra-large pyramidal cells in layer V are stained only slightly more darkly than the large ordinary and special pyramidal cells and large fusiform cells in layers V and VI. The extra-large pyramidal cells contain a greater quantity of chromophil substance in the cytoplasm than any other cells in this area. Nissl bodies and small clumps of chromophil substance are more numerous and granules are less numerous in the extralarge pyramidal cells in the six-month cortex than in the corresponding cells in this area in the brain of the three-month infant. Granules and clumps of chromophil substance extend into the apical dendrites of these cells in greater quantity and for a greater distance in the six-month cortex than in the threemonth brain. Granules and small clumps of chromophil substance are present in small quantity in the proximal ends of the larger basal dendrites. In almost all of these cells a small clump of darkly stained chromophil substance rests upon the nuclear membrane just under the origin of the apical dendrite. The large ordinary and special pyramidal cells in layers V and V I and the large fusiform cells in VI have more chromophil substance in the cytoplasm than the corresponding cells in the cortex of the three-month infant. The chromophil substance is in the form of granules, small amorphous clumps, and a few small Nissl flakes. The chromophil substance extends into the apical dendrites for a greater distance than in the corresponding cells in this area in the three-month cortex. In practically all of these cells a small, darkly stained clump of chromophil substance rests upon the membrane of the nucleus immediately under the base of the apical dendrite. The large pyramidal cells in lamina IIIc are stained more darkly than the large ordinary pyram-

idal cells in layers V and VI, and are as dark as the extra-large pyramidal cells in layer V. The largest cells in laminae I l i a and I l l b are also stained darkly. The large cells in all three laminae in layer III have more chromophil substance than corresponding cells in this area in the three-month cortex. The chromophil substance is in the form of fine granules, small amorphous clumps, and a few small Nissl flakes. In the six-month cortex the large cells in lamina IIIc contain more chromophil substance than the large ordinary pyramidal and fusiform cells in layers V and VI. The apical dendrites of the large pyramidal cells in IIIc are stained for a greater distance and contain more granules and clumps of chromophil substance than the apical dendrites of the large ordinary pyramidal and fusiform cells in layers V and VI in this area in the six-month brain. No collaterals of apical dendrites of any large cells in layers III, V, or VI are stained. In some of the large cells in lamina IIIc the location of one or two proximal collaterals is indicated by a dark clump of chromophil substance. No basal dendrites are stained on any large cells in layer III, but the points of origin of two or three of these dendrites on the large cells in IIIc are indicated by slight bulges filled with clumps of chromophil substance. The advance in development of the chromophil substance in the large cells in lamina IIIc beyond the condition of the corresponding cells in the three-month cortex is a conspicuous feature of the cortex in this area in the sixmonth infant. The smallest cells in all layers are stained more lightly than any other cells, and contain the least amount of chromophil substance in proportion to the size of the nucleus. The chromophil substance is least in quantity in the smallest cells in layer II and consists of a few small, darkly stained globules adhering to the external surface of the membrane of the nucleus. The membrane is thick and darkly stained. The chromophil substance is slightly greater in quantity in the cells of layer IV than in those of layer II. Some of the large horizontal cells of Cajal in layer I are normal in appearance, while others show signs of degeneration. These cells are present in about the same number and condition as the corresponding cells in area FAy. They are seen less frequently than

LOBUS FRONTALIS similar cells in this area of the cortex in the threemonth infant. No neurofibrils are present in any cells in this area. Longitudinal rows of granules are present in the largest extra-large pyramidal cells. The longitudinal rows of granules are more numerous in these cells than in corresponding cells in area F B in the posterior region of gyrus frontalis superior. In all other cells in this area the cytoplasm is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. Differentiation of neurofibrils in the extra-large pyramidal cells is more advanced than in corresponding cells in this area in the cortex of the threemonth infant. In the six-month cortex the differentiation of neurofibrils in cells in this area is less advanced than in the cells in area F B in the posterior region of gyrus frontalis superior. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are coarser than those of corresponding cells in this area in the three-month cortex. The terminal branches of the apical dendrites in layer I are more numerous than in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and longer, but no more numerous than those of these cells in this area in the three-month brain. Pedunculated bulbs are more numerous on all the dendrites of these cells than in the three-month cortex. In the six-month cortex the bulbs are somewhat more numerous on the dendrites of these cells than on the dendrites of corresponding cells in the posterior part of the superior frontal gyrus. The bulbs are not as numerous as those on the dendrites of the giant pyramidal cells in that part of area F A y which lies posterior to the middle frontal gyrus, the region of the hand. The axons of the extra-large pyramidal cells extend vertically downward in a wavy course and enter the mass of vertical fibers in the core of white substance in the center of the gyrus. There has been no appreciable increase in the amount of myelin on the axons of these cells since the three-month stage. The apical dendrites of the large ordinary and special pyramidal cells in layer V ascend vertically and end as single shafts at various levels in layer

33

III, the longest reaching the lower part of lamina I l i a . The apical dendrites are smaller than those of the extra-large pyramidal cells in layer V in this area, and are smaller than the apical dendrites of the large ordinary and special pyramidal cells in the region of the hand in area F A y . The apical dendrites of these cells are coarser and the basal dendrites are larger and longer than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the large ordinary and special pyramidal cells in layer V than those on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs on the dendrites of these cells are not as numerous as those on the dendrites of corresponding cells in the region of the hand in area F A y . There has been no appreciable advance in the development of the axons of the large ordinary and special pyramidal cells since the three-month stage. The apical dendrites of the large pyramidal cells in all three laminae of layer III are larger than those of corresponding cells in this area in the three-month cortex. Terminal branches of the apical dendrites in layer I are coarser and more numerous than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of these cells are coarser and longer than those of corresponding cells in this area in the three-month cortex. The dendrites of the large pyramidal cells in layer I I I are not as large as those of corresponding cells in the region of the hand in area F A y . Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer III than those on the dendrites of corresponding cells in this area in the three-month brain. In the six-month cortex the bulbs are less numerous on the dendrites of the largest pyramidal cells in layer III than those on the dendrites of the extra-large pyramidal cells in layer V in this area. The bulbs are less numerous on the dendrites of the pyramidal cells in layer III in this area than those of corresponding cells in the region of the hand in area FAy. The pyramidal cells in layer I I I are in approximately the same state of development as the corresponding cells in the posterior region of the superior frontal gyrus. The mesh of fibers formed by the axons of Golgi

34

THE CORTEX OF THE SIX-MONTH INFANT

type II cells is more dense than that in this area of the cortex of the three-month infant. In the sixmonth cortex the mesh in this area is much more dense than that in the region of the hand in area FAy. The mesh is densest in layer IV. I t is much thinner in lamina IIIc, and decreases rapidly in density toward lamina I l i a , where it is thinner than in any other lamina in this region. The mesh in lamina Va is very much thinner than that in layer IV. It increases in density in laminae Vb and Via, but is here much less dense than in layer IV. The mesh in lamina Vlb is thinner than that in lamina Vb, and is of about the same density as the mesh in lamina I l l b . The mesh is continuous throughout all layers of the cortex without any sharp boundaries, except that the lower margin of the mesh in layer IV is rather prominent owing to an abrupt decrease in the number of fibers in the mesh in lamina Va. In no part of the cortex do the horizontal or vertical exogenous fibers contribute any fibers to this mesh. It is formed solely by the axons of the Golgi type II cells. The vertical exogenous fibers which ascend into the cortex have no collateral or terminal branches. They end as very small shafts which are directed vertically upward. Only one horizontal cell of Cajal has been observed in layer I and it presents an appearance of atrophy. In this area the tangential fibers in layer I are less numerous, much smaller, and much more lightly stained than the tangential fibers in layer I in area FAy in the region of the trunk, shoulder, and arm, and in the region of the hand. The tangential fibers are small, intermediate, and large in size. The smallest fibers are so fine and so lightly stained that they are barely visible at a magnification of X600. The fibers are collected chiefly in the outer one-half of layer I. Only a few, very fine fibers are present in the inner one-half. The tangential fibers are largest and most numerous in the walls of sulci, numbering 12 to 14 to the unit. The largest tangential fibers measure ι ·3μ in diameter in the Golgi sections. All the tangential fibers bear varicosities, spines, and thorns. No collaterals have been observed. The horizontal exogenous fibers in layer I I number ι or 2 to the unit, and are lightly stained. Internal to layer II the horizontal exogenous fibers gradually increase in number, size, and depth of color in successively deeper layers. In all the layers the horizontal

exogenous fibers are small, intermediate, and large in size. In lamina I l i a the horizontal fibers number 4 to 6 to the unit. In lamina I l l b these fibers number 8 to 10, and in the outer band of Baillarger in lamina IIIc and layer IV they average 10 to 12 to the unit. Lamina Va contains 8 to 10 horizontal exogenous fibers to the unit. In laminae Vb and Via (the inner band of Baillarger) the horizontal fibers number 10 to 12 to the unit. The fibers are small, intermediate, and large in size. In lamina Vlb the horizontal exogenous fibers number 12 to 14 to the unit. The largest fibers in this lamina measure 1.3 μ in diameter in the Golgi sections. The horizontal fibers in the layer of subcortical association fibers number 16 to 18 to the unit. The fibers are small, intermediate, and large in size. The largest fibers in the Golgi sections measure 1.3^ in diameter between varicosities. All the horizontal and subcortical association fibers bear varicosities, spines, and thorns. No branches have been observed on any of them. The vertical exogenous fibers are most numerous in the core of the gyrus. Here they number 35 to 40 to the unit. They are small, intermediate, and large in size. The largest vertical fibers measure 1 .τ,μ in diameter between varicosities. The vertical exogenous fibers gradually decrease in number, in size, and in depth of stain as they radiate into the cortex. All the vertical fibers bear varicosities, spines, and thorns. Neither collateral nor terminal branches have been observed on any vertical exogenous fibers. Most of the radiating vertical fibers end in layer VI. In the lower one-half of lamina Vlb they number 16 to 18 to the unit, and in the lower part of the inner band of Baillarger the vertical fibers number 6 to 8 to the unit. In the outer band of Baillarger there are but 1 to 3 vertical exogenous fibers to the unit and these are very small. Only occasionally is a vertical fiber seen ending in layer I I I . MYELiNATiON. With reference to the depth of stain in the Weigert sections in this area, the brains fall into the following sequence, beginning with the one with the darkest sections: (1) 41-91, (2) 38-44, (3) 3 8 35) (4) 37-82, (5) 37-49. The decrease in density of stain in the sections of brains 41-91, 38-44, 38-25, and 37-82 is gradual and small, but definite. The sections of brain 37-49 are much lighter in color than

LOBUS FRONTALIS those of brain 37-82, but they are much darker than the sections of brain 34-188, which is the three-month brain having the most myelin in this area. The stain in the sections of brain 4 1 - 9 1 is much darker than that in the sections in the posterior part of the superior frontal gyrus in six-month brain 38-35. The stain in the sections in the posterior part of gyrus frontalis medius is about the same as that in the sections in the region of the hand in area F A y in brain 37-82. The following description is based upon the sections of brain 41-91. Stained tangential fibers are present in the walls of sulci only. They occur in small groups placed at rather wide intervals. In the groups they number 4 or 5 to the unit. They are faintly stained, but the three categories of size — small, intermediate, and large — are present. N o stained horizontal fibers are present in layers II, III, or IV. In laminae V b and V i a there are 2 or 3 very small and very lightly stained horizontal fibers to the unit. Stained horizontal fibers number 6 to 8 to the unit in lamina VIb, and they are larger and more darkly stained than those in V i a . The stained fibers are small, intermediate, and large in size, and all bear varicosities. Stained subcortical association fibers are larger, darker, and more numerous than the stained horizontal fibers in lamina VIb. They number 10 to 12 to the unit, and are small, intermediate, and large in size. Stained vertical fibers are largest, most numerous, and darkest in color in the core of the gyrus. Here they number 30 to 35 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers in the core of the gyrus measure ι.ζμ in diameter between varicosities. As the vertical fibers radiate toward and into the cortex they decrease in number and size, and become lighter in color. The stain ends on most of the fibers in the lower part of lamina VIb. Very few stained vertical fibers reach lamina Va. GYRUS FRONTALIS INFERIOR (PARS OPERCULARIS), AREA FCBm This area is known as Broca's center of coordinated speech. The cortex here measures 2.053 mm. in width. It is illustrated in Figures 29, 30, 31, and 32.

35

CELL-BODIES

Layer I. The molecular layer measures 0.195 m m · width and contains 57 neurons to the unit. Layer II. The external granular layer is visible macroscopically. It measures 0.092 mm. in width and contains 314 neurons to the unit. The cells measure 5/4-9/8/*. Layer III. This layer measures 0.654 mm. in width. The neurons number 61 to the unit in lamina I l i a and measure 10/6-20/10/*. In lamina I l l b the nerve cells number 48 per unit and measure 10/6-23/15/1. In lamina IIIc the neurons number 52 to the unit and range in size from 10/6/1 to 33/15μ. Layer IV. The internal granular layer is more distinct than the external one. It measures 0.236 mm. in width and contains 132 neurons to the unit. The cells range in size from 5/4/1 to 13/10/1. The layer is not invaded by the large cells of laminae IIIc and Va to as great a degree as in area F A y in the region of the head. Layer V. The internal pyramidal layer measures 0.420 mm. in width. Small and large neurons are scattered throughout the layer. The extra-large pyramidal cells are located principally in lamina Vb, but some are present in lamina Va also. The small cells number 55 to the unit in lamina V a and 45 in lamina Vb. The large neurons number 16 per unit in lamina Va and 17 in Vb. The extra-large pyramidal cells number 12 to the unit, and range in size from 33/13/4, to 49/13/t. The small neurons measure 10/616/ιομ, and the large neurons range in size from 2ΰ/ΐΟμ to 26/12/1. Layer VI. This layer measures 0.456 mm. in width. The neurons number 75 to the unit in lamina V i a and 55 in lamina VIb. The pyramidal cells measure 10/6-26/ 13/j, and the spindle cells range in size from 16/6/1 to 29/ιομ. The extra-large pyramidal cells in layer V contain more chromophil substance than any other cells in this area. Most of them are stained more deeply than any other cells, but they are only slightly darker than the largest cells in layer V I and in lamina IIIc. The extra-large pyramidal cells are not particularly conspicuous as to either size or depth of stain. They contain more chromophil substance and are more darkly stained than the extra-large pyramidal cells in layer V in this area of the cortex in the three-month

36

THE CORTEX OF THE SIX-MONTH INFANT

infant. In the six-month cortex the chromophil substance in these cells is in the form of fine, lightly stained granules, small, darkly stained clumps, and a few small Nissl flakes. In most of the cells a small, dark clump of chromophil substance rests upon the nuclear membrane immediately subjacent to the base of the apical dendrite. A few granules and slender, elongated strands of chromophil substance extend into the apical dendrite for a short distance. A small clump of dark chromophil substance indicates the place of origin of one or two of the most proximal collaterals of the apical dendrite. Basal dendrites are faintly stained for a much shorter distance than the apical dendrite, and they contain no chromophil substance. In the cresyl violet sections the cells in lamina IIIc stand out conspicuously because of their large size and dark color. The chromophil substance is in the form of granules, clumps, and a few small Nissl flakes. The clumps are larger than those in the extralarge pyramidal cells in layer V. In nearly all the cells a clump of chromophil substance rests upon the nuclear membrane immediately below the base of the apical dendrite. This clump is cone-shaped, and larger than the corresponding clumps in the extra-large pyramidal cells. Small Nissl flakes invade the apical dendrite for a short distance. These Nissl flakes are smaller and less numerous than those in the apical dendrites of the extra-large pyramidal cells. In many of the apical dendrites a small clump of chromophil substance indicates the place of origin of one or two of the most proximal collaterals, but the collaterals are not stained. The basal dendrites are faintly stained for a short distance from the cell-body. The large cells in lamina I l l b contain less chromophil substance and are stained less darkly than the large cells in IIIc. Nissl flakes are less in quantity, and clumps and granules are correspondingly more numerous than in the large cells in IIIc. The large cells in I l i a have less chromophil substance and are stained more lightly than the large cells in I l l b . Granules are more numerous and clumps are proportionately less numerous than in the cells in Illb. Nissl flakes are very few in number. The chromophil substance is greater in quantity and more advanced in differentiation in the large cells in layers III, V, and VI than in corresponding cells in

this area in the cortex of the three-month infant. The large cells in lamina IIIc are conspicuously more advanced in development in the six-month cortex than those in the three-month brain. In the six-month cortex the chromophil substance in the large pyramidal cells in lamina IIIc is less advanced in development in every respect in area FCBm than it is in the corresponding cells in the posterior region of gyrus frontalis medius, area FB. The cells in layers II and IV have more chromophil substance than corresponding cells in the three-month cortex, and the cells in layer IV have more chromophil substance than those in layer II. The large horizontal cells of Cajal in layer I are present in approximately the same condition and number as in area FAy. A few longitudinal rows of granules can occasionally be seen in the apical dendrite of an extra-large pyramidal cell in layer V, but the granules are chiefly scattered irregularly throughout the cell-body and processes. The dendrites and axons of the extra-large cells are slightly darker than the cell-bodies. The cytoplasm in all other cells in this area is filled with granules which are scattered throughout the cell-body and processes. Differentiation of neurofibrils in the cells in this area is less advanced than it is in the cells in area FB in the posterior region of gyrus frontalis medius. The apical dendrites of the extra-large pyramidal cells in layer V are coarser than those of corresponding cells in this area in the cortex of the three-month infant. The terminal branches of the apical dendrites in layer I are conspicuously more numerous than in the three-month cortex. The collateral branches of the apical dendrites are coarser and longer, but no more numerous than those in the cortex of the three-month infant. The basal dendrites are larger and longer, but no more numerous, than those of corresponding cells in this area in the threemonth cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in layer V in this area are in approximately the same condition of development as those of corresponding cells in area FB in the posterior region of the middle frontal gyrus. Pedunculated bulbs are much more numerous on the dendrites of the extra-large pyramidal cells in FIBERS.

LOBUS FRONTALIS layer V than those on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs on the dendrites of the cells in this area are more numerous than those on the dendrites of corresponding cells in area FB in the posterior region of the middle frontal gyrus. Pedunculated bulbs are much less numerous on the dendrites of the extra-large pyramidal cells in area FCBm than those on the dendrites of the giant pyramidal cells of Betz in area FAy in the region of the head. The axons of the extra-large cells in layer V are but slightly larger and show more evidences of myelin than those of corresponding cells in this area of the cortex in the three-month brain. In the six-month cortex the axons of these cells have much less myelin than the axons of the giant pyramidal cells of Betz in the region of the head in area FAy. The dendrites of the large ordinary and special pyramidal cells in layers V and VI are larger and longer, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The pyramidal cells in layer III are more advanced in development than the corresponding cells in this area in the three-month cortex. The apical dendrites of these cells in the six-month cortex are coarser and have more terminal branches in layer I than corresponding cells in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all cells in layer III than in the threemonth cortex. In the Golgi-Cox preparations of the six-month cortex the largest pyramidal cells in lamina IIIc are as large as some of the extra-large pyramidal cells in layer V, and the apical dendrites are even larger than those of some of the extra-large cells. The dendrites of the large pyramidal cells in lamina IIIc do not have as many pedunculated bulbs as the dendrites of the extra-large cells. In fact, pedunculated bulbs are less numerous on the apical dendrites of the large pyramidal cells in lamina IIIc than those on the apical dendrites of the large ordinary and special pyramidal cells in layer V. A few bulbs are present on the

37

terminal branches of the apical dendrites of even the smallest pyramidal cells in lamina I l i a . The axons of most of the pyramidal cells, large or small, are stained for a short distance only, about two or three times the length of the cell-body. Occasionally an axon of one of the large cells in lamina IIIc can be traced into layer Vb. Some of these long axons have a few small beads on the surface and become slightly enlarged from layer IV to near their terminations, where they decrease in size. The dendrites of the small pyramidal cells in layer IV are coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. A few pedunculated bulbs are sparsely scattered on the dendrites of these cells. The dendrites of the small pyramidal and granular cells in layer II are slightly coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are even fewer in number on the dendrites of the pyramidal cells of layer II than those on the dendrites of the pyramidal cells of layer IV. N o pedunculated bulbs are present on the dendrites of the granular cells in layer II. N o horizontal cells of Cajal have been observed in layer I, and no branches have been observed on any of the tangential fibers. Golgi type II cells are present in all layers except layer I. Small, intermediate, and large cells occur in all layers of the cortex, but the cells in layer II are predominantly small in size. The axons of the Golgi type II cells form a very thin mesh of extremely fine fibers in layer II and in lamina I l i a . The mesh of fibers increases slightly in density in lamina I l l b , and contains some fibers which are larger than any in lamina I l i a . The mesh increases decidedly in density in lamina IIIc, and reaches the maximum density for this area in layer IV. It decreases abruptly in density in lamina Va, then increases in laminae Vb and Via, but is much less dense here than in layer IV. The mesh becomes thinner again in lamina VIb, where it is of about the same density as in lamina IIIc. Many of the fibers of the mesh are so fine as to be just visible with the high dry objective. Large fibers are more numerous in layers IV, V, and VI than in layer III. The mesh receives no fibers from the horizontal or vertical exogenous fibers. N o

38

THE CORTEX OF THE SIX-MONTH INFANT

branches have been observed on any of these fibers in the Golgi-Cox preparations. The vertical exogenous fibers end as vertically directed shafts. The mesh of fibers formed by the Golgi type II cells is somewhat more dense than that in this area in the three-month cortex. In the six-month cortex the mesh in this area is of about the same density as that in area F B in the posterior region of gyrus frontalis medius. The tangential fibers in layer I in this area are smaller and more lightly stained than those in area FA-y in the region of the head. The fibers number 14 to 16 to the unit. The fibers are much larger, more numerous, and more darkly stained in the outer onehalf than in the inner one-half of the layer. The largest fibers in the Golgi preparations measure ι.τ,μ in diameter. All the tangential fibers in the Golgi sections bear varicosities, spines, and thorns. No branches have been observed on any of these fibers, in either the Cajal or the Golgi sections. The horizontal exogenous fibers in layer II number 3 or 4 to the unit. They are small, intermediate, and large in size, and the fibers of each category are about the same size as the tangential fibers in the inner onehalf of layer I, and all are lightly stained. At successively deeper levels in the cortex the horizontal exogenous fibers increase in size and in intensity of stain. In lamina I l i a the horizontal fibers number 6 to 8, and in lamina 111b, 8 to 10 to the unit. The outer layer of Baillarger in lamina IIIc and layer IV contains 10 to 12 horizontal exogenous fibers to the unit. These fibers number 8 to 10 to the unit in lamina Va, and 10 to 12 in the inner band of Baillarger in laminae Vb and Via. In lamina VIb the horizontal exogenous fibers number 12 to 14 to the unit. The subcortical association fibers number 18 to 20 to the unit. The largest horizontal exogenous fibers in lamina VIb measure 1.3/1 in diameter in the Golgi preparations. The largest subcortical association fibers measure ι.ζμ in diameter. No branches have been observed on any of the horizontal or subcortical association fibers. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere. In this location the vertical fibers number 35 to 40 to the unit. As the vertical fibers radiate toward the cortex they are collected more or less into bundles. Most of the

radiating fibers end in lamina VIb, many end in the neuropil in the inner band of Baillarger, and a few end in layer IV. In the lower part of lamina VIb the radiating fibers number 16 to 18 to the unit. In lamina Vb these fibers number 4 to 6, and in the lower part of layer IV, 2 or 3 to the unit. Occasionally a vertical exogenous fiber is seen ending in layer III. The vertical fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. They are largest in the core of the gyrus, and diminish in size as they ascend in the cortex. The largest fibers in the core of the gyrus measure ι.5μ in diameter between varicosities. Neither collateral nor terminal branches have been observed on any vertical exogenous fibers. The stain in the Weigert sections of brain 37-82 is darkest in this area, but it is not much darker than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is much darker than that in the sections of brain 38-128, which has the most myelin in this area at the three-month stage of development. The stain in the Weigert sections of brain 37-82 in area F C B m is but slightly lighter than the stain in the sections in the region of the head in area FAy, but is distinctly lighter than the stain in the sections of this brain in the posterior part of gyrus frontalis medius. The stain in the sections of brain 37-82 in area F C B m is very much lighter than the stain in the sections in the posterior part of gyrus frontalis medius in brain 41-91. The following description is based upon the sections of brain 37-82. MYELiNATiON.

No stained tangential fibers are present in layer I. No stained horizontal fibers are present in the cortex external to layer VI. Scattered, single, short, very small and faintly stained horizontal fibers are present in lamina Via. In lamina VIb the stained horizontal fibers are longer, larger, and more darkly stained, and number 4 or 5 to the unit of width. These fibers are not as large or as darkly stained as the stained horizontal fibers in this lamina in the posterior part of gyrus frontalis medius in brain 37-82. The stained subcortical association fibers are larger, more darkly stained, and more numerous than the stained horizontal fibers in lamina VIb. They are small, intermediate, and large in size, and all bear varicosities. They number 8 to 10 to the unit. They are not as

LOBUS

FRONTALIS

large or as dark as the stained subcortical association fibers in the posterior part of gyrus frontalis medius. The stained vertical fibers are distinctly lighter in color than those in the posterior part of gyrus frontalis medius, and are slightly lighter than the vertical fibers in the posterior part of gyrus frontalis superior. The stained vertical fibers are darkest, largest, and most numerous in the core of the gyrus. In this location they number 25 to 30 to the unit. They are small,

39

intermediate, and large in size, and all bear varicosities. The largest fibers measure 1.3/x between varicosities. The vertical fibers decrease in number and size and become lighter in color as they radiate toward and into the cortex. The stain ends on most of the fibers in the lowermost part of lamina VIb, on a few fibers the stain ends near the middle of layer V I , and occasionally a stained vertical fiber ends in lamina V i a .

MIDDLE REGION OF THE FRONTAL GYRI THE CORTEX in the middle region of the superior, middle, and inferior frontal gyri has the same general characteristics as it has in these locations in the threemonth infant. The granular layers are visible macroscopically and are more prominent than they are in the posterior part of the three frontal gyri. The inner granular layer is more prominent than the outer one. GYRUS FRONTALIS SUPERIOR, AREA FC The cortex in the crown of the gyrus on the superolateral surface measures 2.498 mm. in width. It is illustrated in Figures 33, 34, 35, and 36. CELL-BODIES

Layer I. This layer measures 0.240 mm. in width, and contains 58 neurons to the unit. Large horizontal cells of Cajal are seen occasionally. Layer II. The external granular layer measures 0.118 mm. in width. The neurons number 290 to the unit and measure 4/4-9/8/». The outer border is fairly sharp, but the inner margin is indistinct because the small cells of layer II mingle with the cells of lamina Ilia. Layer III. This layer measures 0.850 mm. in width. The neurons number 57 to the unit in lamina I l i a and measure 10/6-23/ιομ. In lamina I l l b the nerve cells number 52 per unit and measure 10/6-26/13/». The neurons number 59 to the unit in lamina IIIc, and range in size from ιο/6μ to 33/13/*. Layer IV. The internal granular layer measures 0.229 mm. in width, and contains 111 neurons to the unit. The nerve cells range in size from 5/4/» to 12/τομ. The layer is invaded to only a small degree by the large neurons of laminae IIIc and Va.

Layer V. This layer measures 0.497 m m · i n width. Small and large neurons are scattered throughout the layer. The extra-large pyramidal cells are located chiefly, but not entirely, in lamina Vb. The small cells measure ΐ0/6-ι6/ΐ0/α, and number 55 to the unit in lamina Va and 50 in lamina Vb. The large cells measure 20/10-26/12/», and number 21 to the unit in V a and 19 in Vb. The extra-large cells number 13 to the unit and range in size from 29/13/» to 53/i2,u. Layer VI. The polymorphic layer measures 0.564 mm. in width. The neurons number 75 to the unit in lamina V i a and 40 in lamina VIb. The pyramidal cells measure 10/6-26/13/», and the spindle cells measure 16/6-2 9/1 ομ. The extra-large pyramidal cells in layer V contain more chromophil substance and are stained more darkly than any other cells in this area. These cells contain more chromophil substance than the corresponding cells in the three-month cortex. Nissl flakes are more numerous in the six-month cortex, and the chromophil substance extends into the apical and basal dendrites in greater quantity and for a greater distance than in the three-month brain. In the sixmonth cortex the chromophil substance in the extralarge pyramidal cells in this area is less advanced in differentiation in every respect than in the corresponding cells in the posterior region of gyrus frontalis superior. Some of the large ordinary pyramidal cells in layers V and V I and some of the large fusiform cells in V I are darkly stained, while others are light in color. These cells contain more chromophil substance than the corresponding cells in this area of the cortex in the three-month infant. The chromophil substance is

40

THE CORTEX OF THE SIX-MONTH INFANT

in the form of granules, small clumps, and small Nissl flakes. The latter are more numerous than those in the corresponding cells in this area of the cortex of the three-month infant. Granules, small clumps, and small Nissl flakes invade the apical dendrites in greater quantity and for a greater distance than in these cells in the three-month cortex. The basal dendrites of the largest cells only are stained for a very short distance at their proximal ends, and they contain a few scattered granules and small clumps of chromophil substance. The large pyramidal cells in lamina IIIc are stained more darkly than any other cells in layer III, and are about the same color as the darkest large ordinary pyramidal cells in layers V and VI. Some of the largest cells in lamina I l l b are stained approximately as darkly as the large cells in IIIc. Chromophil substance is present in greater quantity in the large cells in IIIc than in corresponding cells in this area in the three-month cortex. The chromophil substance is in the form of granules, flakes, and small darkly stained clumps. It extends into the apical dendrites in greater quantity and for a greater distance than in the threemonth cortex. The basal dendrites of the large pyramidal cells in IIIc are stained for a very short distance from the cell-body. Rarely, a few granules of chromophil substance are seen in the most proximal portion of a basal dendrite. As the cells in layer III become smaller toward layer II the quantity of chromophil substance in proportion to the size of the nucleus diminishes. In all the cells in layers III and II granules of chromophil substance are less in quantity and small clumps are more numerous than in the corresponding cells in this area in the three-month cortex. No neurofibrils are present in any cells in this area of the cortex. Longitudinal rows of granules are present in the extra-large pyramidal cells, and are of greater quantity than in the extra-large pyramidal cells in area FCBm. The cytoplasm in all other cells is filled with granules, and these are scattered irregularly throughout the cell-bodies and their processes. Differentiation of neurofibrils is not as advanced in the cells in this area as it is in corresponding cells in area FB in the posterior region of gyrus frontalis superior.

FIBERS. In the Golgi-Cox preparations the apical and basal dendrites of the extra-large pyramidal cells in layer V are larger and more compact in structure than those of corresponding cells in this area in the threemonth cortex. The terminal branches of the apical dendrites in layer I are more numerous than in the three-month cortex. The terminal branches have numerous varicosities and present a general appearance of immaturity. The apical dendrites of many of the extra-large pyramidal cells do not subdivide dichotomously before reaching layer I, but end as single, vertically directed shafts. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than in the three-month cortex. The bulbs are more numerous on the apical dendrites than on the basal dendrites or collateral branches of the apical dendrites. The bulbs are most numerous on the apical dendrites as they pass through layer IV and lamina IIIc (the outer band of Baillarger), and they decrease rapidly in quantity toward the distal end of the dendrites. Pedunculated bulbs are few in number and widely scattered on the terminal branches of the apical dendrites in layer I. In the six-month cortex the extra-large pyramidal cells in this area are not as well developed as the corresponding cells in the posterior region of the superior frontal gyrus, area FB. The cell-bodies and dendrites are smaller in area FC than in FB, terminal branches of the apical dendrites in layer I are less numerous in FC, and pedunculated bulbs on all dendrites are less numerous in area FC than in area FB. Pedunculated bulbs are particularly less numerous on the terminations of the apical dendrites in layer I in area FC than in area FB. The axons of the extra-large pyramidal cells in layer V descend vertically in a wavy course and enter the layer of vertical exogenous fibers in the core of the gyrus. Small beads appear on the surface of the axons in lamina Via. The beads gradually increase in size and in number, and the axons become slightly larger as they descend. The surface beads and the increase in size of the axons are interpreted as evidences of myelin. All the axons show about the same quantity of myelin. Two or three very fine collaterals are seen emerging from some of the axons as they

LOBUS FRONTALIS pass through the upper one-half of layer VI. The collaterals always leave the parent axon at right angles. The collaterals extend horizontally, mingling with the horizontal exogenous fibers. The collaterals do not show any evidences of the presence of myelin. The axons of the extra-large pyramidal cells are coarser and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells in the present area are smaller and have less myelin than the axons of corresponding cells in area F B in the posterior region of the superior frontal gyrus. The apical and basal dendrites of the large ordinary and special pyramidal cells in layers V and VI are coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the large ordinary and special pyramidal cells are not as large as those of corresponding cells in the posterior region of gyrus frontalis superior. Pedunculated bulbs are more numerous on the dendrites of the large ordinary and special pyramidal cells in layers V and VI than in the three-month cortex. In the six-month cortex the pedunculated bulbs on the dendrites of these cells in this area are less numerous than those on the dendrites of corresponding cells in the posterior region of gyrus frontalis superior. Pedunculated bulbs are present in much less quantity on the dendrites of the large ordinary and special pyramidal cells than on the dendrites of the extra-large pyramidal cells in this area. The axons of the large ordinary and special pyramidal cells in layers V and VI are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are much smaller and have much less myelin than the axons of the extra-large cells in this area. The axons of the large ordinary and special pyramidal cells in this area are smaller in caliber and have less myelin than the axons of corresponding cells in the posterior region of gyrus frontalis superior. The apical dendrites of the large pyramidal cells in all three laminae of layer I I I end in layer I. The terminal branches in layer I are more numerous than those in this area in the three-month cortex. Also,

41

the apical dendrites are coarser, and the basal dendrites and collateral branches of the apical dendrites are coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the apical and basal dendrites of the pyramidal cells in layer I I I are not as coarse as those of corresponding cells in the posterior region of gyrus frontalis superior. Pedunculated bulbs are more numerous on the apical dendrites of all the pyramidal cells in layer I I I than on the basal dendrites or the collaterals of the apical dendrites. The bulbs are most numerous on the dendrites of the large pyramidal cells in lamina IIIc and gradually decrease in quantity as the cells become smaller in laminae I l l b and I l i a . The bulbs are widely scattered along the apical dendrites of the pyramidal cells in I l i a and are very few in number on the terminal branches. The pedunculated bulbs are more numerous on all the dendrites of all the pyramidal cells in layer I I I than on the dendrites of cells of corresponding size in this layer in this area of the cortex in the three-month infant. Pedunculated bulbs are less numerous on the dendrites of the largest pyramidal cells in lamina IIIc than those on the dendrites of the large ordinary and special pyramidal cells in layer V in the present area. In the six-month cortex the pedunculated bulbs on the dendrites of all pyramidal cells in layer I I I in this area are less in quantity than those on the dendrites of corresponding cells in this layer in the posterior region of gyrus frontalis superior. The axons of the largest pyramidal cells in laminae I l l b and IIIc are the only ones which bear any indications of myelin, and this is represented by a few droplets which appear at widely scattered intervals. The myelin is not appreciably greater in quantity than that present on the axons of corresponding cells in this area in the three-month cortex. In the sixmonth cortex there is much less myelin on the axons of the large pyramidal cells in laminae I l l b and IIIc in this area than is present on the axons of corresponding cells in the posterior region of gyrus frontalis superior. The axons of pyramidal cells in lamina I l i a do not have any indications of myelin. The apical and basal dendrites of the pyramidal cells of layer IV are coarser and longer than those of

42

THE CORTEX OF THE SIX-MONTH INFANT

corresponding cells in this area in the three-month cortex, but they are not as coarse or as long as those of corresponding cells in layer IV in the posterior region of gyrus frontalis superior in the six-month cortex. Pedunculated bulbs are sparsely scattered along the dendrites of these cells, but are more numerous than in the three-month cortex. The bulbs are less numerous than those on the dendrites of corresponding cells in the posterior region of gyrus frontalis superior in the six-month cortex. The apical and basal dendrites of the small cells in layer I I are coarser and longer than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of these cells are not as well developed as those of corresponding cells in the posterior region of the superior frontal gyrus, and also are not as coarse or as long as the dendrites of the pyramidal cells of layer IV in the middle region of this gyrus. Pedunculated bulbs are scattered along the dendrites and their branches even more sparsely than on the dendrites of the pyramidal cells of layer IV in this area. The bulbs are more numerous, however, than those on the dendrites of corresponding cells in the cortex of the three-month infant. In the six-month cortex the bulbs on the dendrites of these cells in this area are fewer than those on the dendrites of corresponding cells in the posterior region of the superior frontal gyrus. A shrunken remnant of a horizontal cell of Cajal is occasionally seen in layer I. No long branches are present on any of the tangential fibers in layer I, but occasionally a very short and delicate twig is observed. Golgi type I I cells are present in all layers in the cortex in this region except layer I. These cells are more numerous in this area than in area F B in the posterior region of gyrus frontalis superior. In the middle region of this gyrus the Golgi type I I cells are more numerous in layer IV than in any other layer. Representatives of all types of these cells occur in all layers. Their dendrites are in approximately the same state of development as the pyramidal cells of comparable size in the particular stratum of a layer in which they are located. The axons of the Golgi type I I cells form a mesh of small and large fibers in all layers except layer I. The mesh is at the maximum

density in layer IV, and rapidly decreases in density externally, and is very thin in lamina I l i a and in layer II. The mesh of fibers is much thinner in lamina Va than in layer IV, and increases considerably in density in laminae Vb and Via, but is not as dense here as in layer IV. The mesh decreases rapidly but gradually in lamina VIb where it is of about the same density as in lamina I l l b . The vertical exogenous fibers do not contribute to this mesh by terminal branching. No terminal or collateral branches have been observed on any of the vertical exogenous fibers. In the Golgi preparations all of the vertical exogenous fibers end as single shafts directed vertically. The mesh of fibers formed by the Golgi type I I cells is more dense in all the layers than that in this area in the cortex of the three-month infant. In the six-month cortex the mesh is somewhat more dense in this area than that in the posterior region of gyrus frontalis superior. The tangential fibers in layer I are larger, more numerous and more darkly stained in the walls of sulci than in the crown of gyrus frontalis superior. In the walls of sulci they number 12 to 14 to the unit. They are small, intermediate, and large in size. The fibers in each category of size are smaller and more lightly stained than the tangential fibers in the posterior region of gyrus frontalis superior, area F B . The largest fibers measure 1.3^ in diameter. The horizontal exogenous fibers in layer I I are very small and lightly stained. They number but 2 or 3 to the unit. In lamina I l i a the horizontal fibers are larger, more darkly stained, and more numerous than those in layer II. They number 4 to 6 to the unit. In lamina I l l b the horizontal exogenous fibers average 6 to 8 to the unit. The horizontal exogenous fibers number 10 to 12 to the unit in the outer band of Baillarger. In lamina Va these fibers number 8 to 10 to the unit. The inner band of Baillarger in laminae Vb and Via contains 10 to 12 horizontal exogenous fibers to the unit. In lamina VIb the horizontal fibers number 10 to 12 to the unit. The subcortical association fibers number 14 to 16 to the unit. In all layers of the cortex the horizontal exogenous fibers are small, intermediate, and large in size. Proceeding inwardly from layer I I the horizontal exogenous fibers gradually increase in size and are largest in layer VI and in the layer of subcortical association

LOBUS FRONTALIS fibers. The largest horizontal fibers in lamina V I b and the largest subcortical association fibers measure ι .^μ in diameter in the Golgi sections. All the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. N o branches have been observed on any of these fibers. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere, and number 35 to 40 to the unit. The fibers are small, intermediate, and large in size. The largest fibers measure 1 .3/1 in diameter in the Golgi sections. The vertical fibers are smaller and more lightly stained than the corresponding fibers in the posterior region of gyrus frontalis superior. The fibers decrease in quantity per unit and become smaller as they radiate toward the cortex. Most of the radiating vertical fibers end in the mid-region of layer V I . Some end in the neuropil in the inner band of Baillarger, and a few end in layer IV. In the lower part of lamina V I b the radiating vertical fibers number 12 to 14 to the unit. In lamina V b the vertical fibers number 4 to 6 to the unit, and in the lower part of layer I V they number but ι or 2 to the unit. Only rarely is a vertical fiber seen in layer III. All the vertical exogenous fibers bear varicosities, spines, and thorns. Neither collateral nor terminal branches have been observed on any vertical exogenous fibers. The brains examined fall into the following sequence as to the depth of stain in the Weigert sections: ( 1 ) 37-82, (2) 38-35, (3) 37-49· The stain in the sections of brain 38-35 is not much lighter than that in the sections of brain 37-82, but the stain in the sections of brain 37-49 is much lighter than that in the sections of brain 38-35. The sections of brain 37-49 are much darker in color than those of brain 38-128, the three-month brain having the most myelin in this area. The stain in the sections of brain MYELiNATioN.

37-82 is about the same color as that in the sections in the posterior part of gyrus frontalis superior in this brain. The sections of brain 37-82 in the middle of gyrus frontalis superior are slightly lighter in color than those in the posterior part of this gyrus in brain 38-35. The following description is based upon the sections of brain 37-82. N o stained tangential fibers are present in layer I in the crown of the superior surface of the gyrus. A

43

few widely scattered, small, lightly stained fibers are seen in layer I on the medial surface of the gyrus. N o stained horizontal fibers are present in the cortex external to layer V. A few widely scattered small, lightly stained fibers are seen occasionally in lamina Va. In laminae V b and V i a the stained fibers are larger, darker, and more numerous, averaging 3 or 4 to the unit. The stained fibers are yet larger and darker in lamina VIb, and they number 5 or 6 to the unit. The stained subcortical association fibers are larger and more darkly stained than the horizontal fibers in lamina VIb. The stained subcortical fibers number 8 to 10 to the unit. The stained horizontal and subcortical association fibers are small, intermediate, and large in size. The stained vertical exogenous fibers are largest, darkest, and most numerous in the core of the gyrus. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 25 to 30 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The smallest stained fibers are very fine. The largest are the most darkly stained, and they measure 1.5/1 in diameter between varicosities. The vertical fibers gradually become smaller and lighter in color as they radiate toward and ascend in the cortex. The stain ends on most of the fibers in the lowermost part of lamina VIb, and on a few fibers it ends in the middle of layer VI. A stained vertical fiber is occasionally seen ending in the lower part of layer V. GYRUS FRONTALIS MEDIUS, AREA FD The cortex in the crown of the gyrus measures 2.235 m m · i n width. It is illustrated in Figures 37, 38,39, and 40. CELL-BODIES

Layer I. This layer measures 0.233 mm. in width. The neurons number 53 to the unit. Layer II. The outer granular layer measures 0.096 mm. in width. The neurons of this layer number 293 to the unit and measure 4/4-8/8/1. Layer III. The external pyramidal layer measures 0.730 mm. in width. In lamina I l i a the neurons number 57 to the unit and measure 10/6-23/12/1. In lamina I l l b there are 50 neurons to the unit, and they range in size from 10/6/1 to 26/ 13/x. The nerve cells

44

THE CORTEX OF THE SIX-MONTH INFANT

in lamina IIIc number 56 to the unit and measure 10/6-33/13/4.

Layer IV. The inner granular layer is more distinct than the outer one. Fewer large cells of laminae IIIc and Va are intermingled with the small cells of layer IV than in the posterior part of gyrus frontalis medius. The layer measures 0.208 mm. in width. The neurons number 122 to the unit and measure 5/412/10/4.

Layer V. This layer measures 0.421 mm. in width. Small and large neurons are scattered throughout the layer, but the extra-large pyramidal cells occur chiefly in lamina V b . The small cells number 55 to the unit in lamina Va and 45 in lamina Vb. The large cells number 23 per unit in Va and 20 in Vb. The small cells measure 10/6-16/10/1, and the large neurons range in size from 20/10/4 to 26/12/4. The extra-large pyramidal cells number 10 to the unit, and measure 29/12-33/13/4.

Layer VI. This layer measures 0.547 mm. in width. The neurons number 75 to the unit in lamina V i a and 45 in lamina VIb. The pyramidal cells measure 10/6-26/15/4, and the spindle cells range in size from 16/6/4 to 29/12/4.

The extra-large pyramidal cells in layer V are stained more darkly than other cells in this layer and in layer V I . They contain more chromophil substance than any other cells in these two layers, and the chromophil substance extends into the apical dendrites farther than in any of the other cells. The chromophil substance in the extra-large pyramidal cells is greater in quantity than in the corresponding cells in the three-month cortex. The chromophil substance is chiefly in the form of clumps, but granules and a few small Nissl bodies are present. The clumps are small and large, the latter size predominating in number. Granules, small clumps, and elongated strands of chromophil substance invade the proximal end of the apical dendrites. N o collaterals of the apical dendrites are stained, but the point of origin of each of the most proximal two or three collaterals is indicated by a small clump of darkly stained chromophil substance. Basal dendrites are stained for a short distance, and have a few granules of chromophil substance at their proximal ends. A small disc or cone of darkly stained chromophil substance rests upon the nuclear membrane immediately under the

origin of the apical dendrite. The stance in the extra-large pyramidal is less in quantity and less advanced than it is in corresponding cells in gion of gyrus frontalis medius.

chromophil subcells in this area in differentiation the posterior re-

The large pyramidal cells in lamina IIIc are stained as darkly as the extra-large pyramidal cells in layer V. The largest cells in IIIc contain as much chromophil substance as the smaller extra-large pyramidal cells in V. The largest cells in laminae I l l b and I l i a are stained almost as darkly as the large cells in IIIc. The cells in all three laminae of layer I I I contain more chromophil substance than corresponding cells in this area in the three-month cortex. Nissl bodies are more numerous in all cells in layer I I I than in the cortex of the three-month infant. In the six-month cortex the chromophil substance in the cells in layer I I I in this area is not as advanced in differentiation as it is in corresponding cells in the posterior region of gyrus frontalis medius. More of the chromophil substance is in the form of large clumps in the cells in the middle region than in corresponding cells in the posterior region of the gyrus. The small cells in layer I V contain more chromophil substance in proportion to the size of the nuclei than cells of comparable size in layer II, and the chromophil substance is stained more darkly in the cells in layer IV. The chromophil substance in the cells in both of the granular layers in the six-month cortex is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the cortex of the three-month infant. In the Cajal sections of the cortex in this area the cytoplasm in all cells is filled with granules which are scattered irregularly throughout the cell-bodies and processes. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are coarser than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of the extra-large cells in the cortex of the three-month infant. The terminal branches of the apical dendrites are more numerous than in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells

LOBUS FRONTALIS in this area are not as coarse as those of corresponding cells in the posterior region of gyrus frontalis medius. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than on the dendrites of any other cells in this area. The bulbs are more numerous on the apical dendrites than on the basal dendrites or on the collateral branches of the apical dendrites, and are present on the apical dendrites in greatest quantity as they pass through layer IV and lamina IIIc. Distal to this location the bulbs decrease in quantity rapidly but evenly, and are least in number on the terminal branches of the apical dendrites in layer I, where they are widely scattered. The pedunculated bulbs are much less numerous than those on the dendrites of the extralarge pyramidal cells in the posterior region of gyrus frontalis medius. The axons of the extra-large pyramidal cells are only slightly larger than those of corresponding cells in this area in the three-month cortex. The quantity of myelin has not appreciably increased since the three-month stage. In the six-month cortex the axons of these cells are smaller and less compact in structure and have less myelin than those of corresponding cells in the posterior region of gyrus frontalis medius. The dendrites of the large ordinary and special pyramidal cells in layers V and VI are coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex, but the dendrites are not as well developed in these respects as those of the large ordinary and special pyramidal cells in layers V and VI in the posterior region of gyrus frontalis medius in the cortex of the six-month infant. Pedunculated bulbs are much less in quantity on the dendrites of the large ordinary and special pyramidal cells than on the dendrites of the extra-large pyramidal cells in this area. The bulbs are more numerous on the dendrites of the large ordinary and special pyramidal cells than on the dendrites of corresponding cells in this area in the threemonth cortex. In the six-month cortex the pedunculated bulbs are much less numerous on the dendrites of these cells than on the dendrites of corresponding cells in this area in the posterior region of gyrus frontalis medius. The axons of the large ordinary and special pyram-

45

idal cells in layers V and VI are somewhat coarser and more compact in structure than those of corresponding cells in this area in the three-month cortex. The axons of all of these cells decrease in size as they descend, but some of them have very small droplets on their surfaces which may represent myelin. In the six-month cortex the axons of these cells are not as well developed as those of corresponding cells in the posterior region of gyrus frontalis medius. The apical dendrites of all the pyramidal cells in layer I I I are coarser and more compact in structure and have more terminal branches than those of corresponding cells in layer I I I in this area in the threemonth cortex. The basal dendrites and the collateral branches of the apical dendrites of these cells are coarser, longer, and more compact in structure, but no more numerous than those of corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the dendrites of the pyramidal cells in layer I I I are not as coarse, and the terminal branches of the apical dendrites in layer I are not as numerous, as those of corresponding cells in the posterior region of gyrus frontalis medius. Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer I I I than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs are less numerous on the dendrites of the pyramidal cells in layer I I I in this area than on the dendrites of corresponding cells in the posterior region of gyrus frontalis medius. In the middle region of gyrus frontalis medius pedunculated bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of any other cells in layer III. The bulbs gradually decrease in number as the cells become progressively smaller in laminae I l l b and I l i a . A few widely scattered bulbs are present even on the terminal branches of the apical dendrites of the pyramidal cells of lamina I l i a . Pedunculated bulbs are much more numerous on the dendrites of the large ordinary and special pyramidal cells in layers V and VI than on the dendrites of even the largest pyramidal cells of lamina IIIc. The axons of the pyramidal cells in all three laminae of layer I I I are coarser, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. The axons of the

46

THE CORTEX OF THE SIX-MONTH INFANT

large pyramidal cells in lamina IIIc bear evidences of myelin in the form of small beads and amorphous clumps on the surface. The dendrites of the small pyramidal cells in layer IV are coarser and longer, but no more numerous, than those of corresponding cells in this area of the three-month cortex. Pedunculated bulbs are widely scattered along the dendrites of these cells, but they are somewhat more numerous than on the dendrites of corresponding cells in the three-month cortex.

accumulated chiefly in the mid-region of layer I. The fibers are small, intermediate, and large in size. Those of each category of size are smaller and more lightly stained than the tangential fibers in the walls of sulci in the posterior region of gyrus frontalis medius. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure 1.3m in diameter between varicosities. No branches have been observed on any tangential fibers.

The dendrites of the small pyramidal cells of layer II are coarser and longer than those of corresponding cells in the three-month cortex. The dendrites of the cells in layer I I are not as coarse or as long as those of the pyramidal cells belonging to layer IV. Pedunculated bulbs are very sparsely scattered along the apical dendrites of the cells in layer II.

Very small, lightly stained solitary horizontal exogenous fibers are widely scattered in layer II. In lamina I l i a horizontal exogenous fibers are larger than those in layer II, and are also widely scattered, in lamina I l l b the horizontal fibers are larger and are more darkly stained than those in lamina I l i a . They number 3 or 4 to the unit. In layer IV the horizontal fibers number 8 to 10 to the unit. In lamina Va the horizontal fibers number 6 to 8, and in the inner band of Baillarger they number 10 to 12 to the unit. In lamina V I b the horizontal fibers number 8 to 10 to the unit, and the subcortical association fibers number 10 to 12. In all layers the horizontal exogenous fibers are small, intermediate, and large in size. The fibers gradually increase in size and in depth of stain from layer I I to the layer of subcortical association fibers. In the Golgi preparations the largest horizontal fibers in layer V I and the largest subcortical association fibers measure 1.3/4 in diameter. All the horizontal exogenous fibers and all the subcortical association fibers in the Golgi sections bear varicosities, spines, and thorns. N o branches have been observed on any of these fibers.

N o horizontal cells of Cajal have been observed in layer I. The dendrites of the Golgi type II cells are of about the same size and have about the same quantity of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size in this area. The dendrites of the Golgi type II cells are coarser and longer and have more pedunculated bulbs than the dendrites of corresponding cells in this area in the three-month cortex. The mesh of fibers formed by the axons of the Golgi type II cells in this area is much thinner and less extensive than that in the posterior region of gyrus frontalis medius. In the present area (middle region of gyrus frontalis medius) the mesh of fibers is more dense in laminae V b and V i a than in any other strata of the cortex. It is slightly less dense in lamina VIb, and is much thinner in lamina Va. The mesh is very thin in layer IV, and external to this layer the axons of Golgi type II cells are not sufficiently numerous to form a mesh. The mesh is not any more dense than that in this area in the three-month cortex. The tangential fibers in layer I are more numerous, larger, and more darkly stained in the walls deep within sulci than in the crown of the gyrus. The fibers in the crown are so small and so lightly stained that they are barely visible with the low power. Where they are most numerous in the walls of sulci the tangential fibers number 12 to 14 to the unit. They are

The vertical exogenous fibers number 35 to 40 to the unit in the core of the gyrus. The fibers are small, intermediate, and large in size. All bear varicosities, spines, and thorns. The largest vertical fibers measure ι.ΐμ. in diameter between varicosities. As the vertical exogenous fibers radiate toward the cortex they are chiefly collected in bundles. Most of the radiating fibers end in lamina VIb. In the lower part of this lamina they number 12 to 14 to the unit. In lamina V b the vertical exogenous fibers number 6 to 8 to the unit, and in the lower part of layer IV they number but 3 or 4 to the unit, and they are very small. Only an occasional vertical exogenous fiber is seen in

LOBUS

47

FRONTALIS

layer III. No branches have been observed on any vertical exogenous fibers. The brains examined fall into the following sequence with reference to the amount of myelin as revealed by the Weigert sections: ( i ) 3782, (2) 38-35, (3) 37-49. The sections of brain 37-82 are distinctly darker in color than those of brain 38-35. The sections of brain 37-49 are much lighter than those of brain 38-35. The sections of brain 37-49 are much darker than those of brain 34188, which is the three-month brain having the most myelin in this area. The sections in this area of brain 37-82 are lighter than those in the middle region of gyrus frontalis superior in this brain. Each of the three brains has less myelin in the middle region of gyrus frontalis medius than in the posterior region of this gyrus. The following description is based upon the sections of brain 37-82. MYELINATION.

No stained tangential fibers are present in layer I, and no stained horizontal fibers are present in the cortex external to layer V. A very small, short, lightly stained horizontal fiber is occasionally seen in lamina Va. In laminae Vb and V i a (the inner band of Baillarger) the stained fibers number 2 or 3 to the unit. They are small and lightly stained, but larger, longer, and darker than the stained fibers in Va. In lamina VIb the stained horizontal fibers are yet larger and darker, and they number 4 or 5 to the unit. The stained horizontal fiber is occasionally seen in lamina and darker, and number 6 to 8 to the unit. The horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. Stained vertical exogenous fibers are largest, darkest, and most numerous in the core of the gyrus. Here they number 25 to 30 to the unit. The stained fibers are small, intermediate, and large in size and all bear varicosities. The largest fibers measure 1 in diameter between varicosities. As the stained fibers radiate toward and into the cortex they gradually decrease in size and number and become lighter in color. The stain on most of them ends in the lower part of lamina VIb, it remains on some few fibers to lamina Via, and very lightly stained fibers are occasionally seen in lamina Va.

GYRUS FRONTALIS INFERIOR (PARS TRIANGULARIS), AREA F ü r

The cortex in the crown of the gyrus measures 2.212 mm. in width. It is illustrated in Figures 41, 42, 43, and 44. CELL-BODIES

Layer I. This layer measures 0.214 mm. in width and contains 53 neurons to the unit. Horizontal cells of Cajal are seen occasionally. Layer II. The external granular layer measures 0.101 mm. in width. The neurons number 296 to the unit and measure 4/4-9/8^. The outer margin of the layer is fairly sharp, but the internal boundary is indistinct because the neurons of layer II intermingle with those belonging to lamina I l i a . Layer III. This layer measures 0.739 m m · width. In lamina I l i a the neurons number 58 to the unit and measure 10/6-23/12/*. The neurons number 48 per unit in lamina IHb, and range in size from το/6μ to 26/13/*. In lamina IIIc the neurons number 51 to the unit and measure 10/6-33/13/1. Layer IV. The internal granular layer is more distinct than the external one. In the crown of the gyrus the layer measures 0.251 mm. in width. The neurons measure 5/4-10/6/1 and number 132 to the unit. Not as many neurons of laminae IIIc and Va invade layer IV as in the posterior part of this gyrus. Layer V. The internal pyramidal layer measures 0.388 mm. in width. The small neurons number 55 to the unit in lamina Va and 50 in lamina Vb. The large neurons number 21 to the unit in lamina Va and 19 in Vb. The extra-large cells number 12 to the unit, and are located chiefly in lamina Vb. The small neurons measure 10/6-16/ιομ, the large ones measure 20/10-26/12^1, and the extra-large pyramidal cells range in size from 29/12/i to 39/15/1. Layer VI. This layer measures 0.519 mm. in width. The neurons number 75 to the unit in lamina V i a and 45 in lamina VIb. The pyramidal cells measure 10/6-23/12μ and the spindle cells range in size from 16/6μ to 29/10μ. The extra-large pyramidal cells in layer V contain more chromophil substance and are more darkly stained than any other cells in layers V and VI. Some

48

THE CORTEX OF THE SIX-MONTH INFANT

of the extra-large pyramidal cells are located in the external part of layer VI. The chromophil substance is more abundant and more advanced in differentiation than that in corresponding cells in this area in the three-month cortex. It is in about the same state of differentiation as the chromophil substance in the extra-large pyramidal cells in area FCBm. The chromophil substance is in the form of granules, flakes, and clumps. The clumps are chiefly small, but some are large. The large cells in lamina IIIc are stained about as darkly as the extra-large cells in layer V. The largest cells in IIIc are as large and contain as much chromophil substance as the extra-large pyramidal cells in layer V. All the cells in layer III have more chromophil substance than corresponding cells in this area in the three-month cortex. In the six-month brain the chromophil substance in the cells in layer III is present in about the same quantity and condition as that in corresponding cells in this layer in area FCBm. The cytoplasm in all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. The apical dendrites of the extra-large pyramidal cells in layer V are coarser and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells are not as large as those of corresponding cells in the posterior region of gyrus frontalis inferior, area FCBm.

FIBERS.

Pedunculated bulbs are more numerous on the apical dendrites of the extra-large pyramidal cells in layer V than on the dendrites of any other cells in the cortex in this area. Bulbs are present on the terminal branches of the apical dendrites in much less quantity than on the collateral branches or on the basal dendrites. Pedunculated bulbs are more numerous on all the dendrites of the extra-large pyramidal cells than those on the dendrites of corresponding cells in this area in the three-month cortex. In the cortex of the six-month infant pedunculated bulbs are less numer-

ous on the dendrites of the extra-large pyramidal cells in layer V in this area than those on the dendrites of corresponding cells in the posterior region of gyrus frontalis inferior, area FCBm. The axons of the extra-large pyramidal cells in layer V decrease in size as they descend. They are larger than the axons of corresponding cells in this area in the three-month cortex, but are smaller than the axons of these cells in the posterior region of gyrus frontalis inferior in the six-month cortex. The only evidences of myelin are a few very small, widely scattered droplets on the surface of the axons. The apical dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are coarser than those of the corresponding cells in this area in the three-month cortex. The apical dendrites end as single shafts at various levels in layers V, IV, and III. The apical dendrites are smaller than the apical dendrites of the extra-large pyramidal cells in layer V. The basal dendrites and the collateral branches of the apical dendrites of the large ordinary and special pyramidal and spindle cells are coarser and longer than those of corresponding cells in this area in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. In the six-month cortex the pedunculated bulbs are not as numerous on the dendrites of the large ordinary and special pyramidal and spindle cells as on the dendrites of the extra-large pyramidal cells in layer V. The bulbs are not as numerous on the dendrites of the large ordinary and special pyramidal and spindle cells in this area as those on the dendrites of corresponding cells in area FCBm. The large size of some of the pyramidal cells in layer III is especially evident in the Golgi-Cox preparations. The largest cells in this layer are located in lamina IIIc. Proceeding externally in layer III the size of the large cells decreases slightly and gradually. The largest pyramidal cells in lamina IIIc are actually larger than the smallest extra-large pyramidal cells in layer V, the former measuring 24/18)11 and the latter measuring 2 2/16ft. The apical dendrites of the pyramidal cells in layer I I I are larger and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical

LOBUS FRONTALIS dendrites are coarser and longer, but no more numerous than those of corresponding cells in the cortex of the three-month infant. In the six-month cortex the apical dendrites of the largest pyramidal cells in lamina IIIc are almost as large as the apical dendrites of the extra-large pyramidal cells in layer V. There is very slight, if any, difference in the size and length of the dendrites of the pyramidal cells in this area and those of corresponding cells in area FCBm. Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer I I I than on the corresponding dendrites of corresponding cells in this area in the three-month cortex. The bulbs are more numerous on the apical dendrites than on the basal dendrites or on the collateral branches of the apical dendrites. The bulbs are most numerous on the dendrites of the large pyramidal cells in lamina IIIc, and gradually decrease in number as the cells become smaller in laminae I l l b and I l i a . The bulbs are present in least quantity on the terminal branches of the apical dendrites of the pyramidal cells in lamina I l i a , but even here the bulbs are fairly numerous. The pedunculated bulbs are present on the dendrites of the pyramidal cells in layer I I I in this area in about the same quantity as on the dendrites of the pyramidal cells of corresponding location and size in layer I I I in area FCBm. The longest axons of the largest pyramidal cells in lamina IIIc descend vertically and can be traced into layer VI. These axons bear a few very small droplets scattered at wide intervals along the surface. The axons of other pyramidal cells in layer I I I are smaller and shorter than those of the large cells in lamina IIIc and do not bear any evidences of myelin. The dendrites of the small pyramidal cells belonging to layer IV are coarser and longer than those of corresponding cells in this area in the three-month cortex. The dendrites of the small pyramidal and granular cells in layer I I are larger and longer than those of corresponding cells in this area in the threemonth cortex, but they are smaller and shorter than the dendrites of the small pyramidal cells of layer IV in the six-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells of layer IV than on the dendrites of the small pyramidal and granular cells of layer II. The bulbs are more numerous on the dendrites of the cells of layers I I and

49

IV than on the dendrites of corresponding cells in the cortex of the three-month infant. A few bulbs are present on the terminal branches of the apical dendrites of the small pyramidal and granular cells in layer II. No horizontal cells of Cajal have been observed in layer I. No branches have been observed on any tangential fibers. Golgi type I I cells are present in all layers of the cortex in this area, and their dendrites are in approximately the same state of development as the dendrites of pyramidal cells of corresponding size and location. The mesh of fibers formed by the axons of the Golgi type I I cells is of about the same density in this area as that in area FCBm. In area F D r , however, the mesh is more dense in layer IV than in laminae Vb and Via. The axons of Golgi type I I cells are not sufficiently numerous to form a mesh in layers I I or III. The vertical exogenous fibers do not contribute any fibers to the mesh. The vertical fibers end as single shafts and have no collateral or terminal branches. The tangential fibers in the walls deep in sulci are larger and more darkly stained than those in the crown of the gyrus. The fibers are smaller and more lightly stained than the tangential fibers in the sulci in the opercular part of gyrus frontalis inferior, area FCBm. Deep in the walls of a sulcus in the middle of the triangular part of gyrus frontalis inferior the tangential fibers number 12 to 14 to the unit. They are small, intermediate, and large in size. The fibers are more numerous, larger, and more darkly stained in the middle of the layer than in the outer or inner portions. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure 0.9/x in diameter between varicosities. No horizontal fibers are present in layer II. In lamina I l i a horizontal fibers number 1 or 2 to the unit in the walls of sulci. They are very small and lightly stained. Horizontal exogenous fibers in lamina I l l b number 3 or 4 to the unit, and they are larger and more darkly stained than those in lamina I l i a . The outer band of Baillarger in lamina IIIc and layer IV contains 10 to 12 horizontal exogenous fibers to the unit. These fibers are larger and more darkly stained than the horizontal fibers in lamina

so

THE CORTEX OF THE SIX-MONTH INFANT

I l l b . In layer Va the horizontal fibers number 8 to io, and in the inner band of Baillarger in laminae Vb and V i a they number 12 to 14 to the unit. In lamina VIb the horizontal exogenous fibers number 10 to 12 to the unit. The horizontal fibers in layer V I are larger and more darkly stained than those in layer V. The subcortical association fibers number 16 to 18 to the unit. In the Golgi preparations all the horizontal exogenous fibers and subcortical association fibers bear varicosities, spines, and thorns. No branches have been observed on any of these fibers. The largest horizontal exogenous fibers in lamina VIb and the largest subcortical association fibers measure ι.ομ in diameter between varicosities. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere. Here they number 30 to 35 to the unit. The fibers are small, intermediate, and large in size. They are smaller and more lightly stained than the vertical fibers in the core of the opercular part of gyrus frontalis inferior, area FCBm. The vertical fibers are chiefly collected in bundles as they radiate toward the cortex. Most of the radiating fibers end in the mid-region of lamina VIb, some end in the neuropil in the inner band of Baillarger, and a few end in layer IV. In the lower part of lamina VIb the radiating vertical exogenous fibers number 12 to 14 to the unit. In the lower part of lamina Vb they number 6 to 8, and in the lower part of layer IV they average 3 or 4 to the unit. A vertical fiber is occasionally seen in layer III. In the Golgi preparations all the vertical exogenous fibers bear varicosities, spines, and thorns. No branches, either collateral or terminal, have been observed on any vertical exogenous fibers. The largest fibers in the core of the gyrus measure τ.ομ in diameter between varicosities. The vertical fibers decrease in size as they ascend in the cortex, and all end eventually as small, straight fibers. Using the darkness and extent of the stain in the Weigert sections as a criterion for the amount of myelination, the brains examined fall into the following sequence, beginning with the one whose MYELiNATiON.

sections are the darkest: (1) 37-82, (2) 41-91, (3) 38-35, (4) 37-49· The sections of brain 37-82 are but slightly darker than those of brain 41-91, and there is not much difference between the sections of brain 41-91 and the sections of brain 38-35. The sections of brain 37-49 are distinctly lighter than those of brain 38-35. The stain in the sections of brain 37-49 is much darker and more extensive than that in the sections of brain 34-188, which is the three-month brain having the most myelin. In each of the six-month brains the stain in the vertical fibers in the core of the gyrus is slightly lighter than that in the sections of the corresponding brain in the posterior region of gyrus frontalis inferior, but it is slightly darker than that in the sections in the middle region of gyrus frontalis medius. The following description is based upon the sections of brain 37-82. Small, short, lightly stained tangential fibers are scattered at wide intervals along layer I in the walls of sulci, but no stained fibers are present in this layer in the crown of the gyrus. No stained horizontal fibers are present in the cortex external to layer VI. Very short, small, lightly stained horizontal fibers are scattered at wide intervals in lamina Via. In lamina VIb stained horizontal fibers number 2 or 3 to the unit, and they are very light in color. The stained subcortical association fibers are larger and darker than the stained horizontal fibers in lamina VIb. They number 4 or 5 to the unit. The stained vertical fibers are largest, darkest, and most numerous in the core of the gyrus. Where they are most numerous in the middle of the core of the gyrus the stained vertical fibers number 20 to 25 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1 in diameter. As the vertical fibers radiate toward and into the cortex they gradually decrease in size and become lighter in color. The stain on most of the fibers ends in the lowermost part of lamina VIb, on a few fibers the stain ends in the middle of layer VI, and occasionally a stained vertical fiber is seen in lamina V i a .

LOBUS

FRONTALIS

51

ANTERIOR REGION OF THE FRONTAL GYRI THE CORTEX in the anterior part of the three frontal gyri has the same general characteristics as in the three-month brain. The outer and inner granular layers are more prominent than they are in the middle region of the frontal gyri. GYRUS FRONTALIS SUPERIOR, AREA FDm The cortex in the crown of the gyrus on the superolateral surface measures 2.135 mm. in width, and is illustrated in Figures 45, 46, 47, and 48. CELL-BODIES

Layer I. This layer measures 0.215 mm. in width, and contains 52 neurons to the unit. Large cells of Cajal are seen occasionally. Layer II. The external granular layer is distinctly visible macroscopically. The layer measures 0.093 mm. in width and contains 290 neurons to the unit. The cells range in size from 4/4^ to 8/8/1. Layer III. This layer measures 0.688 mm. in width. The neurons in lamina I l i a number 57 to the unit and measure 10/6-23/io/t. In lamina I l l b the neurons number 52 to the unit and measure 10/626/12/1. In lamina IIIc the neurons number 52 to the unit and range in size from ιο/6/ι. to 33/13/1. Layer IV. The internal granular layer is more prominent than the external one. The neurons number 141 to the unit and measure 5/4-12/10/1. Only a few neurons of laminae IIIc and V a invade layer IV. The layer measures 0.204 mm. in width. Layer V. This layer measures 0.443 m m · i n width. The small neurons number 55 to the unit in lamina Va and 50 in lamina Vb. The large neurons number 20 to the unit in both Va and Vb. The extra-large pyramidal cells number 14 to the unit. The small neurons measure 10/6-15/10//., the large ones measure 18/10-26/12/4, and the extra-large pyramidal cells range in size from 29/12/1 to 53/13/4. Layer VI. The layer of polymorphic cells measures 0.492 mm. in width. The neurons number 80 to the unit in lamina V i a and 55 in lamina VIb. The pyramidal cells measure 10/6-26/13/1, and the spindle cells measure 16/6-29/10/1. The extra-large pyramidal cells in layer V contain

more chromophil substance and are stained more darkly than any other cells in the cortex in this area. The contrast in density of stain between these cells and large ordinary pyramidal cells in layers V, VI, and I I I is not great. The chromophil substance is in the form of granules, clumps, and a few Nissl flakes. The clumps are principally large in size. In almost every cell a large cone-shaped clump rests upon the nuclear membrane immediately under the base of the apical dendrite. Granules and elongated strands of chromophil substance extend into the proximal end of the apical dendrite of each extra-large pyramidal cell. A dark clump of chromophil substance indicates the points of origin of the most proximal three or four collaterals of each apical dendrite, but the collaterals are not stained. A very few granules are present in the proximal ends of the larger basal dendrites. The chromophil substance in the large ordinary pyramidal cells in layers V and V I and in the large spindle cells in V I is chiefly in the form of large clumps, but some small clumps and a few Nissl flakes are present. The largest pyramidal cells in laminae IIIc and I l l b are stained almost as darkly as the extra-large pyramidal cells in layer V. Large clumps of chromophil substance are more common in these cells than in the extra-large pyramidal cells. Small clumps and a few Nissl flakes are present. Chromophil substance in the apical dendrites is less in quantity and less extensive in distribution than in the apical dendrites of the extra-large pyramidal cells in layer V. The chromophil substance in all cells is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the three-month cortex. In the six-month cortex large clumps of chromophil substance predominate in number in the cells in this area, whereas small clumps are more numerous than large ones in the cells in the middle region of gyrus frontalis superior. This difference is interpreted as indicating that the chromophil substance is less advanced in differentiation in cells in the anterior region of this gyrus than it is in corresponding cells in the middle region.

52

THE CORTEX OF THE SIX-MONTH INFANT

The cytoplasm in all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in this area are in about the same state of development as to size and length as those of the corresponding cells in the middle region of gyrus frontalis superior. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than they are on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the extra-large pyramidal cells in this area than on the dendrites of corresponding cells in the middle region of gyrus frontalis superior. The axons of the extra-large pyramidal cells in layer V bear only slightly more myelin than those of corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the axons of these cells have less myelin than the axons of corresponding cells in the middle region of gyrus frontalis superior. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are coarser, longer, and more compact in structure than those of corresponding cells in this area in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corresponding cells in the three-month cortex. In the six-month cortex the bulbs are less numerous on the dendrites of these cells in this area than on the dendrites of corresponding cells in the middle region of gyrus frontalis superior. The apical dendrites of the large pyramidal cells in laminae IIIc, I l l b , and I l i a are larger and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the

apical dendrites are coarser and longer than those of corresponding cells in this area in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of all the pyramidal cells in layer I I I than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the pyramidal cells in layer I I I than on corresponding cells in the middle region of gyrus frontalis superior. The axons of all the pyramidal cells in layer I I I decrease in size as they descend. A few small, widely scattered droplets on the axons of the large pyramidal cells in lamina I I I c are the only indications of the presence of myelin. The dendrites of the small pyramidal cells belonging to layer IV are coarser and longer than the dendrites of corresponding cells in this area in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites than in the three-month cortex, but they are few in number. The dendrites of the small cells belonging to layer I I are coarser and longer and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The bulbs are scattered at wide intervals on the dendrites, and are less numerous than those on the dendrites of the small pyramidal cells of layer IV in this area. The dendrites of Golgi type I I cells in this area are in about the same state of development as to size, length, and quantity of pedunculated bulbs as the dendrites of pyramidal cells of comparable size in corresponding location in the cortex. The axons of the Golgi type I I cells form a mesh of fibers which is of maximum density in the outer band of Baillarger in lamina I I I c and layer IV. The mesh is thinner in laminae Vb and Via, the inner band of Baillarger. It decreases in density in laminae VIb and IIIc, is very thin in lamina I l l b , and is absent in lamina I l i a and in layer II. The mesh of axons of the Golgi type I I cells is more dense than that in this area in the cortex of the three-month infant. In the six-month cortex the mesh in this area is not as dense as that in the middle region of gyrus frontalis superior. The mesh does not receive any contribution from the vertical exogenous fibers. The latter end as single shafts, and have no terminal or collateral branches. A horizontal cell of Cajal is occasionally observed

LOBUS FRONTALIS in layer I. Some of these cells have four or five branches which extend horizontally in layer I, and some of these branches subdivide into two branches which also extend horizontally in the molecular layer. No collateral branches have been observed on any of the tangential fibers. The tangential fibers in layer I are more numerous, larger, and more darkly stained in the walls deep within sulci than elsewhere. They are not as large or as darkly stained as corresponding fibers in the midregion of gyrus frontalis superior. Deep within the sulci the tangential fibers number 12 to 14 to the unit. They are collected chiefly in the middle of layer I. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. They are small, intermediate, and large in size. The largest fibers measure 0.9/i in diameter between varicosities. Single, widely scattered horizontal exogenous fibers are present in layer II. The horizontal fibers number 1 or 2 to the unit in lamina I l i a and 3 or 4 in lamina I l l b . In the outer band of Baillarger in lamina IIIc and layer I V the horizontal exogenous fibers number 10 to 12 to the unit, and they are larger and more darkly stained than the horizontal fibers in lamina I l l b . In lamina Va the horizontal exogenous fibers number 8 to 10 to the unit, and in the inner band of Baillarger in laminae V b and V i a these fibers number 12 to 14. These fibers number 10 to 12 to the unit in lamina VIb. The horizontal fibers in layer V I are larger than those in layer IV. The subcortical association fibers number 12 to 14 to the unit, and they are about the same size as the horizontal exogenous fibers in lamina VIb. In the Golgi preparations all the horizontal exogenous fibers and subcortical association fibers bear varicosities, spines, and thorns. N o branches have been observed on any of these fibers. The largest horizontal exogenous fibers in lamina V I b and the largest subcortical association fibers measure 0.9/u. in diameter between varicosities. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere. Here they number 30 to 35 to the unit. All the fibers in the Golgi preparations bear varicosities, spines, and thorns. The largest fibers measure ι.ομ between varicosities. As the vertical fibers radiate toward the cortex they are chiefly collected in bundles. Most of the radiating fibers end in the middle of layer V I ,

S3

some end in lamina Vb, and a few ascend to layer IV. In the lower part of lamina V I b the vertical exogenous fibers number 12 to 14 to the unit. In the lower part of lamina V b the radiating fibers number 6 to 8, and in the lower part of layer IV, 3 or 4 to the unit. All the ascending vertical exogenous fibers bear varicosities and can thereby be distinguished from ascending endogenous axons. All the radiating vertical fibers decrease in size as they ascend, and are smallest at their terminations, but even here they fall into three categories of size, small, intermediate, and large. N o branches, either collateral or terminal, have been observed on any radiating vertical exogenous fibers. MYELiNATiON. With reference to the amount of myelin as revealed in the Weigert sections the brains examined fall into the following sequence: ( 1 ) 37-82, (2) 38-44, (3) 38-35, (4) 37-49· The stain in the sections of brain 37-82 is much darker than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is distinctly darker and more extensive than that in brain 34-188, the three-month brain with the most myelin in this area. The stain in the sections of each of the six-month brains is distinctly lighter and less extensive than the stain in the sections of the corresponding brain in the middle region of gyrus frontalis superior and in the middle region of gyrus frontalis inferior. The following description is based upon the sections of brain 37-82. No stained tangential fibers are present in layer I. No stained horizontal fibers are present in the cortex external to layer V I . A small, short, lightly stained horizontal fiber is occasionally seen in lamina V i a . In lamina V I b the stained horizontal fibers number 2 or 3 to the unit, and they are larger, longer, and more darkly stained than those in lamina V i a . The stained subcortical association fibers are larger, darker, and more numerous than the stained horizontal fibers in lamina VIb. They number 5 or 6 to the unit, and are small, intermediate, and large in size. The stained vertical fibers are largest, darkest, and most numerous in the core of the gyrus. In the center of the core the stained vertical fibers number 20 to 25 to the unit. As the vertical fibers radiate toward and into the cortex they decrease in size and number, and become lighter in color. The stain ends on most of

T H E CORTEX OF THE SIX-MONTH INFANT

54

the fibers among the subcortical association fibers immediately under the lower boundary of lamina VIb. On a few vertical fibers the stain ends in the middle of lamina VIb, and a stained vertical fiber is seen occasionally ending in lamina V i a . The stained vertical fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure τ.ΐμ. GYRUS FRONTALIS MEDIUS, AREA FDA The cortex in the crown of the gyrus measures 2.048 mm. in width. It is illustrated in Figures 49, 50, 51, and 52. The two granular layers are not any more prominent than those in the anterior part of the superior and inferior frontal gyri or in the frontal pole. CELL-BODIES

Layer I. This layer measures 0.213 mm. in width, and contains 59 neurons to the unit. Large cells of Cajal are seen occasionally. Layer II. The external granular layer measures 0.097 mm. in width. The neurons number 308 to the unit and measure 4/4-9/8/*. Layer III. This layer measures 0.673 mm. in width. In lamina I l i a the neurons number 58 to the unit and measure 10/6-23/10/j,. In lamina I l l b the neurons number 54 per unit and measure 10/6-26/12^. The neurons number 53 to the unit in lamina IIIc and range in size from το/6μ to 33/13/n. Layer I I I does not appear to be any more granular in this region than in the anterior part of the superior and inferior frontal gyri. Layer IV. The internal granular layer is more prominent macroscopically than the external one, even though the cells are fewer per unit. The layer measures 0.219 mm. in width and contains 150 neurons to the unit. Very few neurons of laminae IIIc and V a invade the granular layer. The neurons range in size from 5/4/j, to ιο/6μ. Layer V. The internal pyramidal layer measures 0.377 mm. in width. Small and large neurons are scattered throughout the layer, numbering 55 and 20, respectively, in lamina Va, and 45 and 19 in lamina Vb. The small neurons measure 1 0 / 6 - 1 3 / ι ο μ , and the large ones measure 16/10-26/13^. The extralarge pyramidal cells are located chiefly in lamina Vb,

but some are present in lamina Va also. These neurons number 12 to the unit, and range in size from 29/Ι2/Λ to 36/13/A. Layer VI. This layer measures 0.469 mm. in width. The neurons number 80 to the unit in lamina V i a and 55 per unit in lamina VIb. The pyramidal cells measure 10/6-23/12/X, and the spindle cells measure 16/6-29/IOJU.

The contrast between the extra-large pyramidal cells in layer V and large ordinary pyramidal cells in layers V, V I , and I I I as to size, amount of chromophil substance, and density of stain is much less in this area than in any area in the middle and posterior regions of the frontal gyri. The largest of the ordinary pyramidal cells in layers V and V I and in lamina IIIc are comparable in all these respects with the extra-large cells in V. Small clumps of chromophil substance and Nissl flakes predominate in the extralarge pyramidal cells, while large clumps and few Nissl flakes characterize the large ordinary pyramidal cells in layers V, V I , and III. The chromophil substance in the small cells in layers II, III, and I V is scant in quantity and is chiefly in the form of small, darkly stained beads which are clinging to the external surface of the nuclear membrane. A few granules and beads invade the proximal end of the apical dendrites. The chromophil substance is greater in quantity and more advanced in differentiation in all cells than in corresponding cells in this area in the three-month cortex. In the six-month cortex the chromophil substance in the cells in this area is less advanced in development than that in corresponding cells in the middle region of gyrus frontalis medius. In the Cajal sections the cytoplasm in all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer than those of the extra-large pyramidal cells in layer V in this area in the cortex of the threemonth infant.

FIBERS.

LOBUS FRONTALIS Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs are more numerous on the dendrites of the extra-large pyramidal cells than those on the dendrites of any other cells in this area. Pedunculated bulbs are less numerous on the dendrites of the extra-large pyramidal cells than on the dendrites of corresponding cells in the mid-region of gyrus frontalis medius. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area of the three-month cortex. The axons of these cells are larger and have more myelin than those of any other cells in this area. The apical dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are much more slender than those of the extra-large pyramidal cells in layer V. The apical and basal dendrites of the large ordinary and special pyramidal and spindle cells are coarser and more compact in structure than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than they are on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs are less numerous on the dendrites of these cells than they are on the dendrites of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal cells in layers V and VI are coarser than those of corresponding cells in this area of the three-month cortex. The axons of these cells are smaller and have less myelin than those of the extra-large pyramidal cells in layer V. The apical dendrites of the pyramidal cells in layer I I I are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and more compact in structure, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer I I I than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the bulbs on the

55

largest pyramidal cells in layer I I I are less numerous than those on the dendrites of the large ordinary and special pyramidal cells in layers V and VI. The dendrites of the pyramidal cells in layer I I I are smaller and have fewer pedunculated bulbs than those of corresponding cells in the mid-region of gyrus frontalis medius. The axons of the pyramidal cells in layer I I I are coarser than those of corresponding cells in this area in the three-month brain. The axons of the large pyramidal cells in laminae I l i a and I l l b bear no indications of myelin. A few small globules widely scattered along the axons of the large pyramidal cells in lamina IIIc probably indicate the presence of a small amount of myelin. The dendrites of the small pyramidal cells of layer IV are coarser, longer, and have more pedunculated bulbs than those of the small pyramidal and granular cells of layer II. The dendrites of the cells of both the granular layers are larger, longer, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the Golgi type I I cells in all layers of the cortex in this area are in about the same state of development as the basal dendrites of pyramidal cells of comparable size. The mesh of fibers formed by the axons of the Golgi type I I cells is of approximately the same density as that in this area in the three-month cortex. The mesh of fibers receives no contributions from the vertical exogenous fibers. No terminal or collateral branches have been observed on any of these fibers. No horizontal cells of Cajal have been observed in layer I in this area. A very short branch is occasionally seen on a tangential fiber in the molecular layer, but no long branches have been observed. The tangential fibers in layer I are most numerous, largest, and most darkly stained in the walls deep in sulci. Here they number 10 to 12 to the unit. They are smaller and more lightly stained than corresponding fibers in the mid-region of gyrus frontalis medius. The fibers are much more numerous in the middle of layer I than in the outer or inner margins. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure 0.75/x in diameter between varicosities. No branches have been observed on any tangential fibers. Horizontal exogenous fibers are more numerous,

56

THE CORTEX OF THE SIX-MONTH INFANT

larger, and more darkly stained in the walls of sulci than in the crown of the gyrus. In the walls there are no horizontal fibers in layer II or in lamina I l i a . In the lower part of lamina I l l b horizontal exogenous fibers number 2 or 3 to the unit. In layer IV they number 6 to 8 to the unit, and they are larger and darker than those in I l l b . In lamina Va the horizontal fibers average 5 to 6, and in the inner layer of Baillarger they number 6 to 8 to the unit. These fibers in lamina VIb number 6 to 8 to the unit. The subcortical association fibers number 8 to 10 to the unit. The horizontal exogenous fibers and the subcortical association fibers are small, intermediate, and large in size. The fibers in layer VI and the subcortical association fibers are larger than those in layer IV. In the Golgi preparations all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest horizontal fibers in lamina VIb and the largest subcortical association fibers measure 0.75/x between varicosities. No branches have been observed on any horizontal exogenous or subcortical association fibers. The vertical exogenous fibers are most numerous in the core of the gyrus, and number 2 5 to 30 to the unit. As the vertical fibers radiate toward the cortex they collect in bundles. Most of the radiating fibers end in the middle of layer VI, some end in lamina Vb, and a few ascend to layer IV. The radiating exogenous fibers number 14 to 16 to the unit in the lower part of lamina VIb, 8 to 10 in lamina Vb, and 5 or 6 to the unit in the lower part of layer IV. Single vertical exogenous fibers are occasionally seen in the lower part of layer III. In the Golgi preparations all the vertical exogenous fibers bear varicosities, spines, and thorns. They are small, intermediate, and large in size. The largest fibers in the core of the gyrus measure 0.75/01 in diameter between varicosities. The radiating fibers decrease in size as they ascend in the cortex. Each bundle of radiating fibers usually contains small, intermediate, and large fibers. No collateral or terminal branches have been observed on any of the vertical exogenous fibers. MYELiNATioN. Using the depth and extent of the stain in the Weigert sections as a measure of the amount of myelin, the brains examined fall into the following sequence, beginning with the one having

the most myelin: (1) 41-91, (2) 37-82, (3) 38-35, (4) 38-44, (5) 37-49. The stain in the sections of brain 37-49 is much lighter and less extensive than that in the sections of brain 41-91. The stain in the sections of the brains in this area is lighter than that in the sections in the anterior region of gyrus frontalis superior and in the middle region of gyrus frontalis medius. The stain in the sections of brain 37-49 is much darker and more extensive than that in the sections of brain 38-128, which is the threemonth brain having the most myelin in this area. The following description is based upon the sections of brain 41-91. No stained tangential fibers are present in layer I. No stained horizontal fibers are present anywhere in the cortex. Stained subcortical association fibers number 4 to 6 to the unit. They are small and lightly stained, but the three categories of size, small, intermediate, and large, can be identified. Stained vertical exogenous fibers are most numerous in the core of the gyrus, where they number 20 to 25 to the unit. The stained fibers are small, intermediate, and large in size, but they are smaller and more lightly stained than the corresponding fibers in the anterior region of gyrus frontalis superior or in the mid-region of gyrus frontalis medius. The stained vertical fibers gradually decrease in size and number, and become lighter as they radiate toward and ascend within the cortex. The stain on most of the fibers ends just before they reach the lower margin of lamina VIb. The stain continues on a few fibers as far as the middle of lamina VIb, and occasionally a stained vertical fiber reaches lamina V i a . All the stained vertical fibers bear varicosities, and the varicosities are more prominent than those on the stained vertical fibers in the middle part of this gyrus. The largest stained vertical fibers measure ι.τμ in diameter between varicosities. GYRUS FRONTALIS INFERIOR, AREA FDp The cortex in the crown of the gyrus on the lateral surface measures 1.967 mm. in width. It is illustrated in Figures 53, 54, 55, and 56. CELL-BODIES

Layer I. This layer measures 0.213 mm. in width, and contains 54 neurons to the unit. Layer II. The outer granular layer measures 0.107

LOBUS FRONTALIS mm. in width. The neurons number 296 to the unit and measure 4/4-8/8/*. The small cells of this layer mingle with the larger cells of lamina I l i a to a lesser degree than in the middle region of this gyrus. Layer III. This layer measures 0.603 m m · in width. The neurons number 58 to the unit in lamina I l i a and measure 10/6-23/10/1. In lamina I l l b the cells number 51 per unit and they measure 10/6-26/13/1. In lamina IIIc there are 53 neurons to the unit, and they range in size from ιο/6μ to 33/13/Λ. Layer IV. The inner granular layer is more prominent than the outer one. The layer measures 0.205 mm. in width. The neurons number 133 to the unit and measure 5/4-10/6/1. Very few neurons of laminae IIIc and Va invade the inner granular layer. Layer V. This layer measures 0.393 m m · width. Small neurons number 55 to the unit in lamina Va and 50 in lamina Vb. Large neurons number 20 per unit in Va and 18 in Vb. The extra-large pyramidal cells number 10 to the unit, and are located chiefly in lamina Vb, but some are present in lamina Va also. The small neurons measure 10/6-13/ιομ, the large ones measure 15/10-23/12111, and the extra-large pyramidal cells range in size from 26/12/1 to 33/13/1. Layer VI. This layer measures 0.446 mm. in width. The neurons number 80 to the unit in lamina V i a and 55 per unit in lamina VIb. The pyramidal cells measure 10/6-2 6/12 μ, and the spindle cells range in size from ι6/6μίο 26/6μ. The extra-large pyramidal cells in layer V and the largest cells in laminae I l l b and IIIc are approximately similar in regard to quantity of chromophil substance and depth of stain. These cells are stained more darkly than any other cells in this area. The chromophil substance in the extra-large cells is chiefly in the form of granules and small clumps, with a few Nissl flakes. In the large cells in laminae I l l b and IIIc large clumps predominate, whereas small clumps and Nissl flakes are few in number. Proceeding externally in layer III the chromophil substance decreases in quantity with decrease in size of the cells. In the smallest cells in layer II the chromophil substance is in the form of dark beads clinging to the external surface of the nuclear membrane. In all cells the chromophil substance is greater in quantity and more advanced in differentiation than

57

in corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the chromophil substance in the cells in this area is less advanced in differentiation than that in corresponding cells in the middle region of gyrus frontalis inferior. The cytoplasm in all cells in this area in the Cajal sections is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than on the dendrites of these cells in this area in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in this area are larger and have more pedunculated bulbs than those of corresponding cells in area FDA. In area F D p the dendrites of the extra-large pyramidal cells in layer V are larger, longer, and have more pedunculated bulbs than those of any other cells. On all pyramidal cells in this area pedunculated bulbs are more numerous on the apical dendrites than on their collateral branches or on the basal dendrites. The dendrites of the extralarge pyramidal cells in this area are smaller and have fewer pedunculated bulbs than those of corresponding cells in the mid-region of gyrus frontalis inferior, area F D r . FIBERS.

The axons of the extra-large pyramidal cells are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The axons of these cells decrease in size as they descend, but small globules are more numerous on their surfaces than in the three-month cortex. From one to three collaterals are present on some of the axons. The collaterals are very slender, but are larger than those on the axons of corresponding cells in this area of the three-month cortex. No indications of myelin are present on the collaterals.

58

THE CORTEX OF THE SIX-MONTH INFANT

The apical and basal dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger and more compact in structure than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the apical dendrites of these cells are much more slender than those of the extra-large pyramidal cells in layer V. Pedunculated bulbs are more numerous on the dendrites of the large ordinary and special pyramidal and spindle cells than those on the dendrites of corresponding cells in this area in the three-month cortex. The bulbs are more numerous on the apical dendrites than on the basal dendrites or on the collaterals of the apical dendrites. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI are much more slender than those of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special cells are larger than those of corresponding cells in this area in the three-month cortex. A few very small, widely scattered globules on the surfaces of the axons are the only indications of the presence of myelin. The apical dendrites of the pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer III than on the dendrites of corresponding cells in this area in the three-month cortex. The bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large pyramidal cells in lamina I l l b , and are more numerous on the latter cells than on the dendrites of the large pyramidal cells in lamina Ilia. The apical dendrites of the large pyramidal cells in lamina IIIc are larger, but have fewer pedunculated bulbs, than the apical dendrites of the large ordinary pyramidal cells in layer V. The dendrites of the pyramidal cells in layer III in this area are smaller and have fewer pedunculated bulbs than the dendrites of corresponding cells in the mid-region of gyrus frontalis inferior, area F D r .

The axons of the pyramidal cells in layer III are larger than those of corresponding cells in this area in the three-month cortex. The axons of the pyramidal cells in lamina I l i a are very small and bear no indications of myelin. The axons of the pyramidal cells in lamina I l l b are larger than those of the pyramidal cells in lamina I l i a , and bear a few small widely scattered globules on their surfaces. The axons of the large pyramidal cells in lamina IIIc are larger than those of the large pyramidal cells in lamina I l l b . They decrease in size as they descend. Small globules scattered along the surfaces of the axons are the only indications of myelin, and they are more numerous than those on the axons of the large pyramidal cells in lamina I l l b . The dendrites of the small pyramidal cells of layer IV are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the granular and small pyramidal cells of layer II are coarser, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the threemonth cortex. The dendrites of the small cells of layer II are not as large or as long as those of the small pyramidal cells of layer IV. The dendrites of the Golgi type II cells are about the same size and have approximately the same quantity of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size. The axons of these cells form a thin mesh of fibers in layers IV, V, and VI. The mesh is of about the same density as that in this area in the three-month cortex. No horizontal cells of Cajal have been observed in layer I. An inverted pyramidal cell is occasionally seen in layer VI. The tangential fibers are largest, darkest, and most numerous in the floors and walls of sulci. Here they number ι ο to 12 to the unit. The fibers are small, intermediate, and large in size. The largest fibers measures 0.75^ in diameter. No branches have been observed on any tangential fibers. Horizontal fibers are larger, darker, and more numerous in the cortex in the walls of sulci than in the crown of the gyrus. No horizontal fibers are present in the cortex external to lamina I l l b . In the lower part of this lamina the horizontal fibers number 2 or

LOBUS

FRONTALIS

3 to the unit. In lamina IIIc and layer IV (the outer band of Baillarger) the horizontal fibers number 6 to 8 to the unit, and they are larger and more darkly stained than those in lamina I l l b . In lamina Va the horizontal fibers number 5 or 6 to the unit, and in laminae Vb and V i a (the inner band of Baillarger) they number 6 to 8 to the unit. These are larger and darker than the fibers in lamina Va. In lamina VIb the horizontal fibers number 6 to 8 to the unit. The horizontal fibers are small, intermediate, and large in size, and all bear varicosities. The largest fibers in lamina VIb measure 0.75/x between varicosities. No branches have been observed on any horizontal fibers. The subcortical association fibers number 6 to 8 to the unit. They are about the same size as the horizontal fibers in lamina VIb. They are small, intermediate, and large in size, and all bear varicosities. The vertical exogenous fibers number 25 to 30 to the unit in the core of the gyrus. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 0.75/x in diameter between varicosities. As the vertical fibers radiate toward and into the cortex they decrease in size. Most of the ascending fibers end in the lower part of layer VI, some end in lamina Via, and occasionally a vertical fiber is seen ending in lamina Va. MYELINATioN. With reference to the depth and extent of the stain in the Weigert sections the brains fall into the following sequence: (1) 41-91, (2) 3 7 82, (3) 38-35, (4) 38-44, (5) 37-49· I " the first four brains the decrease in the depth of stain is small and gradual, but the stain in the sections of brain

59

3 7-49 is much lighter and less extensive than that in brain 38-44. The stain in the sections of brain 37-49 is much darker and more extensive than that in the sections of brain 38-128, the three-month brain having the most myelin. The stain in each of the sixmonth brains is slightly darker in color than that in the sections of the corresponding brain in the anterior region of gyrus frontalis medius, and it is only slightly lighter than that in the middle region of gyrus frontalis inferior. The following description is based upon the sections of brains 41—91 and 37-82. No stained tangential fibers are present in layer I, and no stained horizontal fibers are present in the cortex external to lamina Via. Small, short, lightly stained horizontal fibers are seen occasionally in this lamina. In lamina VIb the stained fibers are larger, longer, darker, and more numerous, averaging 2 or 3 to the unit. The stained subcortical association fibers are larger and darker than the stained horizontal fibers in lamina VIb. The stained association fibers number 4 to 6 to the unit. They are small, intermediate, and large in size. The stained vertical fibers are largest, darkest, and most numerous in the core of the gyrus. Here they number 20 to 25 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure ι.ΐμ in diameter. The stained vertical fibers decrease in size and number, and become lighter in color as they radiate toward and into the cortex. The stain on most of them ends in the region of the subcortical association fibers. The stain remains on a few fibers to the lower part of lamina VIb, and a stained vertical fiber is occasionally seen in the upper part of layer VI.

ORBITAL GYRI, AREA FF

RECENT experimental and clinical evidence indicates that the cortex on the orbital surface of the frontal lobe may represent a cortical center for the visceral nervous system. The cortex in the crowns of the orbital gyri measures 2.031 mm. in width. It is illustrated in Figures 57, 58, 59, and 60. CELL-BODIES

Layer I. This layer measures 0.217 mm. in width, and contains 49 neurons to the unit. Large horizontal cells of Cajal are seen occasionally.

Layer II. The external granular layer is visible macroscopically. It measures 0.089 m m · i n width. The neurons number 306 to the unit and measure 4/48/8/*. Layer III. This layer measures 0.618 mm. in width in the crowns of the gyri. In lamina I l i a the neurons number 55 to the unit and measure 10/6-23/10^. In lamina I l l b the neurons number 48 per unit and range in size from io/βμ to 26/12/Λ. The nerve cells in lamina IIIc number 50 to the unit and measure ι0/6-29/ι3μ.

60

THE CORTEX OF THE SIX-MONTH INFANT

Layer IV. The internal granular layer is more prominent than the external one. It measures 0.234 mm. in width and contains 129 neurons to the unit. The neurons are small, measuring 5/4-8/6/i. Very few large cells of laminae IIIc and Va invade the granular layer. Layer V. This layer measures 0.413 mm. in width. The neurons are small, large, and extra-large. The small and large nerve cells are distributed throughout the width of the layer, numbering 55 and 17, respectively, in lamina Va, and 50 and 14 in lamina Vb. The extra-large pyramidal cells are located chiefly in lamina Vb, but some are present in lamina Va. The extra-large neurons number 12 to the unit, and range in size from 26/12/* to 36/13/*. The small neurons measure 10/6-13/10/*, and the large ones measure 15/l 0-2 3/12/1. Layer VI. This layer measures 0.460 mm. in width. The neurons number 85 to the unit in lamina V i a and 50 to the unit in lamina VIb. The pyramidal cells measure 10/6-23/13/1, and the spindle cells measure ι6/6-26/1ομ. The extra-large pyramidal cells in layer V contain more chromophil substance than any other cells in this area, but they are stained about the same as the largest ordinary pyramidal cells in layers V, V I and III. The chromophil substance in all cells is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the threemonth cortex. In the six-month cortex the chromophil substance in the cells in this area is in about the same state of development as that in corresponding cells in the anterior region of gyrus frontalis inferior. The chromophil substance in the extra-large pyramidal cells in layer V is in the form of granules, Nissl flakes, and clumps, the latter being the most numerous. Small clumps are more numerous than large ones. In almost every cell a darkly stained cone of chromophil substance rests upon the nuclear membrane immediately under the base of the apical dendrite. The chromophil substance in the large pyramidal cells in layer III is in the same form as that in the extra-large pyramidal cells in layer V, except that large clumps predominate in number, and Nissl flakes are less numerous in the cells in layer III. Furthermore, the chromophil substance invades the apical dendrites in greater quantity and for a greater dis-

tance in the extra-large pyramidal cells than in the largest cells in layer III. In most of the cells in layer I I I a cone of chromophil substance is resting on the nuclear membrane immediately under the apical dendrite. The small cells in layer IV have more chromophil substance than those of comparable size in layer II. In all of the small cells in all layers the chromophil substance is in the form of darkly stained beads and small clumps. In the smallest cells these are adhering to the external surface of the nuclear membrane. In all cells the karyoplasm is stained much more lightly than the chromophil substance in the cytoplasm. The nucleolus in all cells, however, is stained as darkly as any of the clumps of chromophil substance. In the Cajal sections the cytoplasm in all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites of these cells are coarser, longer, and more compact in structure, but no more numerous, than in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. In the six-month cortex the dendrites of the extralarge pyramidal cells in layer V are larger and have more pedunculated bulbs than those of any other cells in this area. FIBERS.

The axons of the extra-large pyramidal cells in layer V decrease in size as they descend. A few small globules scattered along the surfaces of the axons are the only indications of the presence of myelin. The apical dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are much more slender than those of the extra-large pyramidal cells in layer V. The basal dendrites of these cells are smaller and shorter than those of the extra-large cells. All the dendrites of the large ordinary and special pyramidal and spindle cells have fewer pedunculated bulbs than the dendrites of the extra-large pyramidal cells. The dendrites of the

LOBUS FRONTALIS large ordinary and special pyramidal and spindle cells in layers V and VI are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The apical dendrites of the pyramidal cells in all three laminae of layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer III than in the three-month cortex. The bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large pyramidal cells in lamina I l l b , and are more numerous on the dendrites of the latter cells than on the dendrites of the pyramidal cells in lamina I l i a . Pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI. The axons of the large pyramidal cells in all three laminae of layer III are coarser than those of corresponding cells in this area in the three-month cortex. Only the axons of the large pyramidal cells in lamina IIIc bear any indications of myelin, and this is in the form of a few globules widely scattered along the surfaces of the axons. The axons of all the pyramidal cells in layer III decrease in size as they descend. The dendrites of the small pyramidal cells of layer IV are longer, larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the small pyramidal cells of layer IV are longer, larger, and have more pedunculated bulbs than those of the small pyramidal and granule cells belonging to layer II. The dendrites of the cells of layer II have very few pedunculated bulbs. In the six-month cortex the dendrites of the cells in area FF are smaller and have fewer pedunculated bulbs than those of corresponding cells in area FDp.

61

No horizontal cells of Cajal have been observed in layer I. The tangential fibers in layer I are larger and more darkly stained deep in the walls of sulci than in the crowns of gyri. In the walls of sulci they number 14 to 16 to the unit. They are located principally in the middle of layer I. In the Golgi preparations all the fibers bear varicosities, spines, and thorns. The fibers are small, intermediate, and large in size. The largest fibers measure 0.75/u in diameter between varicosities, but fibers of this size are few in number, most of them being smaller. No branches have been observed on any of the tangential fibers. No horizontal exogenous fibers are present in layer II. A solitary horizontal fiber is occasionally seen in lamina I l i a . In lamina I l l b horizontal exogenous fibers number 3 or 4 to the unit. In the outer band of Baillarger in lamina IIIc and layer IV the horizontal exogenous fibers number 6 to 8 to the unit. In lamina Va the horizontal fibers number 4 to 6 to the unit. The inner band of Baillarger in laminae Vb and V i a contains 8 to 10 horizontal exogenous fibers to the unit. In VIb the horizontal fibers number 8 to 10 to the unit. The subcortical association fibers number 10 to 12 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size. The horizontal fibers gradually increase in size and depth of stain proceeding inward from layer IV, and the fibers are largest in lamina VIb and in the layer of subcortical association fibers. In the Golgi preparations all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest fibers in lamina VIb and the largest subcortical association fibers measure 0.75/x in diameter between varicosities. Fibers of this size are more numerous in the subcortical layer than in lamina VIb. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere. Here they number 30 to 35 to the unit. They are small, intermediate, and large in size. All the vertical fibers bear varicosities, spines, and thorns. The largest fibers measure 0.75/i in diameter between varicosities. In ascending toward and into the cortex the radiating vertical fibers are chiefly collected in bundles. Usually each bundle contains small, intermediate, and large fibers. All the radiating fibers decrease in size

62

T H E CORTEX OF

THE SIX-MONTH

and number as they ascend in the cortex, and all end as straight, vertical fibers. Most of the radiating fibers end in the lower part of lamina VIb, some end in lamina Vb, and a few ascend as far as lamina Va. In the lower part of lamina VIb the vertical exogenous fibers number 12 to 14 to the unit, and in lamina Vb the vertical fibers number 4 to 6 to the unit. No branches, either collateral or terminal, have been observed on any vertical exogenous fiber. MYELINATION. The brains examined fall into the following sequence as to the amount of myelin as revealed by the Weigert sections: ( 1 ) 37-82, (2) 3835) (3) 37-49· The stain in the sections of brain 37-82 is distinctly darker than that in the sections of brain 38-35, and the stain in the sections of brain 38-35 is much darker than that in the sections of brain 37-49. The stain in the sections in this area of each of these brains is lighter than that in the sections in area F D p in the anterior region of the inferior frontal gyrus of the corresponding brain. The stain in the sections in area F F of brain 37-49 is much darker and more extensive than that in brain 38-128, the three-month brain having the most myelin in this area. The following description is based upon the sections of brain 37-82.

INFANT

No stained tangential fibers are present in layer I, and no stained horizontal fibers are present in the cortex external to lamina VIb. A very small, short, lightly stained horizontal fiber is occasionally seen in the upper part of lamina VIb. In the lower part of the lamina stained horizontal fibers number 2 or 3 to the unit. They are small and lightly stained. Stained subcortical association fibers number 3 or 4 to the unit. They are slightly larger and more darkly stained than the horizontal fibers in the lower part of lamina VIb. The stained vertical fibers are largest, darkest, and most numerous in the centers of the cores of the gyri. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1 .ομ in diameter between varicosities. In the center of the cores of gyri the stained vertical fibers number 20 to 25 to the unit of width. The stained vertical fibers decrease in size and number as they radiate toward and ascend in the cortex. The stain ends on most of the fibers immediately below the lower margin of lamina VIb. On a few fibers the stain ends in the lower part of lamina VIb, and occasionally a very small, lightly stained fiber is seen in the upper part of this lamina.

POLUS F R O N T A L I S , A R E A F E

THE CORTEX on the lateral surface of the polar region measures 2.013 mm. in width. It is illustrated in Figures 61, 62, 63, and 64. CELL-BODIES

Layer I. This layer measures 0.205 mm. in width, and contains 50 neurons to the unit. Horizontal cells of Cajal are seen occasionally. Layer II. This layer is visible macroscopically. It measures 0.093 mm. in width, and contains 317 neurons to the unit. The cells measure 4/4-8/8/*. Layer III. The external pyramidal layer measures 0.648 mm. in width. In lamina I l i a the neurons number 53 to the unit and measure 10/6-20/io/*. In lamina I l l b the neurons number 48 to the unit and measure 10/6-23/12/*. The nerve cells number 50 per unit in lamina IIIc, and range in size from 10/6/* to 26/15/*.

Layer IV. The inner granular layer is more prominent than the outer one. The layer measures 0.206 mm. in width. The neurons number 143 to the unit, and measure 5/4-8/6/*. Very few cells of laminae IIIc and Va invade the granular layer. Layer V. The internal pyramidal layer measures 0.369 mm. in width. Small and large neurons are present in all parts of the layer, numbering 55 and 16 to the unit, respectively, in lamina Va, and 50 and 15 in lamina Vb. The extra-large pyramidal cells are located chiefly in lamina Vb, but some are present in Va. They number 10 to the unit, and range in size from 26/12/* to 36/13/1. The small neurons measure 10/6-13/1 ομ, and the large ones measure 16/1023/12/*. Layer VI. This layer measures 0.492 mm. in width. The neurons number 85 per unit in lamina V i a and 55 in lamina VIb. The pyramidal cells measure

LOBUS FRONTALIS 10/6-26/13/*,, and the spindle cells measure 1 6 / 6 26/τομ. The chromophil substance is approximately the same in quantity and in depth of stain in the extralarge pyramidal cells in layer V, the large ordinary pyramidal cells in V, the largest pyramidal and spindle cells in layer V I , and the largest pyramidal cells in lamina IIIc. In all large cells in all layers the chromophil substance is chiefly in the form of large, darkly stained clumps. Small clumps and a few Nissl flakes are also present in the large cells. In the smallest cells in all layers the chromophil substance is in the form of small, darkly stained beads closely packed on the external surface of the nuclear membrane. The karyoplasm is stained more darkly in the small cells than it is in the large cells. Chromophil substance is present in greater quantity in the small cells in layer IV than in cells of comparable size in layer II. In all cells the chromophil substance is more advanced in differentiation than it is in corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the chromophil substance in the cells in this area is less advanced in differentiation than that in corresponding cells in the anterior region of the three frontal gyri. The cytoplasm in all cells in this area in the Cajal sections is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. FIBERS.

The axons of the extra-large pyramidal cells in layer V are larger than those of corresponding cells in this area in the three-month cortex. The axons decrease in size as they descend, and the only indications of myelin on them are some small globules widely scattered along their surfaces.

63

The apical dendrites of the large pyramidal cells in all three laminae of layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area of the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. The bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large pyramidal cells in lamina I l l b . The bulbs are less numerous on the dendrites of the large pyramidal cells in lamina I l i a than on the dendrites of the large pyramidal cells in lamina I l l b . The bulbs are less numerous on the dendrites of the pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI. The apical dendrites of the large pyramidal cells in lamina IIIc are larger than those of the large ordinary and special pyramidal cells in layer V. The axons of the pyramidal cells in layer I I I are larger than those of corresponding cells in this area in the three-month cortex. The axons of the large pyramidal cells in lamina IIIc are larger than those of the large pyramidal cells in lamina I l l b . The axons of large pyramidal cells in lamina I l i a are smaller than those of the large pyramidal cells in I l l b . Only the axons of the large pyramidal cells in lamina IIIc bear any indications of myelin, and this is in the form of a few small globules on the surface. The dendrites and axons of the cells in this area are in about the same state of development as those of the cells in the anterior regions of the three frontal gyriTangential fibers in layer I are larger, more numerous, and more darkly stained in the walls of sulci than elsewhere. Here they number 12 to 14 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure 0.75/n. No branches have been observed on any of the tangential fibers. N o horizontal exogenous fibers are present in layer II or in lamina I l i a . There are 3 or 4 horizontal fi-

64

THE CORTEX OF THE SIX-MONTH INFANT

bers to the unit in lamina I l l b . In the outer band of Baillarger in lamina IIIc and layer IV the horizontal exogenous fibers number 8 to ι ο to the unit. In lamina Va the horizontal exogenous fibers number 6 to 8 to the unit, and 8 to io in the inner band of Baillarger in laminae Vb and Via. The horizontal exogenous fibers in lamina VIb and the subcortical association fibers average 8 to ι ο to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size. The fibers of all these three categories gradually increase in size toward the inner boundary of the cortex. In the Golgi preparations all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. No branches have been observed. The largest horizontal exogenous and subcortical association fibers measure ο.ηζμ between varicosities. The vertical exogenous fibers are most numerous in the cores of the gyri. Here they number 30 to 35 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the vertical fibers bear varicosities, spines, and thorns. The largest fibers in the cores of gyri measure 0.75/i. in diameter between varicosities. As the vertical exogenous fibers radiate toward the cortex they are chiefly collected in bundles. The fibers decrease in number and in size as they ascend in the cortex. Most of the radiating vertical fibers end in lamina VIb, some end in lamina Vb, and a few end in lamina Va. In the lower part of lamina VIb the radiating fibers number 12 to 14 to the unit, and in the lower part of Vb they number 4 to 6 to the unit. Only occasionally does an ascending vertical exogenous fiber reach layer IV.

Neither collateral nor terminal branches have been observed on any vertical exogenous fiber. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: (1) 3 7-82, (2) 38-44, (3) 38-35, (4) 37-49· The stain in the sections of brain 37-49 is much lighter than that in the sections of brain 37-82, but is much darker and more extensive than that in the sections of brain 34188, the three-month brain with the most myelin in this area. The stain in the sections in area FE of each brain is distinctly lighter than that in areas F F and FDp of the corresponding brain, but is slightly darker than that in the sections in area FDA. The following description is based upon the sections of brain 37-82. No stained tangential fibers are present in layer I, and no stained horizontal fibers are present anywhere in the cortex. A few subcortical association fibers are stained, 3 or 4 to the unit of width. They are small and lightly stained, and all bear varicosities. MYELiNATiON.

Stained vertical fibers are largest, darkest, and most numerous in the center of the cores of the gyri. Here they number 20 to 25 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure ι.ομ in diameter between varicosities. As the stained vertical fibers radiate toward the cortex they decrease in size and number. The stain on most of the fibers ends among the subcortical association fibers below the inner boundary of lamina VIb. A few stained fibers end in the lower part of lamina VIb, and occasionally a small, lightly stained vertical fiber ends in the upper part of this lamina.

LOBUS

PARIETALIS

GYRUS CENTRALIS POSTERIOR AREA POSTCENTRALIS ORALIS GRANULOSA, PB THE AREA of koniocortex has the same characteristics and distribution in the six-month brain as it has in the cortex of the three-month infant. The region of the trunk, shoulder, arm, and forearm is selected as typical of area P B for the description of the koniocortex in the cresyl violet sections. Each functional region of area P B is described separately, however, for the state of development of the fibers. The cortex in the four functional regions in area P B is illustrated in Figures 65 to 80, inclusive. The cortex in the upper two-thirds of the anterior wall of gyrus centralis posterior in the region of the trunk, shoulder, and arm, measures 2.276 mm. in width. CELL-BODIES

Layer I. The molecular layer measures 0.255 mm. in width. The neurons number 50 to the unit. A large Cajal cell is seen occasionally. Layer II. The external granular layer is very prominent. The layer measures 0.121 mm. in width. The neurons number 280 to the unit and measure 4 / 4 8/4μ. The external margin of the layer is sharp, but internally the small cells of the granular layer invade lamina I l i a to such a degree that the internal boundary is indistinct. Layer III. The external pyramidal layer measures 0.746 mm. in width. Small neurons ranging in size from 4/4μ to 6/4/x are present in all three laminae. These small cells number 61 per unit in lamina I l i a , 51 in I l l b , and 55 in IIIc. In addition to these small cells there are 45 neurons to the unit measuring 8/612/ιομ in I l i a , 39 cells measuring 8/6-20/ιομ in I l l b , and 37 cells measuring 10/6-2 3/12 μ in lamina IIIc. Layer IV. The internal granular layer is more prominent than the outer one. The layer measures 0.241 mm. in width. The neurons number 150 to the unit and measure 6/4-8/6^. Only a few neurons of laminae I I I c and V a invade the granular layer. Layer V. The internal pyramidal layer measures 0.409 mm. in width. The small and large neurons are

distributed throughout the layer. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina V b also. The small cells measure 8 / 6 - 1 3 / ι ο μ , and number 60 to the unit in lamina Va and 48 per unit in Vb. The large neurons measure 16/10-26/13^, and number 16 to the unit in each lamina. The extra-large pyramidal cells range in size from 2$/12μ to 49/15/x, and number 8 to the unit. Layer VI. This layer measures 0.504 mm. in width. The neurons number 80 to the unit in lamina V i a and 39 in lamina VIb. The pyramidal cells measure 1 0 / 6 - 2 9 / ΐ 2 μ , and the spindle cells range in size from ι6/8μ to 26/τομ. The extra-large pyramidal cells in layer V have more chromophil substance and are stained more darkly than any other cells in this area. The chromophil substance is in the form of small and large clumps and Nissl bodies. The Nissl bodies are more numerous than the clumps. Elongated strands of chromophil substance and small Nissl bodies extend into the proximal part of the apical dendrite. The chromophil substance is greater in quantity and extends into the apical dendrites in greater quantity and for a greater distance than in the extra-large pyramidal cells in layer V in this area in the cortex of the three-month infant. A few granules and small clumps of chromophil substance extend for a short distance into the largest basal dendrites. Nissl bodies are more numerous in these cells in the six-month cortex than in the three-month brain. The chromophil substance is not as advanced in differentiation in the extra-large pyramidal cells in area P B as it is in the giant pyramidal cells of Betz in area F A y . Small and large darkly stained clumps of chromophil substance are more numerous and Nissl bodies are proportionately less numerous in the large ordinary pyramidal and spindle cells in layers V and V I than in the extra-large pyramidal cells in V. The large pyramidal cells in laminae IIIc and I l l b contain less chromophil substance in proportion to the size of the nucleus than the large ordinary pyram-

66

THE CORTEX OF THE SIX-MONTH INFANT

idal and spindle cells in layers V and V I . The chromophil substance in the large cells in IIIc and I l l b is chiefly in the form of clumps with only a few Nissl flakes. The clumps are small and large and are darkly stained, but the large cells are not conspicuously darker than other cells in layer I I I excepting the smallest ones. In all the large cells in layer I I I the chromophil substance is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the three-month cortex. The chromophil substance in the small cells in layers I V and I I and in the smallest cells in layers III, V, and V I is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the cortex of the three-month infant. In these small cells the chromophil substance is less in quantity in proportion to the size of the nucleus than in any other cells. The chromophil substance is in the form of darkly stained beads and small clumps which are clinging to the external surface of the thick, darkly stained nuclear membrane. The smallest cells in layer I I have less chromophil substance than any other cells, but more than the corresponding cells in this area in the three-month cortex. The karyoplasm of the small cells is not stained as darkly in the sixmonth cortex as it is in the three-month brain. Within area P B there is not much difference between the various functional regions in the state of development of the chromophil substance. The chromophil substance in the large cells in layers III, V, and V I in the region of the hand is more advanced in differentiation than it is in corresponding cells in any other functional region. Small clumps and Nissl flakes are more numerous and large clumps are less numerous in these cells in the region of the hand than in any of the other functional regions in area P B . The other regions fall into the following order: Region of the trunk and arm; region of the head; region of the lower extremity. The chromophil substance in the large ordinary pyramidal cells in layers III, V, and V I in all the functional regions in area P B is neither as great in quantity nor as advanced in differentiation as that in the corresponding cells in the corresponding functional regions in area F A y . A few neurofibrils are present in the extra-large pyramidal cells in layer V in all the various functional

regions of area P B . The neurofibrils are more numerous in these cells in the region of the shoulder and hand than in the regions of the lower extremity and head. The neurofibrils in the extra-large pyramidal cells in each region of area P B are much less in quantity and much less distinctly formed than those in the giant pyramidal cells in the corresponding regions in area F A y . In the six-month cortex the differentiation of neurofibrils in the extra-large pyramidal cells in area P B is more advanced than in corresponding cells in this area in the cortex of the three-month infant. In all other cells in each functional region in area P B the cytoplasm is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. REGION OF THE LOWER EXTREMITY The following description is of the cortex in area P B in the superior part of gyrus centralis posterior and in the adjoining portion of the paracentral lobule. It is illustrated in Figures 65, 66, 67, and 68. The apical dendrites of the extra-large pyramidal cells in layer V are larger and the basal dendrites are larger and longer than those of any other cells in this area of the cortex. The apical dendrites are larger and their terminal branches in layer I are more numerous and coarser than those of the extralarge pyramidal cells in layer V in this area of the cortex in the three-month infant. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those of corresponding cells in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in area P B are not as large as those of the giant pyramidal cells in the region of the lower extremity in area F A y . FIBERS.

Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells in layer V than on the dendrites of any other cells in this area. The bulbs are more numerous on the apical dendrites than on the basal dendrites or collaterals of the apical dendrites. The dendrites of these cells have more pedunculated bulbs than those of corresponding cells in this area of the three-month cortex. In the cortex of the six-month infant pedunculated bulbs are less numerous on the dendrites of the extra-large pyrami-

LOBUS PARIETALIS dal cells in this area than they are on the dendrites of the giant pyramidal cells in the region of the lower extremity in area FAy. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area of the three-month cortex. The collaterals of the axons are no more numerous, but are larger, longer, and have more secondary branches, than those in the three-month cortex. In the six-month cortex the axons of these cells are smaller and have less myelin than the axons of the giant pyramidal cells of Betz in the region of the lower extremity in area FAy. The large ordinary and special pyramidal cells in layers V and VI and the large spindle cells in layer VI are more advanced in development as to the length and coarseness of their dendrites than corresponding cells in this area of the three-month cortex. Pedunculated bulbs are less numerous on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI than on the dendrites of the extra-large pyramidal cells in layer V. The bulbs are more numerous on the dendrites of the large ordinary and special pyramidal and spindle cells than they are on the dendrites of corresponding cells in this area of the three-month cortex. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger and have more myelin than the axons of corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the axons of these cells are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The apical dendrites of the pyramidal cells in layer I I I are generally larger and have more terminal branches in layer I than the apical dendrites of corresponding cells in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and longer, but no more numerous, than those of corresponding cells in the cortex of the three-month infant. In the six-month cortex the apical dendrites of the largest pyramidal cells in lamina IIIc are almost as large as the single-stem apical dendrites of the extra-large pyramidal cells in layer V, and are actually larger than the subdivisions of apical dendrites. The apical dendrites of the large pyramidal cells in lamina IIIc are larger than the

67

apical dendrites of the large ordinary and special pyramidal cells in layers V and VI. The basal dendrites and collateral and terminal branches of the apical dendrites of the largest pyramidal cells in lamina IIIc are not as long or as large as those of the extra-large pyramidal cells in layer V. The size of the cell-bodies and the size and length of the processes of the pyramidal cells in layer I I I gradually decrease toward the external boundary of the layer and are smallest in lamina I l i a . Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer III than they are on the dendrites of corresponding cells in this area of the three-month cortex. In the six-month cortex pedunculated bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of any other cells in layer III. Proceeding toward the external boundary of the layer the pedunculated bulbs gradually decrease in quantity and are least numerous on the dendrites of the cells in lamina I l i a . The bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V, and are much less in quantity than on the dendrites of the extra-large pyramidal cells in layer V. The axons of the large pyramidal cells in lamina IIIc are much smaller and have much less myelin than those of the extra-large pyramidal cells in layer V. The axons of the large cells in lamina IIIc are larger and have more myelin than those of corresponding cells in this area in the cortex of the threemonth infant. The dendrites of the pyramidal cells of layer IV are coarser, longer, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the pyramidal and granular cells of layer I I are coarser and have more pedunculated bulbs than those of correspanding cells in the cortex of the three-month infant. In the six-month cortex the dendrites of the cells in layer II are smaller and shorter and have fewer pedunculated bulbs than those of the cells belonging to layer IV. No horizontal cells of Cajal have been observed in layer I. Golgi type II cells are scattered throughout the

68

THE CORTEX OF THE SIX-MONTH INFANT

cortex, but they are more numerous in layer IV than in any other layer. The axons of these cells form a dense mesh of small, medium, and large fibers in layer IV. The mesh does not appear to be any denser or wider than that in layer I V in the three-month cortex. The tangential fibers in layer I number 14 to 16 to the unit. They are chiefly accumulated in the external one-half of the layer. Tangential fibers are located in the internal one-half but they are much smaller, less numerous, and more lightly stained than those in the outer one-half. The fibers are small, intermediate, and large in size. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure about 1.8/1 in diameter between varicosities, but fibers as large as this are few in number. N o branches have been observed on any tangential fibers.

largest fibers measure τ.8μ in diameter between varicosities. As the vertical fibers radiate toward the cortex they are chiefly collected in bundles. Most of the radiating fibers end at about the middle of layer VI, many end in the inner band of Baillarger, some end in the neuropil in the outer band of Baillarger, and occasionally a vertical fiber is seen in lamina I l i a or in layer II. The vertical exogenous fibers number 12 to 14 to the unit in the lower part of lamina VIb, 8 to 10 in lamina Vb, and 4 to 6 in the lower part of layer IV. N o branches, either collateral or terminal, have been observed on any vertical exogenous fibers. MYELiNATiON. The four brains examined fall into the following sequence as to the depth and extent of the stain in the Weigert sections: ( 1 ) 4 1 - 9 1 , (2) 37-82, (3) 38-35, (4) 37-49- The stain in the sections of brain 4 1 - 9 1 is but slightly darker than that in the sections of brain 37-82. T h e stain in the sections of brain 38-35 is distinctly lighter in color and less extensive than that in the sections of brain 37-82. The stain in the sections of brain 37-49 is much less extensive and much lighter in color than that in the sections of brain 38-35. The stain in the sections of brain 37-49 is clearly lighter in color and less extensive than that in brain 33-197, which is the threemonth brain having the most myelin in this region. The stain in the sections of three-month brain 33-197 is slightly less extensive and lighter in color than the stain in the sections of brain 38-35. The stain in area P B in each of the four six-month brains is slightly lighter in color and less extensive than that in area F A y in the corresponding brain. The following description is based upon the sections of brains 4 1 - 9 1 and 37-82.

Horizontal exogenous fibers number 8 to 10 to the unit in layer II. They are small and lightly stained. The horizontal exogenous fibers number 12 to 14 to the unit in lamina I l i a and 14 to 16 in lamina I H b . The outer band of Baillarger in lamina IIIc and layer IV contains 18 to 20 horizontal exogenous fibers to the unit. In lamina Va the horizontal fibers number 16 to 18 to the unit. In the inner band of Baillarger in laminae V b and V i a these fibers average 18 to 20 to the unit. The horizontal fibers in lamina V I b number 16 to 18 to the unit. The subcortical association fibers number 30 to 35 to the unit. In all layers of the cortex the horizontal exogenous fibers are small, intermediate, and large in size, but the fibers of all the categories gradually increase in size from the outer to the inner boundaries, and are largest in layer V I and in the layer of subcortical association fibers. In the Golgi preparations all horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest horizontal exogenous fibers in layer V I and the largest subcortical association fibers measure 1.8/1 in diameter between varicosities, but the majority of the large-sized fibers are somewhat smaller than this.

Stained tangential fibers in layer I are darkest, largest, and most numerous in the wall of the superior end of sulcus centralis. In this location the stained tangential fibers number 10 to 12 to the unit of width. The stained fibers are small, intermediate, and large in size, and all have varicosities.

The vertical exogenous fibers in the core of gyrus centralis posterior under area P B number 40 to 45 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the vertical exogenous fibers bear varicosities, spines, and thorns. The

N o stained horizontal fibers are present in the cortex external to lamina IIIc. In lamina IIIc and layer IV (the outer band of Baillarger) stained horizontal fibers number 3 or 4 to the unit. They are small and lightly stained. In the inner band of Baillarger the

LOBUS PARIETALIS stained horizontal fibers number 6 or 7 to the unit, and they are larger and darker than the stained fibers in the outer band. In lamina VIb the stained horizontal fibers are larger and darker than those in the inner band of Baillarger and they number 8 to 10 to the unit. Stained subcortical association fibers number 14 to 16 to the unit. They are larger and more darkly stained than the stained horizontal fibers in lamina VIb. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers are largest, most numerous, and darkest in the core of the gyrus immediately subjacent to the koniocortex, area PB. Here they number 25 to 30 to the unit. Proceeding toward the posterior wall of the gyrus the stained vertical fibers in the core become fewer in number, smaller, and lighter in color. The vertical fibers are not stained as darkly as the vertical fibers in the core of gyrus centralis anterior in the region of the lower extremity in area FAy. The stained fibers in the core of gyrus centralis posterior are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure 1.95/n between varicosities. Large stained fibers are not as numerous in this region as they are in area F A y in the region of the lower extremity. As the stained vertical fibers radiate toward and ascend into the cortex they decrease in size and number, and become lighter in color. The stain ends on most of the ascending fibers in the lower part of layer VI, on some fewer it ends in lamina V i a , and occasionally a stained vertical fiber ends in lamina Va. REGION OF THE UPPER TRUNK, SHOULDER, ARM, AND FOREARM

The cortex in this region of area P B is illustrated in Figures 69, 70, 71, and 72. The apical dendrites of the extra-large pyramidal cells in layer V are larger and have more terminal branches in layer I than those of corresponding cells in this area in the three-month brain. The basal dendrites and the collateral branches of the apical dendrites are larger and longer, but no more numerous, than those in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in this area in general appear to be FIBERS.

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larger than those of corresponding cells in the region of the lower extremity in area PB. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than those on the dendrites of corresponding cells in this region of area P B in the cortex of the three-month infant. In the six-month cortex the dendrites of these cells have more pedunculated bulbs than the dendrites of any other cells in this region of area PB. The bulbs are more numerous on the apical dendrites than on either the basal dendrites or the collateral branches of the apical dendrites. In general, the dendrites of the extra-large pyramidal cells in layer V in this region of area P B have more pedunculated bulbs than the dendrites of corresponding cells in the region of the lower extremity, but the dendrites of some cells in the latter region appear to have as many bulbs as the dendrites of any of these cells in the region of the trunk, shoulder, and arm. Pedunculated bulbs are less numerous on the dendrites of the extra-large pyramidal cells in this region of area P B than they are on the dendrites of the giant pyramidal cells in the same region in area F A y . The axons of the extra-large pyramidal cells in layer V descend more or less tortuously into the subjacent white substance. The axons of these cells are larger and have more myelin than the axons of any other cells in the cortex in this region of area PB. The axons of these cells are larger and have more myelin than the axons of corresponding cells in this region of area P B in the cortex of the three-month infant. In the six-month cortex the axons of the extra-large pyramidal cells in this region of area P B are smaller and have less myelin than the axons of the giant pyramidal cells of Betz in the region of the trunk, shoulder, and arm in area FA-y. The collateral branches of the axons of the extra-large pyramidal cells are larger but no more numerous than those of the axons of corresponding cells in the region of the trunk, shoulder, and arm in the three-month cortex. The apical dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are more slender and many of them are shorter than the apical dendrites of the extra-large pyramidal cells in layer V. The basal dendrites are smaller and shorter than those of the extra-large pyramidal cells. Pedunculated bulbs are less numerous on the den-

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THE CORTEX OF THE SIX-MONTH INFANT

drites of the large ordinary and special pyramidal and spindle cells than those on the dendrites of the extra-large pyramidal cells in layer V. The apical dendrites of these cells are larger and their basal dendrites are coarser and longer than those of corresponding cells in this region of area P B in the three-month cortex. Pedunculated bulbs are more numerous on all the dendrites of these cells than in the cortex of the three-month infant. The axons of the large ordinary and special pyramidal cells are coarser and have more myelin than those of corresponding cells in the three-month cortex. In the six-month cortex the axons of these cells are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. No inverted pyramidal cells have been observed in layers V or VI in this area. The apical dendrites of the pyramidal cells in layer III are larger and have more terminal branches in layer I, and the basal dendrites are coarser and longer than those of corresponding cells in this region of area P B in the three-month cortex. In the six-month cortex the apical dendrites of the largest pyramidal cells in lamina IIIc are almost as large as the apical dendrites of some of the extra-large pyramidal cells in layer V. The basal dendrites of the largest pyramidal cells in lamina IIIc are smaller and shorter than the basal dendrites of the extra-large pyramidal cells. Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer I I I than on the dendrites of corresponding cells in this region of area P B in the three-month cortex. The bulbs are much more numerous on the terminal branches of the apical dendrites than in the three-month cortex. In the six-month cortex pedunculated bulbs are more numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of large cells in lamina I l l b , and the bulbs are less in quantity on the pyramidal cells in lamina I l i a than on the dendrites of cells in I l l b . The apical dendrites of the large pyramidal cells in lamina IIIc are larger than those of the large ordinary and special pyramidal cells in layer V, but do not have as many pedunculated bulbs as the latter. The axons of the pyramidal cells in layer I I I are larger than those of corresponding cells in the threemonth cortex. Myelin is present on the axons of the

larger cells in laminae I l l b and IIIc only, and it is much less in quantity than that on the axons of the extra-large pyramidal cells in layer V. The dendrites of the pyramidal cells in layer IV are larger and longer and have more pedunculated bulbs than those of corresponding cells in this region of area P B in the three-month cortex. The apical and basal dendrites of the pyramidal and granular cells in layer II likewise are larger, longer, and have more pedunculated bulbs than those of corresponding cells in the cortex of the three-month infant. Pedunculated bulbs are less numerous on the dendrites of the pyramidal cells of layer I I than on the dendrites of the pyramidal cells of layer IV and of lamina I l i a . No horizontal cells of Cajal have been observed in layer I. Occasionally a short branch is seen on one of the tangential fibers, but no long branches are present on any of these fibers. Golgi type I I cells are present throughout all layers of the cortex except layer I. The dendrites of these cells are in approximately the same state of development as to size and pedunculated bulbs as the dendrites of pyramidal cells of comparable size. The axons of the Golgi type II cells form a dense mesh of fibers in layer IV. The mesh in this layer is of about the same density as that in layer IV in this region of area P B in the cortex of the three-month infant. The mesh does not receive any fibers from the vertical exogenous fibers which enter this region of the cortex. No branches, either terminal or collateral, have been observed on any of the vertical exogenous fibers. All of these fibers end as single, vertically directed shafts. The dense mesh of axons of Golgi type II cells in layer IV ends abruptly at the junction of areas P B and P C near the anterior lip of the posterior central gyrus. The mesh of fibers in layer IV decreases gradually in density in the lower part of the anterior wall of the posterior central gyrus and in the floor of the central sulcus (Economo's area PA), then decreases still more in density but is quite evident in layer IV in area FAy in the posterior wall of gyrus centralis anterior. In area P B the inner margin of the mesh of fibers in layer IV is rather well defined because of the more or less abrupt decrease in quantity of the fibers in lamina Va. The mesh increases in density in the inner band of Baillarger in laminae Vb and Via, but is not as dense as in layer IV. The mesh decreases

LOBUS PARIETALIS

71

slightly in density in lamina VIb. Externally the mesh in layer IV decreases gradually in density in lamina IIIc, then decreases very rapidly in lamina I l l b . The mesh is quite thin in lamina I l i a , and in layer II the axons of Golgi type I I cells are not sufficiently numerous to form a mesh.

subcortical association fibers bear varicosities, spines, and thorns. The largest horizontal exogenous fibers in lamina V I b and the largest subcortical association fibers measure 1.8μ in diameter between varicosities. No branches have been observed on any of the horizontal exogenous or subcortical association fibers.

The tangential fibers in layer I in area P B in the middle two-fourths of the anterior wall of gyrus centralis posterior are less numerous, smaller, and more lightly stained than the tangential fibers in layer I in the middle two-fourths of the posterior wall of gyrus centralis anterior, area F A y . The tangential fibers abruptly become smaller and less numerous at the juncture of area P B with Economo's area P A in the lower one-third of the anterior wall of gyrus centralis posterior. Area P A is a continuation of the area of giant pyramidal cells ( F A y ) across the floor of the central sulcus and into the lower one-third of the anterior wall of gyrus centralis posterior. In area P B the tangential fibers in the middle of layer I are more numerous, larger, and more darkly stained than those in the outer and inner parts of the layer. In the middle of the layer the fibers number 12 to 14 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure ι.ζμ in diameter between varicosities.

The vertical exogenous fibers number 35 to 40 to the unit in the core of gyrus centralis posterior subjacent to area P B . They are small, intermediate, and large in size. In the Golgi preparations all the vertical exogenous fibers bear varicosities, spines, and thorns. The largest fibers measure 1 .Βμ in diameter between varicosities. The vertical fibers are chiefly collected in bundles as they radiate toward the cortex. Most of the radiating fibers end in the lower part of lamina V i a , some end in lamina Va, and a few end in layer IV. The radiating vertical fibers number 12 to 14 to the unit in the lower part of VIb, 6 to 8 in the lower part of lamina Vb, and 4 to 6 in the lower part of layer IV. A vertical fiber is occasionally seen in the mid-region of layer III. No collateral or terminal branches have been observed on any vertical exogenous fibers.

Horizontal exogenous fibers in layer II number 2 or 3 to the unit. They are small and lightly stained. In lamina I l i a the horizontal fibers number 5 to 7 to the unit, and are of about the same size as those in layer II. The horizontal fibers in lamina I l l b are larger and more darkly stained than those in lamina I l i a , and number 8 to xo to the unit. In the outer band of Baillarger in lamina IIIc and layer I V the horizontal exogenous fibers are larger than those in lamina I l l b , and they number 14 to 16 to the unit. These fibers number 12 to 14 to the unit in lamina Va, and 16 to 18 in the inner band of Baillarger. In lamina V I b the horizontal exogenous fibers number 14 to 16 to the unit. The horizontal exögenous fibers are small, intermediate, and large in size, and those in the inner band of Baillarger and in lamina V I b are larger than those in layer IV. The subcortical association fibers number 2 5 to 30 to the unit, and are of about the same size as the horizontal fibers in lamina VIb. In the Golgi sections all the horizontal exogenous and

MYELiNATiON. The brains fall into the following sequence with reference to the amount of myelin shown in the Weigert sections: ( 1 ) 37-82, (2) 38-35, (3) 37-49. The stain in the sections of brain 37-82 is darker and more extensive than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is much darker and more extensive than that in the sections of brain 38-128, the three-month brain with the most myelin in this region. In each of the brains the stain in the sections in the region of the trunk is darker than that in the sections in the region of the lower extremity. The stain in the sections of brain 37-82 in the region of the trunk is slightly lighter in area P B than it is in area FAy. The following description is based upon the sections of brain 37-82.

Stained tangential fibers in layer I number 2 or 3 to the unit. They are smaller and lighter in color than the stained tangential fibers in layer I in the region of the trunk in area FAy. No stained horizontal fibers are present in the cortex external to lamina I l l b , where a single stained fiber is occasionally seen. In lamina I I I c and layer

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THE CORTEX OF THE SIX-MONTH INFANT

IV (the outer band of Baillarger) stained horizontal fibers number 6 to 8 to the unit. In lamina Va the stained horizontal fibers number 4 or 5 to the unit. In laminae V b and V i a (the inner band of Baillarger) stained horizontal fibers are larger and darker than those in the outer band, and they number 7 or 8 to the unit. The stained horizontal fibers in lamina V I b number 8 to 10 to the unit, and they are larger and more darkly stained than those in lamina V i a . The stained subcortical association fibers number 10 to 12 to the unit, and they are larger and more darkly stained than the horizontal fibers in lamina VIb. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers are darkest and largest in the core of the gyrus subjacent to area P B . The stained fibers become smaller and lighter in color toward the posterior wall of the gyrus. In the anterior part of the core of the gyrus the stained vertical fibers number 25 to 30 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1.9/x in diameter between varicosities. The vertical fibers in the core of the gyrus subjacent to area P B are stained as darkly as the vertical fibers subjacent to area F A y in this region. The stained vertical fibers decrease in number and size, and become lighter in color as they radiate into the cortex. On most of the fibers the stain ends in the middle of layer V I , on some it ends in the inner band of Baillarger, and on a very few fibers the stain continues to the lower border of layer I V . REGION OF THE HAND The cortex in this region of area P B is illustrated in Figures 73, 74, 75, and 76. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this region of P B in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites of these cells are coarser, longer, and more compact in structure, but no more numerous, than those of corresponding cells in the cortex of the three-month infant. FIBERS.

The dendrites of the extra-large pyramidal cells have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The axons of the extra-large pyramidal cells are larger and have more myelin than those of corresponding cells in this region of area P B in the threemonth cortex. In the six-month cortex the axons of these cells have less myelin than the axons of the giant pyramidal cells of Betz in the region of the hand in area F A y . The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are coarser, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this region in the three-month cortex. Only one inverted pyramidal cell has been observed in this region of area P B and that is a large ordinary pyramidal cell in layer VI. The dendrites of the large pyramidal cells in layer I I I are conspicuously larger than those of corresponding cells in this region of area P B in the threemonth cortex. The apical dendrites have more terminal branches in layer I than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. The axons of the large pyramidal cells in laminae I l l b and IIIc have more myelin than the axons of corresponding cells in the cortex of the three-month infant. In the six-month cortex the pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V. The proximal parts of the apical dendrites of the largest pyramidal cells in lamina IIIc are as large as the smaller apical dendrites of the extra-large pyramidal cells in layer V, but pedunculated bulbs are much more numerous on the latter. The dendrites of the pyramidal cells of layer IV are coarser, longer, and have more pedunculated bulbs than those of corresponding cells in the threemonth cortex. The dendrites of the pyramidal and granular cells of layer I I are smaller and shorter than those of the pyramidal cells in layer IV, but are coarser and longer and have more pedunculated bulbs than those of corresponding cells in layer I I in this region of area P B in the three-month cortex.

LOBUS PARIETALIS No horizontal cells of Cajal have been observed in layer I. Golgi type II cells are present in all layers of the cortex except layer I. These cells are in the same state of development as to size of dendrites and quantity of pedunculated bulbs as pyramidal cells of comparable size. The mesh of fibers formed in layer IV by the axons of Golgi type II cells is more dense than that in this layer in this region of area PB in the three-month cortex. In the six-month cortex the mesh of fibers formed by the axons of Golgi type II cells in layer IV in this region of area PB is more dense than that in this layer in either the region of the lower extremity or the region of the trunk, shoulder, and arm. The mesh ends abruptly at the junction of area PB with area PC in the anterior lip of the posterior central gyrus, but the mesh gradually decreases in density in the lower one-fourth of the anterior wall of this gyrus and in the floor of the central sulcus. The mesh in layer IV continues into area FAy in the posterior wall of gyrus centralis anterior, but here it is very much thinner than that in area PB in the anterior wall of gyrus centralis posterior. The vertical exogenous fibers do not contribute any fibers to the mesh formed by the axons of the Golgi type II cells. No branches, either collateral or terminal, have been observed on any of the vertical exogenous fibers. All of these fibers end as single, vertical shafts. The tangential fibers in layer I in this region number 14 to 16 to the unit. They are small, intermediate, and large in size. The largest fibers measure 1.5/x in diameter between varicosities. In the Golgi sections all the tangential fibers bear varicosities, spines, and thorns. The tangential fibers in this area are not as large or as darkly stained as those in the region of the hand in area FAy. No branches have been observed on any of the tangential fibers. The tangential fibers are located chiefly in the middle of layer I. Horizontal exogenous fibers numbers or 3 to the unit in layer II, and 4 to 6 in lamina I l i a . In lamina IHb the horizontal fibers number 6 to 8 to the unit, and they are larger and more darkly stained than the fibers in layer II and lamina I l i a . The horizontal fibers in lamina IIIc and layer IV (the outer band of Baillarger) number 12 to 14 to the unit, and they are larger and more darkly stained than the horizontal fibers in lamina IHb. In lamina Va the horizontal

73

exogenous fibers number 10 to 12 to the unit. The inner band of Baillarger in laminae Vb and Via contains 14 to 16 horizontal exogenous fibers to the unit, and in lamina VIb these fibers number 12 to 14 to a unit. The subcortical association fibers average 25 to 30 to the unit. The horizontal exogenous fibers in the inner band of Baillarger and in lamina VIb are larger and more darkly stained than those in the outer band of Baillarger. The subcortical association fibers are about the same size as the fibers in lamina VIb. The horizontal exogenous fibers in all layers of the cortex and the subcortical association fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. No branches have been observed on any of these fibers. The largest horizontal exogenous fibers in lamina VIb and the largest subcortical association fibers measure τ.$μ in diameter between varicosities. The vertical exogenous fibers immediately subjacent to area PB in the core of the gyrus number 30 to 35 to the unit. The fibers are small, intermediate, and large in size. All the fibers bear varicosities, spines, and thorns. The largest fibers in the Golgi preparations measure 1.8μ in diameter between varicosities. As the vertical fibers radiate toward the cortex they are chiefly collected in bundles. Most of the radiating fibers end in the middle of layer VI, many end in the neuropil in the inner band of Baillarger, some end in the outer band, and a few end in the upper part of lamina IIIc. The ascending vertical exogenous fibers number 8 to 10 to the unit in the lower part of lamina VIb, 4 or 5 in the lower part of lamina Vb, and 2 or 3 in the lower part of layer IV. No collateral or terminal branches have been observed on any vertical exogenous fibers. MYELiNATioN. With reference to the depth of the stain in the Weigert sections the brains fall into the following sequence: (1) 37-82, (2) 38-44, (3) 3835» (4) 37_49- The stain in the sections of brain 37-82 is much darker than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is distinctly darker and more extensive than that in the sections of brain 33-197, the three-month brain with the most myelin in this region of area PB. The stain in area PB is about the same in depth of color as that in area F A y in the region of the hand. The

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THE CORTEX OF THE SIX-MONTH INFANT

stain in the sections in area P B in each brain is slightly lighter in the region of the hand than in the region of the trunk, shoulder, and arm, but is darker in the region of the hand than in the region of the lower extremity. The following description is based upon the sections of brain 37-82. A small, lightly stained tangential fiber is seen occasionally in layer I. No stained horizontal fibers are present in the cortex external to lamina I l l b . A small, lightly stained horizontal fiber is seen occasionally in the lower part of lamina I l l b . In lamina IIIc and layer IV (the outer band of Baillarger) stained horizontal fibers number 3 or 4 to the unit. In lamina Va the stained horizontal fibers number 4 or 5 to the unit, and they are larger and darker than those in layer IV. In laminae Vb and V i a (the inner band of Baillarger) the stained horizontal fibers are larger and darker than those in lamina Va, and they number 6 or 7 to the unit. In lamina VIb the stained horizontal fibers number 7 or 8 to the unit, and they are larger and darker than the stained horizontal fibers in lamina Via. The stained subcortical association fibers are larger and darker than the stained horizontal fibers in lamina VIb. The stained subcortical association fibers number 10 to 12 to the unit. The stained vertical fibers are darkest, largest, and most numerous in the core of the gyrus immediately subjacent to area PB. The stained vertical fibers gradually decrease in size and number, and become lighter in color toward the crown and posterior wall of gyrus centralis posterior. Immediately subjacent to area PB the stained vertical fibers number 25 to 30 to the unit of width. The stained fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure 1.95^ in diameter between varicosities. The stained vertical fibers decrease in size and number and become lighter in color as they radiate into the cortex. The stain on most of the fibers ends in the middle of layer VI, on a few it ends in layer V, and occasionally a small stained vertical fiber ends in the lower part of layer IV. REGION OF THE HEAD The cortex in this region of area PB is illustrated in Figures 77, 78, 79, and 80.

The apical dendrites of the extra-large pyramidal cells in layer V are larger and have more terminal branches in layer I than those of corresponding cells in this region of area P B in the threemonth cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and longer than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex.

FIBERS.

The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this region of area P B in the cortex of the three-month infant. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are larger, longer, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The axons of these cells are larger and have more myelin than those of corresponding cells in this region of area P B in the three-month cortex. In the six-month cortex the axons of these cells are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The apical dendrites of the large pyramidal cells in all three laminae of layer I I I are larger, more compact in structure, and have more terminal branches in layer I than corresponding cells in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, more compact in structure, and longer than those of corresponding cells in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer III than in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V. The bulbs are less numerous on the dendrites of the large pyramidal cells in lamina I l l b than on the dendrites of the large pyramidal cells in IIIc, and bulbs are less in quantity on the dendrites of the larger pyramidal cells in lamina I l i a than on the dendrites of the larger cells in lamina I l l b . Pedunculated bulbs are sparsely scattered along the terminal branches of the apical dendrites of the larger pyramidal cells in lamina I l i a .

LOBUS PARIETALIS The axons of the largest cells in layer I I I , those in lamina I I I c , are smaller and have much less myelin than the axons of the extra-large pyramidal cells in layer V. Some of the large pyramidal cells in lamina M b have a small amount of myelin. The axons of the large pyramidal cells in laminae M b and I I I c are larger and have more myelin than the axons of corresponding cells in this region of area P B in the threemonth cortex. The dendrites of the pyramidal cells of layers I I and IV are coarser, longer, and have more pedunculated bulbs than corresponding cells in this region of area P B in the three-month cortex. No horizontal cells of Cajal have been observed in layer I. No long branches have been observed on any of the tangential fibers in layer I, but occasionally a short branch is seen. Golgi type I I cells are present throughout all layers of the cortex except layer I. The dendrites of these cells are in approximately the same state of development as to size, length, and quantity of pedunculated bulbs as the dendrites of pyramidal cells of comparable size. The axons of these cells are not sufficiently numerous to form a mesh of fibers in layer I I or in lamina I l i a , but they do form a thin mesh in lamina M b . The mesh rapidly but gradually increases in density toward layer IV, where it reaches a maximum density and forms a dark horizontal band in this layer. The external boundary of this dense mesh in layer IV is not sharply defined, but the inner margin is, because the mesh abruptly becomes much less dense in lamina Va. The mesh increases in density in the inner band of Baillarger in laminae Vb and V i a , then decreases slightly in lamina V I b where it is of approximately the same density as in the upper part of lamina I I I c . The mesh of fibers does not receive any contributions from the vertical exogenous fibers. No terminal or collateral branches have been observed on any vertical exogenous fiber. These fibers end as single shafts directed vertically. The dense mesh of axons of Golgi type I I cells in layer IV ends abruptly at the junction of area P B with area P C in the superior one-third of the anterior wall of gyrus centralis posterior. Ventrally the mesh continues across the floor of the central sulcus and into the lower one-third of area F A y in the posterior wall of gyrus centralis anterior. In the floor of the central

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sulcus (Economo's area P A ) the mesh is of the same density as it is in area P B in the anterior wall of gyrus centralis posterior. In the lower one-third of area F A y the mesh rapidly decreases in density, and becomes quite thin, but is still present throughout the extent of area F A y in the posterior wall of gyrus centralis anterior. In the lower part of the central sulcus in the lateral surface of the opercular part of gyrus centralis posterior the dense mesh of fibers in layer IV becomes more deeply placed in the anterior wall of this gyrus, and at the lowermost part, where the gyrus becomes quite small, the mesh is confined to the floor of the central sulcus. The mesh gradually decreases in density from the upper to the lower part of the region of the head. The mesh finally disappears in the lowermost part of the central sulcus, and is not present at all in any of the gyri on the deep surface of the opercular portions of the parietal and frontal lobes in the vicinity of the lower end of the central sulcus. This mesh of fibers in the region of the head in area P B is more dense in the six-month cortex than that in the same region in the three-month cortex. The mesh of fibers in layer IV in area P B in the region of the head is more dense than the mesh in the region of the lower extremity or in the region of the hand, and is only slightly more dense than that in the region of the trunk, shoulder, and arm. With reference to the density of this mesh of fibers in layer IV the various regions of area P B fall into the following sequence, beginning with the one in which the density is least: Region of the lower extremity; region of the hand; region of the trunk, shoulder, and arm; region of the head. No inverted pyramidal cells have been observed anywhere in this area of the cortex. The tangential fibers in layer I number 14 to 16 to the unit. They are more numerous, larger, and more darkly stained in the middle of the layer than in the outer and inner margins. The fibers are small, intermediate, and large in size, and all of them bear varicosities, spines, and thorns. The largest fibers in the Golgi preparations measure 1.3^ in diameter between varicosities. No branches have been observed on any of the tangential fibers. The fibers in area P B are generally smaller and more lightly stained than the tangential fibers in area F A y in the region of the head.

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THE CORTEX OF THE SIX-MONTH INFANT

The horizontal exogenous fibers number 2 or 3 to the unit in layer II, and 3 or 4 in lamina I l i a . They are small and lightly stained. In lamina I l l b the horizontal fibers number 6 to 8 to the unit. They are larger and more darkly stained than the horizontal fibers in lamina I l i a . The outer band of Baillarger in lamina IIIc and layer IV contains 14 to 16 horizontal exogenous fibers to the unit, and they are larger and more darkly stained than the horizontal fibers in lamina I l l b . The horizontal exogenous fibers number 12 to 14 to the unit in lamina Va and 16 to 18 in the inner band of Baillarger in laminae Vb and Via. In lamina VIb the horizontal exogenous fibers number 14 to 16 to the unit. The subcortical association fibers number 18 to 20 to the unit. The horizontal exogenous fibers in layer V I and the subcortical association fibers are coarser and more darkly stained than the horizontal exogenous fibers in layers V and IV. All these fibers are small, intermediate, and large, and all bear varicosities, spines, and thorns. The largest fibers in layer V I and the largest subcortical association fibers measure 1.5/x in diameter between varicosities in the Golgi preparations. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers in the core of the gyrus immediately subjacent to area P B number 30 to 35 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure ι.ζμ between varicosities. As they radiate toward and into the cortex the vertical fibers are chiefly collected in bundles. Most of the radiating fibers end in the middle of lamina VIb, some end in the neuropil in the inner band of Baillarger, and a few end in layer IV. The ascending vertical fibers number 10 to 12 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 3 or 4 in the lower part of layer IV. Occasionally a vertical fiber is seen in the mid-region of layer III. In the Golgi preparations all the ascending vertical exogenous fibers bear varicosities, spines, and thorns. No branches, either collateral or terminal, have been observed on any of these fibers. MYELINATION. The following description is based upon the sections of brain 37-82. The stain in the

Weigert sections is much darker than that in the sections of brain 33—197, the three-month brain with the most myelin in this region of area PB. The stain in the sections of brain 37-82 in area PB is lighter in color in the region of the head than in the region of the lower extremity. The stain in the region of the head is about the same color in area P B as that in this region in area F A y . No stained tangential fibers are present in layer I. No stained horizontal fibers are present in the cortex external to layer VI. In lamina V i a stained horizontal fibers number 2 or 3 to the unit. They are very small and very lightly stained. In lamina VIb the stained horizontal fibers number 6 or 7 to the unit, and they are larger and darker than those in lamina Via. Stained subcortical association fibers number 8 to 10 to the unit, and they are darker and larger than the stained horizontal fibers in lamina VIb. The stained vertical fibers are largest, darkest, and most numerous in the core of the gyrus subjacent to area PB. In this location they number 20 to 25 to the unit. The stained vertical fibers are small, intermediate, and large in size, and all bear varicosities. They are smaller and lighter in color than the stained vertical fibers subjacent to area P B in the region of the lower extremity. As the stained vertical fibers radiate toward the cortex they become smaller and lighter in color. The stain ends on most of the fibers in the region of the subcortical association fibers. On a few fibers the stain ends in the lower part of lamina VIb. No stained vertical fibers are present external to lamina VIb. AREA POSTCENTRALIS INTERMEDIA, PC C R O W N OF G Y R U S C E N T R A L I S POSTERIOR

This area has the same characteristics and distribution as in the three-month cortex. The cell-bodies as revealed by the cresyl violet stain present no especial differences in the four functional regions of area PC. The region of the trunk, shoulder, arm, and forearm is selected as typical for the description of the cell-bodies. There are some differences between the four functional regions in the development of the fibers, therefore each region is described separately. The cortex in the four functional regions in area P C is illustrated in Figures 81 to 96, inclusive.

LOBUS PARIETALIS The cortex in area PC in the region of the trunk, shoulder, arm, and forearm measures 2.092 mm. in width. CELL-BODIES

77

form of granules, small clumps and elongated strands. A darkly stained flat or cone-shaped clump rests upon the nuclear membrane immediately below the base of the apical dendrite. The chromophil substance in the extra-large pyramidal cells is greater in quantity and more advanced in development than in corresponding cells in this region of area PC in the three-month cortex.

Layer I. This layer measures 0.258 mm. in width, and contains 44 neurons to the unit. A horizontal cell of Cajal is seen occasionally. Layer II. The external granular layer is faintly visiThe large ordinary pyramidal cells in layers V and ble macroscopically. The layer measures 0.104 mm. VI and the large spindle cells in VI contain less in width. The neurons number 2 68 to the unit, and chromophil substance in proportion to the size of the range in size from 4/4,0, to 8/4/11. nucleus than the extra-large pyramidal cells, and they Layer III. This layer measures 0.658 mm. in width. are not stained as darkly as the latter. The chromoIn lamina I l i a the neurons number 60 to the unit, phil substance is in the form of fine granules, a few and measure 8/6-23/12^. In lamina I l l b they num- Nissl flakes, and small and large clumps. The large ber 52 per unit, and measure 10/6-26/15^1. The neu- clumps predominate in number. The chromophil rons number 51 to the unit in lamina IIIc, and range substance invades the apical dendrite in less quanin size from ιο/6μ to 28/16/i. tity and for a shorter distance than in the extra-large Layer IV. The inner granular layer is distinctly visi- pyramidal cells. In almost all of these cells a small ble macroscopically, but it is not as prominent as it is cone of darkly stained chromophil substance is restin area PB. The layer measures 0.235 mm. in width. ing on the nuclear membrane immediately below the The cells number 149 to the unit, and measure 6/4- base of the apical dendrite. ιο/6μ. A few cells of laminae IIIc and Va invade the The large pyramidal cells in lamina IIIc are congranular layer. spicuous because of their dark color. They are Layer V. The internal pyramidal layer measures stained as darkly as the extra-large pyramidal cells in 0.381 mm. in width. The small neurons measure layer V. Some of these cells are as large as the large 8/6-13/ιομ,, and number 57 to the unit in lamina Va ordinary pyramidal cells in layers V and VI. The and 50 in Vb. The large neurons measure 16/10- chromophil substance is present in approximately the 26/14/x, and number 17 to the unit in each lamina. same quantity and condition as in the large ordinary The extra-large pyramidal cells are located chiefly in pyramidal and spindle cells in layers V and VI. The lamina Va, but some are present in Vb also. They chromophil substance is greater in quantity and more number 9 to the unit and range in size from 29/12 μ advanced in differentiation in all the cells in layer III than in corresponding cells in this region of area PC to 49/13/*· Layer VI. This layer measures 0.456 mm. in width. in the three-month cortex. The neurons number 77 to the unit in lamina V i a and 52 in lamina VIb. The pyramidal cells measure 10/6-29/15μ, and the spindle cells range in size from ΐ6/8μ

to

2β/ΐ2μ.

The extra-large pyramidal cells in layer V contain more chromophil substance in proportion to the size of the nucleus and are more darkly stained than any other cells in this area. In these cells the chromophil substance is chiefly in the form of small darkly stained clumps; Nissl flakes are less numerous than the small clumps, and large clumps are few in number. The chromophil substance extends into the proximal end of the apical dendrite where it is in the

The chromophil substance is greater in quantity and more advanced in differentiation in the small cells in all layers than it is in corresponding cells in this region of area PC in the cortex of the three-month infant. The small cells in layer IV have more chromophil substance than the cells in layer II. As to both the quantity and the state of differentiation of the chromophil substance, the functional regions of the hand and the shoulder and arm closely resemble each other, the former being somewhat more advanced in development. Both these regions are more advanced in development than either the region of the head or the region of the lower extremity.

THE CORTEX OF THE SIX-MONTH INFANT

78

A few neurofibrils and longitudinal rows of granules are present in the extra-large pyramidal cells in layer V. The neurofibrils are more easily distinguished in the dendrites than in the cell-bodies and axons. The dendrites and axons are stained more darkly than the cell-bodies. Still fewer neurofibrils and longitudinal rows of granules are present in the large ordinary pyramidal cells in layers V and V I and in the large spindle cells in VI. A few longitudinal rows of granules are present in the large pyramidal cells in lamina IIIc, and in the largest of these cells a few neurofibrils can be observed. The neurofibrils are not as numerous or as distinctly formed as those in the large ordinary pyramidal and spindle cells in layers V and VI. The axons and dendrites are stained but slightly more darkly than the cell-bodies. The cytoplasm in all other cells in area P C is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. Differentiation of neurofibrils is more advanced in area P C than in area PB. Within area P C the differentiation of neurofibrils is more advanced in the cells in the functional regions of the arm and hand than in corresponding cells in the regions of the head and lower extremity, except that neurofibrils appear to be as well differentiated in the large cells in lamina IIIc in the region of the head as in corresponding cells in the regions of the arm and hand. Differentiation of neurofibrils is more advanced in all the functional regions in area P C in the cortex of the six-month infant than in corresponding regions of this area in the three-month cortex. REGION OF THE LOWER EXTREMITY The cortex in this region of area PC is illustrated in Figures 81, 82, 83, and 84. The boundary line between areas P B and P C in the upper one-third of the anterior wall of gyrus centralis posterior is marked by the abrupt termination of the dense mesh of fibers formed by the axons of Golgi type II cells in layer IV in area P B and also by an especially large number of very large pyramidal cells in lamina IIIc. The apical dendrites of the extra-large pyramidal cells in layer V are coarser than those of correspondFIBERS.

ing cells in this region of area P C in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corresponding cells in the cortex of the three-month infant. In the six-month cortex the dendrites of the extra-large pyramidal cells in layer V in this region of area PC are in general larger and longer, but have fewer pedunculated bulbs, than the dendrites of corresponding cells in the region of the lower extremity in area PB. The axons of the extra-large pyramidal cells are larger and have more myelin than those of corresponding cells in this region of area P C in the cortex of the three-month infant. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger, longer, and have more pedunculated bulbs than those of corresponding cells in this region of area PC in the three-month cortex. In the six-month cortex the dendrites of these cells do not have as many pedunculated bulbs as the dendrites of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells are coarser and have more myelin than the axons of corresponding cells in this region of area PC in the three-month cortex. In the six-month cortex the axons of these cells are smaller and have less myelin than those of the extra-large pyramidal cells in layer V. The large pyramidal cells in lamina IIIc are larger than the large pyramidal cells in this lamina in this region of area PC in the three-month cortex. The apical dendrites are larger and have more terminal branches, and the basal dendrites and collateral branches are larger and longer, but no more numerous, than those in the three-month cortex. The dendrites of these cells have more pedunculated bulbs than the dendrites of corresponding cells in this region of area PC in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V. The dendrites of the large pyramidal cells in all three

LOBUS PARIETALIS laminae of layer I I I are coarser, longer, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. In the six-month cortex the dendrites of the larger pyramidal cells in all three laminae of layer I I I are larger and longer, but have fewer pedunculated bulbs, than those of corresponding cells in this region of area PB. Especially large cells occur in all three laminae in layer I I I throughout the extent of area PC, but these large cells are more numerous at the junction of area PC with area P B than elsewhere. The apical dendrites of the largest pyramidal cells in lamina IIIc are as large as the apical dendrites of some of the extra-large pyramidal cells in layer V, but have far fewer pedunculated bulbs than the latter. The axons of the pyramidal cells in layer I I I are larger and have more myelin than those of corresponding cells in layer I I I in this region of area PC in the cortex of the three-month infant. The axons of the large pyramidal cells in lamina IIIc are larger and have more myelin than those of large pyramidal cells in lamina I l l b , and the axons of large pyramidal cells in lamina I l i a are smaller and have less myelin than those in I l l b . The only evidences of myelin on the axons of the larger pyramidal cells in I l i a are a few small, widely scattered beads. The axons of the largest pyramidal cells in lamina IIIc are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The dendrites of the pyramidal and granular cells in layers I I and IV are larger, longer, and have more pedunculated bulbs than those of corresponding cells in this region of area PC in the three-month cortex. No horizontal cells of Cajal have been observed in layer I. No inverted pyramidal cells have been seen in any layer of the cortex. The dendrites of Golgi type I I cells in all parts of the cortex are in about the same state of development as those of pyramidal cells of comparable size. The axons of the Golgi type I I cells form a thin mesh of fibers in layers IV, V, and VI, but are not sufficiently numerous to form a mesh in layers I I and I I I . No terminal or collateral branches have been observed on any of the vertical exogenous fibers. Tangential fibers in layer I number 14 to 16 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the fibers bear varicosi-

79

ties, spines, and thorns. The largest fibers in the Golgi sections measure 1.3/1. in diameter between varicosities. No branches have been observed on any tangential fibers. Horizontal exogenous fibers number 8 to 10 to the unit in layer II. The fibers are small and lightly stained. In lamina I l i a the horizontal exogenous fibers number 12 to 14 to the unit. They are slightly larger and more darkly stained than the fibers in layer II. In lamina I l l b the horizontal exogenous fibers number 12 to 14 to the unit and they are larger and more darkly stained than those in lamina I l i a . The outer band of Baillarger contains 16 to 18 horizontal exogenous fibers to the unit, and they are larger and more darkly stained than those in lamina I l l b . In lamina Va the horizontal fibers number 12 to 14 to the unit, and in the inner band of Baillarger in laminae Vb and Via these fibers average 16 to 18 to the unit. The horizontal exogenous fibers in lamina VIb number 14 to 16 to the unit, and the subcortical association fibers 16 to 18 to the unit. The horizontal fibers in layer VI are larger and more darkly stained than those in layer IV or V; the subcortical association fibers are about the same size and color as the horizontal fibers in layer VI. The horizontal fibers in all layers of the cortex and the subcortical association fibers are small, intermediate, and large in size, and in the Golgi sections all these fibers bear varicosities, spines, and thorns. The largest horizontal exogenous fibers in lamina VIb measure ι .3/1 in diameter and the largest subcortical association fibers are about the same size. No branches have been observed on any horizontal exogenous or subcortical association fibers. The vertical exogenous fibers in the core of gyrus centralis posterior immediately subjacent to the cortex in the crown number 30 to 35 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi sections all the fibers bear varicosities, spines, and thorns. The largest fibers measure τ.2μ in diameter between varicosities. The vertical exogenous fibers are chiefly collected in bundles as they radiate toward and ascend in the cortex. Many of the fibers end in the mid-region of lamina VIb, some end in the neuropil in the inner band of Baillarger, and a few end in the neuropil in the outer band of Baillarger. The ascending vertical exogenous fibers num-

80

THE CORTEX OF THE SIX-MONTH INFANT

ber i2 to 14 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 4 or 5 in the lower part of layer IV. All the vertical fibers decrease in size as they ascend, and all end as straight, vertically directed fibers. No terminal or collateral branches have been observed on any of the vertical exogenous fibers. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 4 1 - 9 1 , (2) 37-82, (3) 38-35, (4) 37-49. There is not much difference in the color and extent of the stain in the sections of brains 4 1 - 9 1 and 37-82. The stain in the sections of brain 38-35 is distinctly lighter than that in the sections of brain 37-82. The stain in the sections of brain 37-49 is much lighter and less extensive than that in the sections of brain 38-35. The stain in the sections of brain 37-49 is clearly lighter and less extensive than that in the sections of 33-197, the three-month brain with the most myelin in this region of area P C . In each of the six-month brains the stain is lighter in color and less extensive in area P C than in area P B . The following description is based upon the sections of brains 4 1 - 9 1 and 37-82. MYELINATION.

The stained tangential fibers in layer I number 6 to 8 to the unit. They are smaller and lighter in color than the stained tangential fibers in layer I in area P B . The stained fibers are small, intermediate, and large in size, and all bear varicosities. N o stained horizontal fibers are present in the cortex external to layer IV. In this layer stained horizontal fibers number 2 or 3 to the unit, and they are small and light in color. The stained horizontal fibers number 3 or 4 to the unit in lamina V a and 5 or 6 in the inner band of Baillarger. In lamina VIb the stained horizontal fibers number 7 or 8 to the unit, and they are larger and darker in color than those in lamina V i a . The stained subcortical association fibers number 8 to 10 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal fibers in the cortex and the subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers number 16 to 18 to the unit in the core of the gyrus immediately below area

P C . The fibers are stained more lightly than the vertical fibers subjacent to area P B . The stained vertical fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure ΐ·95μ, but fibers of this size are few in number. The stained vertical fibers decrease in number and size, and become lighter in color as they radiate into the cortex. The stain on most of the fibers ends in the lower part of layer V I , on a few fibers it ends in layer V, and occasionally a stained vertical fiber is seen ending in the lower part of layer IV. Single stained vertical fibers are seen occasionally in layers I I I and II. These stained fibers are small and light in color, and are not continuous with vertical fibers which ascend into the cortex. They are interpreted as myelinated axons of neurons located in this region of the cortex. REGION OF THE UPPER TRUNK, SHOULDER, ARM, AND FOREARM The cortex in this region of area P C is illustrated in Figures 85, 86, 87, and 88. The apical dendrites of the extra-large pyramidal cells in layer V and the large pyramidal cells in layer I I I are larger and have more terminal branches in layer I than those of corresponding cells in this region of area P C in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites of all these cells are larger and longer, but no more numerous, than those of corresponding cells in the cortex of the three-month infant. The dendrites of the cells in layers V, VI, IV, and II are coarser and longer than those of corresponding cells in the three-month cortex. FIBERS.

Pedunculated bulbs are more numerous on the dendrites of all cells in this region of area P C than on the dendrites of corresponding cells in the threemonth cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of pyramidal cells in layers I I I and V in this region of area P C than on corresponding cells in the region of the trunk, shoulder, and arm in area P B . A thin mesh of fibers is formed by the axons of Golgi type II cells in lamina IIIc, and in layers I V , V, and VI. The mesh contains more fibers in layer IV than elsewhere. The mesh is more dense than that in

LOBUS PARIETALIS this region of area PC in the cortex of the threemonth infant. In the six-month cortex the mesh is slightly more dense in this region of area PC than it is in the region of the lower extremity. The tangential fibers in layer I number 12 to 14 to the unit. They are small, intermediate, and large in size. The fibers in the external one-half of the layer are more numerous, larger, and more darkly stained than those in the internal one-half. In the Golgi preparations all the fibers bear varicosities, spines, and thorns. Horizontal exogenous fibers in layer II number 4 or 5 to the unit. They are very small and lightly stained. The horizontal exogenous fibers gradually become larger and more darkly stained toward the deeper strata of the cortex, and are largest and most darkly stained in lamina VIb and in the layer of subcortical association fibers. The horizontal fibers number 6 to 8 to the unit in lamina I l i a and 8 to 10 in lamina IHb. In the outer band of Baillarger in lamina IIIc and layer IV the horizontal exogenous fibers number 12 to 14 to the unit. In lamina Va the horizontal fibers number 10 to 12 to the unit. These fibers number 14 to 16 in the inner band of Baillarger in laminae Vb and Via, and 12 to 14 in lamina VIb. The subcortical association fibers number 20 to 22 to the unit. In the Golgi preparations all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest fibers measure 1.5^ in diameter between varicosities. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers number 30 to 35 to the unit immediately subjacent to area PC and just before they radiate toward the cortex. In the Golgi preparations all of the vertical fibers bear varicosities, spines, and thorns. The largest fibers measure 1.5/A in diameter between varicosities. The vertical fibers are chiefly collected in bundles as they radiate toward and into the cortex. Many of the radiating fibers end in the mid-region of layer VI, some end in the neuropil in the inner band of Baillarger, and a few end in the outer band of Baillarger. The ascending vertical exogenous fibers number 10 to 12 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 3 or 4 to the unit in the lower part of layer IV. No branches, either collateral or terminal,

81

have been observed on any of the vertical exogenous fibers. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: (1) 37-82, (2) 38-35, (3) 37-49. The stain in the sections of brain 37-49 is distinctly lighter and less extensive than that in the sections of brain 37-82, but it is much darker and more extensive than that in brain 38-128, the three-month brain with the most myelin in this region. In each of the six-month brains the stain in the sections in this region of area PC is darker than that in the sections in the region of the lower extremity. In each of the brains the stain in the sections in this region of area PC is lighter in color than the stain in sections in this region of area PB. The following description is based upon the sections of brain 37-82. MYELiNATioN.

Stained tangential fibers in layer I number 1 or 2 to the unit, and they are very small and very light in color. No stained horizontal fibers are present in the cortex external to lamina IIIc. In lamina IIIc and layer IV (the outer band of Baillarger) there are 2 or 3 very lightly stained horizontal fibers to the unit. In lamina Va stained horizontal fibers number 3 or 4 to the unit, and they are slightly darker and larger than those in layer IV. In laminae Vb and V i a (the inner band of Baillarger) stained horizontal fibers number 5 or 6 to the unit, and they are larger and darker in color than the stained horizontal fibers in lamina Va. In lamina VIb the stained horizontal fibers are larger and darker than those in lamina Via, and they number 7 or 8 to the unit. The stained subcortical association fibers number 8 to 10 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal fibers in the cortex and the stained subcortical fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers in the core of the gyrus subjacent to area PC are stained more lightly than those subjacent to area P B in this region. The stained vertical fibers subjacent to area P C number 16 to 18 to the unit immediately below the place where they begin to radiate toward the cortex. The

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THE CORTEX OF THE SIX-MONTH INFANT

stained vertical fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure 1.95/x in diameter between varicosities. Large fibers are much less numerous than they are in area PB. The stained vertical fibers decrease in size and become lighter in color as they radiate toward and into the cortex. The stain ends on most of the fibers in the middle of layer VI, on some it ends in the upper part of layer VI, and on a very few fibers it ends in the lower part of layer IV. REGION OF THE HAND

The cortex in this region of area PC is illustrated in Figures 89, 90, 91, and 92. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of the corresponding cells in this region of area P C in the cortex of the three-month infant. The basal dendrites and the collateral branches of the apical dendrites of these cells are larger, longer, and more compact in structure, but no more numerous than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells than in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of these cells than on the dendrites of corresponding cells in this region of area PB. FIBERS.

The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this region of area P C in the three-month cortex. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are coarser, longer, and more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this region of area PC in the cortex of the three-month infant. The axons of these cells are larger and have more myelin than those in the three-month cortex, but they are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V in the six-month cortex. The apical dendrites of the large pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of

corresponding cells in this region of area P C in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all pyramidal cells in layer III than on corresponding cells in this region of area PC in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of pyramidal cells in all three laminae of layer III than on the dendrites of corresponding cells in this region of area PB. The axons of the largest pyramidal cells in lamina IIIc are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The axons of the large pyramidal cells in lamina I l l b are smaller and have less myelin than the axons of the large pyramidal cells in lamina IIIc. The axons of the large pyramidal cells in lamina I l i a are smaller than those of the large pyramidal cells in I l l b , and bear a few small, widely scattered globules as the only indications of myelin. The dendrites of the pyramidal cells of layers IV and II are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this region of area P C in the three-month cortex. The dendrites of the Golgi type II cells in all parts of the cortex in this region of area PC are in approximately the same state of development as to size, length, and quantity of pedunculated bulbs as those of pyramidal cells of comparable size. The dendrites of the Golgi type II cells are larger, longer, more compact in structure, and have more pedunculated bulbs than those of Golgi type II cells of comparable size in this region of area PC in the three-month cortex. The axons of the Golgi type II cells form a thin mesh of fibers in lamina IIIc, layer IV, layer V, and layer VI. The mesh contains more fibers in layer IV than in the other layers. The mesh is of about the same density as that in the region of the trunk, shoulder, and arm in area PC. This mesh of fibers in area PC is continuous with that in area PB, but at the boundary between these two areas the mesh of

LOBUS PARIETALIS fibers abruptly becomes much more dense in area P B . The junction of these two areas is also characterized by an unusually large number of large pyramidal cells in layer I I I . The tangential fibers in layer I number 12 to 14 to the unit. They are small, intermediate, and large in size. The fibers are more numerous, larger, and more darkly stained in the outer one-half of layer I than in the inner one-half. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The horizontal fibers in layer I number 6 to 8 to the unit. They are very small and faintly stained. In lamina I l i a the horizontal exogenous fibers number 8 to 10 to the unit, and they are larger and more darkly stained than the fibers in layer I I . The fibers in lamina I l l b are darker and larger than those in I l i a , and number 10 to 12 to the unit. Those in the outer layer of Baillarger in lamina I I I c and layer IV number 12 to 14 to the unit, and they are larger and more darkly stained than the horizontal fibers in lamina I l l b . The horizontal fibers number 10 to 12 to the unit in lamina Va, and 14 to 16 in the inner band of Baillarger. The fibers in the inner band of Baillarger are larger and more darkly stained than those in layer IV. In lamina VIb the horizontal exogenous fibers number 12 to 14 to the unit. The subcortical association fibers number 16 to 18 to the unit. The horizontal exogenous fibers in VIb and the subcortical association fibers are of about the same size and density of stain, and the fibers in both these strata are larger and more darkly stained than the horizontal fibers in the inner band of Baillarger. The largest fibers measure 1.3/j, in diameter in the Golgi preparations. All the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The vertical exogenous fibers in the core of gyrus centralis posterior subjacent to area P C number 35 to 40 to the unit. They are small, intermediate, and large in size. In the Golgi preparations all the fibers bear varicosities, spines, and thorns. The largest fibers measure 1.5^ in diameter between varicosities. As they radiate toward the cortex the fibers are chiefly collected in bundles. Many of the radiating fibers end in the mid-region of layer VI, some end in the inner band of Baillarger, and a few end in the outer band. The radiating vertical fibers number 14

83

to 16 to the unit in the lower part of lamina VI, 8 to 10 in the lower part of lamina Vb, and 4 to 6 in layer IV. No branches, either collateral or terminal, have been observed on any vertical exogenous fibers. MYELiNATioN. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 37-82, (2) 38-44, (3) 38-35, (4) 37-49· There is but little difference between brains 37-82 and 38-44. The stain in the sections of brain 38-44 is distinctly darker than that in the sections of brain 38-35, and the stain in the sections of brain 38-35 is much darker than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is darker than that in the sections of brain 3 3 - 1 9 7 , the three-month brain with the most myelin in this region of area PC. In each of the six-month brains the stain is lighter in color and less extensive in area P C than it is in area P B . The stain in the sections in area P C in each brain is lighter in the region of the hand than in the region of the trunk and arm, but is darker than in the region of the lower extremity. The following description is based upon the sections of brain 37-82. No stained tangential fibers are present in layer I, and there are no stained horizontal fibers in the cortex external to lamina IIIc. In lamina I I I c and layer IV (the outer band of Baillarger) stained horizontal fibers number 2 or 3 to the unit, and they are very small and light in color. In lamina Va the stained horizontal fibers are larger and darker, and they number 3 or 4 to the unit. The stained horizontal fibers number 5 or 6 to the unit in the inner band of Baillarger, and they are larger and darker than those in lamina Va. In lamina VIb the stained horizontal fibers are still larger and darker, and they number 7 or 8 to the unit. The stained subcortical association fibers are larger and darker than the stained horizontal fibers in the lower part of lamina VIb, and they number 8 to 10 to the unit. The stained horizontal fibers in the cortex and the stained subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers in the core of the gyrus are much lighter in color subjacent to area P C than those which lie under area P B . The stained vertical fibers are small, intermediate, and large in size, and

84

THE CORTEX OF THE SIX-MONTH INFANT

all bear varicosities. The largest stained vertical fibers measure τ.ζμ in diameter between varicosities. Just below the place where they begin to radiate toward the cortex the stained vertical fibers number 20 to 22 to the unit of width. As they radiate into the cortex the stained vertical fibers decrease in number and size, and become lighter in color. The stain ends on most of the fibers in the lower part of layer VI; on some fewer fibers it ends in the inner band of Baillarger. A small number of small, lightly stained vertical fibers end in lamina Va, and occasionally a small stained vertical fiber ends in the lower part of layer IV. REGION OF THE HEAD The cortex in this region of area PC is illustrated in Figures 93, 94, 95, and 96. The apical dendrites of the extra-large pyramidal cells are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this region of area PC in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are longer, larger, and more compact in structure than those of corresponding cells in the three-month cortex. Pedunculated bulbs on the dendrites of these cells are but slightly more numerous than in the three-month cortex. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this region of area PC in the three-month cortex. FIBERS.

The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this region of area PC in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of these cells than on the dendrites of the extra-large pyramidal cells in layer V. The apical dendrites of the large pyramidal cells in layer III are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this region of area PC in the cortex of the three-month infant. The basal dendrites and the collateral branches are larger, longer, and

more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are but slightly more numerous on the dendrites of the pyramidal cells in layer III than in the three-month cortex. In the cortex of the six-month infant pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V, and much less numerous than on the dendrites of the extra-large pyramidal cells in layer V. The axons of the large pyramidal cells in laminae I l l b and IIIc are larger and have more myelin than those of corresponding cells in this region of area PC in the three-month cortex. A slight enlargement distal to the cell-body and a few globules are the only indications of myelin on the axons of the pyramidal cells in lamina IIIc, and the axons of the cells in lamina I l l b have still less myelin. There are no indications of myelin on the axons of the larger pyramidal cells in lamina I l i a . The axons of the largest pyramidal cells in lamina IIIc are smaller and have much less myelin than the axons of the extra-large pyramidal cells in layer V. A very thin mesh of fibers is formed in layer IV by the axons of the Golgi type II cells. The mesh is much less dense than that in layer IV in the region of the hand in area PC. A mesh is present in layers V and VI, but it is thinner than that in layer IV. No fibers are contributed to this mesh by the vertical exogenous fibers. No branches, either terminal or collateral, have been observed on the vertical exogenous fibers. All of them end as single, vertically directed shafts. No inverted pyramidal cells have been observed in this region of area PC. The tangential fibers in layer I number 14 to 16 to the unit. The fibers are small, intermediate, and large in size. They are more numerous, coarser, and more darkly stained in the outer one-half of the layer than in the inner one-half. In the Golgi preparations all the fibers bear varicosities, spines, and thorns. The largest fibers measure 1.2 μ in diameter between varicosities. No branches have been observed on any tangential fibers. The horizontal exogenous fibers in layer II number 6 to 8 to the unit. They are small and lightly stained,

LOBUS PARIETALIS resembling the tangential fibers in the inner one-half of layer I. In lamina I l i a the horizontal exogenous fibers number 8 to ro to the unit. They are slightly larger and more darkly stained than the horizontal fibers in layer II. The horizontal exogenous fibers gradually become larger and more darkly stained in the successively deeper laminae of the cortex, and are largest and darkest in lamina VIb and in the layer of subcortical association fibers. The horizontal fibers number 10 to 12 to the unit in lamina I l l b , and 12 to 14 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina Va the horizontal fibers number 10 to 12 to the unit, and in the inner band of Baillarger in laminae Vb and V i a they average 14 to 16 to the unit. The horizontal exogenous fibers in lamina VIb number 12 to 14 to the unit, and the subcortical association fibers number 20 to 22 to the unit. The horizontal exogenous fibers and the subcortical association fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. The largest horizontal exogenous fibers in lamina VIb and the largest subcortical association fibers measure 1.3/1 in diameter between varicosities. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers in the core of gyrus centralis posterior immediately subjacent to area P C number 30 to 35 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all the fibers bear varicosities, spines, and thorns. The largest fibers measure ι.ζμ in diameter between varicosities, but only a few of the fibers are as large as this. As the vertical exogenous fibers radiate toward the cortex they are chiefly collected in bundles. Many of the radiating fibers end in the midregion of layer VI, some end in the neuropil in the inner band of Baillarger and a few end in the outer band, ascending as high as the lower part of lamina IIIc. The vertical exogenous fibers number 12 to 14 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 4 or 5 in the lower

85

part of layer IV. All of the vertical fibers decrease in size as they ascend, and all end as straight fibers directed vertically. No terminal or collateral branches have been observed on any of the vertical exogenous fibers. MYELiNATioN. The following description is based upon the Weigert sections of brain 37-82. The stain in the sections of this brain is much darker than that in the sections of brain 33-197, the three-month brain with the most myelin in this region of area PC. The stain in the sections of brain 37-82 in the region of the head is lighter than that in the region of the lower extremity. The stain is lighter in the region of the head in area PC than it is in the same region in area PB. No stained tangential fibers are present in layer I. No stained horizontal fibers are present in the cortex external to lamina Via. In this lamina a small, lightly stained horizontal fiber is seen occasionally. In lamina VIb there are 3 or 4 small, lightly stained fibers to the unit. Stained subcortical association fibers number 6 or 7 to the unit. They are much smaller and lighter in color than the stained subcortical association fibers in area PB. The stained vertical fibers in the core of the gyrus subjacent to area P C are less numerous, smaller, and lighter in color than the stained vertical fibers subjacent to area PB. Immediately below the point where they begin to radiate into the cortex in area PC the stained vertical fibers number 14 to 16 to the unit. The fibers are small, intermediate, and large in size, and all of them bear varicosities. The largest stained vertical fibers measure τ.ζμ in diameter between varicosities. The stained vertical fibers decrease in size and number, and become lighter in color as they radiate toward and into the cortex. On most of the ascending fibers the stain ends in the region of the subcortical association fibers. No stained vertical fibers are present external to lamina VIb.

86

THE CORTEX LOBULUS

OF T H E

PARIETAL]

THE CORTEX in the crown on the superior surface of the superior parietal lobule measures 2.219 m m width. It is illustrated in Figures 97, 98, 99, and 100. CELL-BODIES

Layer I. This layer measures 0.204 mm. in width, and contains 47 neurons to the unit. A large horizontal cell is seen occasionally. Layer II. The outer granular layer is visible macroscopically. It measures 0.107 mm. in width. The neurons number 259 to the unit and measure 4/4&/6μ.

Layer III. This layer measures 0.679 m m · i n width. In lamina I l i a the neurons number 58 to the unit and measure 8/6-20/ιομ,. In lamina 111b the nerve cells number 47 per unit, and measure 10/6-26/15/x. Lamina IIIc contains 51 neurons to the unit, and the cells range in size from το/6μ to 30/15/a. Layer IV. The inner granular layer is more prominent than the outer one. The layer measures 0.224 mm. in width. The cells number 163 to the unit, and range in size from 6/4/x to ιο/6μ. A few neurons belonging to laminae IIIc and Va invade the granular layer. Layer V. The inner pyramidal layer measures 0.480 mm. in width. Small and large neurons are scattered throughout the width of the layer. The extra-large pyramidal cells are located principally in lamina Va, but some are present in lamina Vb. The small neurons number 56 to the unit in lamina V a and 50 in lamina Vb. The large neurons number 17 to the unit in each lamina. The extra-large pyramidal cells number 9 to the unit, and range in size from 28/12/u to 39/13/11. The small neurons measure 8/6-13/ιομ, and the large ones measure 16/10-26/13/^. Layer VI. This layer measures 0.525 mm. in width. The neurons number 78 to the unit in lamina V i a and 44 per unit in lamina VIb. The pyramidal cells measure 10/6-29/15/n, and the spindle cells measure ι6/6-26/1ομ. The extra-large pyramidal cells in layer V are stained but slightly more darkly than the other large cells in layers V and VI, but they are not any darker in color than the large cells in lamina IIIc. The extra-large pyramidal cells contain more chromophil substance in proportion to the size of the nucleus than

SIX-MONTH

INFANT

SUPERIOR, AREA PE

any other cells in this area of the cortex with the exception of the large cells in lamina IIIc. The chromophil substance is greater in quantity and more darkly stained in some of the large cells in lamina IIIc than in the majority of the extra-large pyramidal cells in layer V. The chromophil substance in the cell-bodies and apical dendrites of the extra-large pyramidal cells in layer V is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the three-month cortex. In the six-month cortex the chromophil substance in these cells is less in quantity and less advanced in differentiation than that in corresponding cells in area P C . The chromophil substance in the extralarge pyramidal cells in area P E is chiefly in the form of very fine granules and large clumps. A few small clumps and Nissl flakes are also present. A darkly stained cone of chromophil substance rests upon the nuclear membrane immediately under the base of the apical dendrite. The chromophil substance extends into the apical dendrite in much less quantity and for a much shorter distance than it does in the extra-large pyramidal cells in layer V in area P C . In the apical dendrites the chromophil substance is chiefly in the form of very fine granules, with a few small clumps and elongated strands. A small, dark clump indicates the point of origin of each of the 3 or 4 most proximal collaterals, but no collaterals are stained. The basal dendrites are stained for a short distance, and each one contains a few granules of chromophil substance. The large ordinary pyramidal cells in layers V and V I and the large spindle cells in V I contain less chromophil substance in proportion to the size of the nuclei than the extra-large pyramidal cells in layer V. The chromophil substance is chiefly in the form of large clumps, but small clumps, granules and a few Nissl flakes are also present. Granules and small clumps of chromophil substance invade the apical dendrites in less quantity and for a shorter distance than in the extra-large pyramidal cells. Basal dendrites are very faintly stained for a very short distance from the cell-body. The chromophil substance in these cells is less in quantity and less advanced in differentiation than that in corresponding cells in area P C in the six-month cortex, but it is greater in quantity and more advanced in differentiation than that in

LOBUS PARIETALIS corresponding cells in area PE in the cortex of the three-month infant. The large cells in lamina IIIc are as darkly stained as the extra-large pyramidal cells in layer V, and some of them are darker in color than the latter. Collectively, the large cells in lamina IIIc are conspicuous because of their dark color. Scattered large cells in laminae I l l b and I l i a are stained as darkly as the large cells in IIIc. The largest cells in IIIc contain more chromophil substance in proportion to the size of the nucleus and are more darkly stained than most of the extra-large pyramidal cells in layer V. The chromophil substance is predominantly in the form of large clumps, but granules, small clumps and a few Nissl flakes are also present. The apical dendrites of the largest cells are invaded by the chromophil substance in greater quantity and for a greater distance than those of the extra-large pyramidal cells in layer V. A dark clump of chromophil substance rests upon the nuclear membrane immediately below the base of the apical dendrite. No collaterals of the apical dendrite are stained, but the point of origin of each of the most proximal ones is indicated by a dark clump of chromophil substance. Basal dendrites are stained more frequently and contain more chromophil granules than those of extra-large pyramidal cells in layer V. The amount of chromophil substance in proportion to the size of the nucleus decreases in quantity as the cells become smaller toward layer II. The chromophil substance in all cells in layer III is greater in quantity and more advanced in differentiation than that in corresponding cells in area PE in the three-month cortex. In the six-month brain the chromophil substance in all cells in layer III in area PE is less in quantity and less advanced in differentiation than in corresponding cells in area PC. The small cells in layer IV have more chromophil substance and are more darkly stained than the small cells in layer II. In all layers the small cells have less chromophil substance in proportion to the size of the nucleus than large cells have. The chromophil substance in the small cells in layers II and IV is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the three-month cortex. In the six-month cortex the chromophil substance in the cells in layers II and IV is less in quantity than that in corresponding cells in area PC.

87

Granules, neurofibrils, and longitudinal rows of granules are present in the extra-large pyramidal cells in layer V. The neurofibrils are less in quantity and less distinctly formed than those in corresponding cells in the region of the trunk and arm in area PC. The neurofibrils are more evident in the apical and basal dendrites than in the cell-bodies. The dendrites and axons are more darkly stained than the cellbodies. Irregularly scattered granules predominate in the large ordinary pyramidal cells in layers V and V I and in the large pyramidal cells in lamina IIIc. A few neurofibrils and longitudinal rows of granules are present, and these are less in quantity than those in the extra-large pyramidal cells in layer V. The axons and dendrites of these cells are stained more darkly than the cell-bodies. In all other cells in this area the cytoplasm is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. Differentiation of neurofibrils is more advanced than in this area in the cortex of the three-month infant. In the six-month cortex differentiation of neurofibrils is less advanced in area PE than in the region of the shoulder and arm in area PC. The apical dendrites of the extra-large pyramidal cells in layer V are more compact in structure, somewhat larger, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are slightly more numerous on the dendrites than in the three-month cortex. In the six-month cortex the dendrites of the extralarge pyramidal cells are smaller and have fewer pedunculated bulbs than those of corresponding cells in area P C in the region of the lower extremity. The axons of these cells are slightly larger and have more myelin than in the three-month cortex, but they are smaller and have less myelin than those in area P C in the region of the lower extremity in the six-month cortex. FIBERS.

The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are larger, more compact in structure, and have more

88

THE CORTEX OF THE SIX-MONTH INFANT

pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the cortex of the six-month infant the dendrites of these cells in this area are smaller and have fewer pedunculated bulbs than those of corresponding cells in area PC in the region of the lower extremity. In area PE pedunculated bulbs are less numerous on the dendrites of these cells than on the dendrites of the extra-large pyramidal cells. The axons of the large ordinary and special pyramidal and spindle cells in layers V and V I are smaller and have less myelin than those of the extra-large pyramidal cells in layer V. The presence of myelin is indicated by a slight increase in size of the axon a short distance distal to its origin and by a few globules on the surface. The axons of the large ordinary and special pyramidal and spindle cells have more myelin than those of corresponding cells in this area in the three-month cortex. The apical dendrites of the pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and more compact in structure, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer III than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the cells in layer III in this area are smaller and shorter and have fewer pedunculated bulbs than those of corresponding cells in the region of the lower extremity in area PC. Pedunculated bulbs are present in less quantity on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layer V. Pedunculated bulbs are less numerous on the dendrites of the pyramidal cells in lamina I l l b than on those of cells in IIIc, and the bulbs are even less numerous on the dendrites of the pyramidal cells in lamina I l i a . The bulbs are less numerous on the terminal branches of the apical dendrites of the pyramidal cells in lamina I l i a than they are on the terminal branches of the apical dendrites of the cells in lamina I l l b .

The axons of the large pyramidal cells in lamina IIIc have a slight amount of myelin. No indications of myelin are present on the axons of pyramidal cells in laminae I l l b or I l i a . The dendrites of the small cells of layers IV and II are more compact in structure and have more pedunculated bulbs than those of corresponding cells in this area in the cortex of the three-month infant. The dendrites of the cells of layer II are smaller and have fewer pedunculated bulbs than those of layer IV. Bulbs are less numerous on the terminal branches of the apical dendrites of the cells belonging to layer II than on the terminal branches of the apical dendrites of the pyramidal cells of lamina I l i a . The dendrites of the Golgi type II cells are in about the same state of development as to size and length, and as to quantity of pedunculated bulbs, as the dendrites of pyramidal cells of comparable size. The mesh of fibers formed by the axons of the Golgi type II cells is but slightly more dense than that in this area of the three-month cortex. In the six-month cortex the mesh is more dense than that in the region of the lower extremity in area PC. A horizontal cell of Cajal is rarely seen in layer I. One cell appears to be in good condition. No long branches have been observed on any tangential fiber in layer I, but a short branch is occasionally present. The tangential fibers in layer I in the crown of the supero-lateral surface of gyrus parietalis superior number 14 to 16 to the unit. They are small, intermediate, and large in size, and are faintly stained. The fibers are larger, more numerous, and more darkly stained in the outer one-half of layer I than in the inner one-half. In the walls of sulci the tangential fibers are larger and more darkly stained, and are slightly more numerous. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The largest fibers measure ι.ομ in diameter between varicosities. No branches have been observed on any tangential fibers. Horizontal exogenous fibers in layer II number 2 or 3 to the unit, and they are so small and faintly stained that they are barely visible with the lowpower objective (No. 10). Proceeding inwardly the horizontal exogenous fibers gradually become larger and more darkly stained, and are largest and darkest

LOBUS

PARIETALIS

in lamina V I b and in the layer of subcortical association fibers. T h e horizontal fibers number 4 or 5 to the unit in lamina I l i a , and 8 to 10 in lamina I l l b . In the outer band of Baillarger in lamina I I I c and layer I V the horizontal exogenous fibers number 10 to 12, and in lamina V a they number 8 to 10 to the unit. T h e horizontal fibers number xo to 12 to the unit in the inner band of Baillarger and in lamina V I b . T h e subcortical association fibers number 14 to 16 to the unit. T h e horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. These fibers are not as large or as darkly stained as corresponding fibers in area P C in the region of the lower extremity. T h e largest fibers in lamina V I b and the largest association fibers measure τ.ομ in diameter between varicosities. T h e vertical exogenous fibers in the core of the gyrus immediately subjacent to the crown number 30 to 35 to the unit. T h e y are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. T h e largest fibers measure τ.ομ in diameter between varicosities. T h e vertical fibers are chiefly collected in bundles as they radiate toward and into the cortex. T h e y decrease in size and number as they ascend in the cortex. M a n y of the ascending vertical fibers end in the middle of lamina V I b , some end in the neuropil in the inner band of Baillarger, and only a very few reach layer I V . T h e ascending fibers number 8 to 10 to the unit in the lower part of lamina V I b , 4 to 6 in the lower part of lamina V b , and 2 or 3 in the lower part of layer I V . N o terminal or collateral branches have been observed on any vertical exogenous fibers. T h e brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 3 7 - 8 2 , (2) 38-35, (3) 37-49. T h e stain in the sections of MYELINATION.

LOBULUS T H E SAME

brain 37-82 is much darker than that in the sections of brain 37-49. T h e stain in the sections of brain 37-49 is very much darker and more extensive than that in 3 3 - 1 9 7 , the three-month brain with the most myelin in this area. T h e stain in the sections of each six-month brain is lighter in area P E than it is in the region of the lower extremity in area P B or P C . T h e following description is based upon the sections of brain 37-82. Deep in the wall of a sulcus a small, lightly stained tangential fiber is seen occasionally in layer I. N o stained tangential fibers are present in the crowns of gyri. There are no stained horizontal fibers in the cortex external to lamina V I b . In the inner part of this lamina there are from 1 to 3 small, lightly stained horizontal fibers to the unit. Stained subcortical association fibers number 5 or 6 to the unit. T h e y are small, intermediate, and large in size, and all bear varicosities. T h e y are much smaller and more lightly stained than the stained subcortical association fibers in area P C in the region of the lower extremity in the superior part of gyrus centralis posterior. T h e stained vertical fibers are most numerous, largest, and darkest in the cores of the gyri. Immediately below the place where they begin to radiate toward the cortex the stained fibers number 16 to 18 to the unit. T h e y are small, intermediate, and large in size, and all bear varicosities. T h e y are more lightly stained than the corresponding fibers in the region of the lower extremity in area P C . T h e largest stained vertical fibers measure τ.τ μ in diameter between varicosities, but fibers of this size are few in number. A s the stained vertical fibers radiate toward and into the cortex they decrease in size and number, and become lighter in color. On most of the fibers the stain ends in the region of the subcortical association fibers. On some fibers the stain ends in the lower part of lamina V I b , and on a very few fibers it ends in lamina V i a .

PARIETALIS

areas of the cortex in the inferior parietal

89

INFERIOR

gyrus angularis. T h e cortex in these areas has the

lobule are selected for description as in the three-

same characteristics as in the brain of the three-

month brain, namely, gyrus supramarginalis and

month infant.

THE CORTEX OF THE SIX-MONTH INFANT

90

GYRUS SUPRAMARGINALIS, AREA PF The cortex in the crown of this gyrus measures 2.173

mm

· in width. It is illustrated in Figures 101,

102, 103, and 104. CELL-BODIES

Layer I. This layer measures 0.242 mm. in width, and contains 44 neurons to the unit. Layer II. The external granular layer measures 0.132 mm. in width. It is visible macroscopically. The neurons number 304 to the unit and measure 4 / 4 η/6μ. Layer III. The external pyramidal layer measures 0.655 m m · i n width. In lamina I l i a the neurons number 60 to the unit and measure 10/6-20/ιομ. In I l l b the neurons number 50 per unit and measure 10/626/τομ. The neurons in lamina IIIc number 51 to the unit and range in size from 10/6μ to 29/13^. Layer IV. The inner granular layer is more prominent than the outer one. The layer is invaded by only a few neurons belonging to laminae IIIc and Va. The boundaries of the layer are fairly distinct, but they are irregular. In the widest places the layer measures 0.204 mm. in width. The neurons number 175 per unit and measure 6/4-8/6/*. Layer V. This layer measures 0.412 mm. in width. Small and large neurons are scattered throughout the layer. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb. The small cells number 64 to the unit in Va and 52 in Vb. The large cells number 16 per unit in Va and 17 in Vb. The extra-large pyramidal cells number 9 to the unit. The small neurons measure 8/613/ιομ, the large ones measure 16/10-26/12/11, and the extra-large pyramidal cells range in size from 29/12/i to 36/12^,. Layer VI. This layer measures 0.528 mm. in width. The neurons number 78 to the unit in lamina V i a and 47 in lamina V l b . The pyramidal cells measure 10/6-23/13/*, and the spindle cells measure 16/426/10/4. The chromophil substance in the cells in this area is in approximately the same state of development as that in corresponding cells in area P E . The large pyramidal cells in lamina IIIc are conspicuous because of their dark stain. The chromophil substance

in the large cells in layers III, V, and V I is chiefly in the form of large clumps, but small clumps, granules, and a few Nissl flakes are also present. In all cells the chromophil substance is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the three-month cortex. A few longitudinal rows of granules are present in the extra-large pyramidal cells in layer V in the Cajal sections, and an occasional neurofibril is seen in the apical dendrite of one of these cells. The dendrites and axons are stained slightly more darkly than the cell-bodies. Some of the largest pyramidal cells in layers V and V I and in lamina IIIc have a few longitudinal rows of granules in the dendrites. The axons and dendrites are stained slightly more darkly than the cell-bodies. In all other cells the cytoplasm is filled with granules which are scattered irregularly throughout the cell-bodies and their processes. Differentiation of neurofibrils is more advanced in this area than it is in the same area in the cortex of the three-month infant, but it is less advanced than in area P E in the six-month cortex. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches than those of corresponding cells in this area in the threemonth cortex. The basal dendrites and the collateral branches of the apical dendrites are larger and more compact in structure, but no more numerous than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the cortex of the three-month infant. In the six-month cortex these cells are in about the same state of development as the corresponding cells in area PE, but they are smaller and have fewer pedunculated bulbs than the corresponding cells in the region of the hand in area P C . FIBERS.

The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The collateral branches of the axons are larger, but no more numerous, than in the threemonth cortex. In the six-month cortex the axons of these cells are in about the same state of development

LOBUS PARIETALIS as those of corresponding cells in area PE, but are smaller and have less myelin than those in the region of the hand in area PC. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The apical dendrites of the pyramidal cells in layer I I I are larger, have more terminal branches in layer I, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of these cells are in about the same state of development as those of corresponding cells in area PE, but are not as advanced in development as those in area PC in the region of the hand. The axons of the large pyramidal cells in lamina IIIc are larger and have more myelin than those of the large pyramidal cells in lamina Illb. The axons of the larger pyramidal cells in lamina I l i a have no indications of myelin. The axons of the pyramidal cells in layer I I I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of the pyramidal cells in layer I I I in this area are in about the same state of development as those of corresponding cells in area PE. The dendrites of the small pyramidal cells belonging to layer IV are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the small pyramidal and granular cells in layer I I are smaller and have fewer pedunculated bulbs than the small pyramidal cells in layer IV. The dendrites of the small cells in layer I I are coarser and have more pedunculated bulbs than those of corresponding cells in this area in the cortex of the three-month infant. The small cells of layers II and IV in this area are in about the same state of development as the corresponding cells in area PE. The dendrites of Golgi type I I cells are in about the same state of development as to size and length and as to the number of pedunculated bulbs as those of pyramidal cells of comparable size. The mesh of fibers which is formed by the axons of these cells is

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more extensive and more dense than that in area P E and is much more dense than the mesh in the region of the hand in area PC. In area P F the axons of Golgi type I I cells do not form a mesh in layer II. A very thin mesh is present in lamina I l l b . The mesh increases gradually in density in lamina IIIc and reaches a maximum in layer IV. It decreases in density in lamina Va, then increases in lamina Vb and in layer VI, where it is less dense than in layer IV. The mesh of fibers formed by the axons of Golgi type I I cells is slightly more dense than that in this area in the cortex of the three-month infant. Rarely a horizontal cell of Cajal is seen in layer I. No long branches have been observed on any tangential fibers in layer I, but short branches are occasionally seen. No inverted pyramidal cells have been observed in any part of the cortex in this area. The tangential fibers in layer I are larger, more numerous, and more darkly stained in the walls of sulci than in the crown of gyrus supramarginalis. In the walls of sulci they number 10 to 12 to the unit. They are larger, more numerous, and more darkly stained in the middle of layer I than in the outer and inner margins. The fibers are small, intermediate, and large, and those of each category of size are smaller and more lightly stained than the tangential fibers in layer I in the crown of the gyri on the supero-lateral surface of the superior parietal lobule. In the Golgi preparations all the fibers bear varicosities, spines, and thorns. The largest fibers measure 0.75/A in diameter between varicosities. No branches have been observed on any tangential fibers. No horizontal exogenous fibers are present in layer II, and a single very small fiber is seen only occasionally in lamina I l i a . The horizontal fibers number 4 or 5 to the unit in lamina I l l b and 6 to 8 in the outer band of Baillarger in lamina IIIc and layer IV. The fibers are small and lightly stained. In lamina Va they number 8 to 10 to the unit. They are larger and more darkly stained than those in layer IV. The horizontal exogenous fibers number 10 to 12 to the unit in the inner band of Baillarger and in lamina VIb. The horizontal exogenous fibers in layer VI are larger and more darkly stained than those in lamina

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THE CORTEX OF THE SIX-MONTH INFANT

Va. The subcortical association fibers number 12 to 14 to the unit. They are of about the same size and density of stain as the horizontal fibers in lamina VIb. The horizontal fibers are small, intermediate, and large in size, and all of them bear varicosities, spines, and thorns in the Golgi preparations. The fibers of each category of size are not as large or as darkly stained as those in the superior parietal lobule or as those in area PC in the region of the hand. The largest horizontal exogenous fibers in lamina VIb measure 0.9/u, and the largest subcortical association fibers measure 1.0μ in diameter between varicosities. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers number 35 to 40 to the unit in the core of gyrus supramarginalis immediately below the point where the fibers begin to radiate toward the cortex. The fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. The largest fibers measure 0.9/A in diameter between varicosities, but fibers as large as this are few in number. As the vertical fibers radiate toward and into the cortex they are chiefly collected in bundles. Each bundle contains about 4 to 8 fibers, and the fibers are usually small, intermediate, and large in size. Many of the radiating vertical fibers end in the lower part of lamina VIb, some end in layer V, and a few end in the lower part of layer IV. The vertical exogenous fibers number 12 to 14 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 4 to 6 in the lower part of layer IV. All of the ascending fibers decrease in size as they ascend, and all end as straight, vertically directed fibers without any terminal branching. No collateral branches have been observed on any vertical exogenous fibers. MYELiNATioN. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 3 7 - 8 2 , ( 2 ) 38-35, (3) 37-49· The stain in the sections of brain 37-82 is slightly darker than that in the sections of brain 38-35. The stain in the sections of brain 37-49 is much lighter and less extensive than that in the sections of brain 38-35. The stain in the sections of brain 37-49 is but slightly darker and more extensive than that in the sections of brain

34-188, the three-month brain with the most myelin in this area. The stain in the sections of each sixmonth brain is slightly darker and more extensive in area P F than it is in area PE. In each brain the stain is lighter and less extensive in area P F than that in areas PB and P C in the region of the hand. The following description is based upon the sections of brain 37-82. In layer I in the walls of sulci there are from 1 to 3 small, lightly stained tangential fibers to the unit. No stained tangential fibers are present in the crowns of the gyri. A small, lightly stained horizontal fiber is seen occasionally in layer IV, but no stained horizontal fibers are present external to this layer. In lamina Va stained horizontal fibers number 2 or 3 to the unit, and they are small and light in color. In laminae Vb and V i a (the inner band of Baillarger) stained horizontal fibers number 4 or 5 to the unit, and they are larger and darker than those in lamina Va. In lamina VIb the stained horizontal fibers number 5 or 6 to the unit, and they are larger and darker than those in lamina Via. The stained subcortical association fibers are larger and darker than the stained horizontal fibers in lamina VIb. They number 8 to 10 to the unit, are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers are most numerous, largest, and darkest in the core of the gyrus. They are more lightly stained than the stained vertical fibers in the core of gyrus centralis posterior in the region of the hand. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 20 to 22 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure ι.ομ in diameter between varicosities. As they radiate toward and into the cortex the stained vertical fibers decrease in number and size, and become lighter in color. The stain ends on most of the fibers in the lowermost part of layer VI, on some fewer fibers it ends in the middle of this layer, and on a very few vertical fibers the stain continues to lamina Via. GYRUS ANGULARIS, AREA PG

The cortex in this area closely resembles that in area PF. On the crown of the gyrus the cortex meas-

LOBUS PARIETALIS ures 2.116 mm. in width. It is illustrated in Figures 105, 106, 107, and 108. CELL-BODIES

Layer I. The molecular layer measures 0.212 mm. in width, and contains 47 neurons to the unit. A horizontal cell of Cajal is seen occasionally. Layer II. The external granular layer measures ο. 115 mm. in width, and contains 287 neurons to the unit. The neurons measure 4/4-9/6/*. The outer margin of the layer is fairly sharp, but the inner boundary is indistinct because the small cells of the granular layer intermingle with the cells of lamina I l i a . Layer III. This layer measures 0.628 mm. in width. The neurons number 62 to the unit in lamina I l i a and measure 10/6-23/ιομ. In lamina I l l b the neurons number 49 to the unit and measure 10/629/12/1. In lamina IIIc the neurons number 54 per unit and range in size from 10/6/1 to 29/13//.. Layer IV. The internal granular layer is more prominent than the external one. Very few cells of laminae IIIc and Va invade the layer. The layer measures 0.200 mm. in the widest places. The neurons number 179 to the unit, and they measure 6/4-9/6//,. Layer V. This layer measures 0.437 m m · i n width. The small neurons number 53 to the unit in lamina Va and 48 in lamina Vb. The large neurons number 16 to the unit in each lamina. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb. These cells number 9 to the unit, and range in size from 29/10/4 to 36/13/1. The small neurons measure 8/6-13/1 o/i, and the large ones measure 16/10-25/12/1. Layer VI. The layer of fusiform cells measures 0.524 mm. in width. The neurons number 77 to the unit in lamina V i a and 45 in lamina VIb. The pyramidal cells measure 10/6—26/13/1, and the spindle cells measure 16/8-2 6/1 $μ. The state of development of the chromophil substance in the cells in this area is approximately the same as in area PF. The large cells in lamina IIIc are conspicuous because of the dark stain. The chromophil substance is greater in quantity and more advanced in differentiation in all cells than in corresponding cells in this area in the three-month cortex. In the Cajal sections a few longitudinal rows of granules are present in the extra-large pyramidal cells

93

in layer V, and occasionally one or two neurofibrils can be identified in one of the largest of these cells. The axons and dendrites are stained slightly more darkly than the cell-bodies. The processes of the large ordinary pyramidal cells in layers V and VI and in lamina IIIc are stained only a little more darkly than the cell-bodies. No longitudinal rows of granules or neurofibrils are present in any cells in this area other than the extra-large pyramidal cells in layer V. The differentiation of neurofibrils in the cells in this region is more advanced than in corresponding cells in the same region in the three-month cortex, but less advanced than in region PF in the cortex of the six-month infant. FIBERS. The dendrites and axons of the cells in this area are in approximately the same state of development as corresponding cells in area PF. The dendrites are coarser, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in area PG in the three-month cortex. The axons of the pyramidal cells in layers III, V, and VI are larger and have more myelin than those of corresponding cells in the three-month cortex. The mesh of fibers formed by the axons of Golgi type II cells is more dense than that in area PG in the threemonth cortex. In the six-month cortex the mesh is approximately the same in density and extent as that in area PF.

No horizontal cells of Cajal have been observed in layer I in this area. Short branches are occasionally seen on tangential fibers in layer I, but no long branches have been observed. The tangential fibers are slightly more numerous and much more darkly stained in the walls of sulci than in the crown of gyrus angularis. In the walls of sulci the tangential fibers number 16 to 18 to the unit. The are small, intermediate, and large in size, and in Golgi preparations all the fibers bear varicosities, spines, and thorns. The fibers are more numerous, larger, and more darkly stained in the outer one-half of layer I than in the inner one-half. The largest fibers in the Golgi preparations measure 0.9/i in diameter between varicosities, but the majority of the large fibers are slightly smaller than this. No horizontal exogenous fibers are present in layer

94

T H E C O R T E X OF T H E SIX-MONTH I N F A N T

II. These fibers number 2 or 3 to the unit in lamina I l i a , and 3 or 4 in lamina I l l b . T h e y are very small and lightly stained in I l i a but are slightly larger and darker in I l l b . In the successively deeper laminae the horizontal exogenous fibers gradually become larger and more darkly stained. T h e horizontal fibers number 8 to 10 to the unit in the outer band of Baillarger in lamina I I I c and layer I V , 8 to 10 in lamina Va, 12 to 14 in the inner band of Baillarger, and 10 to 12 in lamina V I b . T h e subcortical association fibers number 10 to 12 to the unit. T h e horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. T h e fibers are smaller and more lightly stained than the corresponding fibers in area P C in the region of the head. T h e y are of about the same size and density of stain as the corresponding fibers in area P F . T h e largest fibers in lamina V I b and the largest subcortical association fibers in the Golgi preparations measure ι.ομ in diameter between varicosities. N o branches have been observed on any of these fibers. T h e vertical exogenous fibers number 35 to 40 to the unit in the core of the gyrus immediately before the fibers begin to radiate toward the cortex. The fibers are small, intermediate, and large in size, and in the Golgi preparations all bear varicosities, spines, and thorns. T h e largest fibers measure ι.ομ in diameter between varicosities, but most of the large fibers are somewhat smaller. A s the vertical fibers radiate toward the cortex they are chiefly collected in bundles containing about 4 to 8 fibers which are small, intermediate, and large in size. M a n y of the ascending fibers end in lamina VIb, some end in the inner band of Baillarger, and a few end in the outer band. T h e ascending fibers number 16 to 18 to the unit in the lower part of lamina V I b , 8 to 10 in the lower part of lamina V b , and 4 to 6 to the unit in the lower part of layer I V . A very few fibers end in the lower part of lamina IIIc. T h e vertical exogenous fibers decrease in size as they ascend, and are very small and lightly stained at their ends. All of them end as straight vertical fibers without any terminal branches. N o collateral branches have been observed on any vertical exogenous fibers.

MYELiNATioN. T h e brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 37-82, (2) 38-35, (3) 37-49. T h e stain in the sections of brain 37-82 is much darker and more extensive than that in the sections of brain 37-49. T h e stain in the sections of brain 37-49 is distinctly darker and more extensive than that in the sections of brain 34-188, the three-month brain with the most myelin in this area. In each six-month brain the stain in the sections in this area is slightly lighter and less extensive than that in area P F . T h e following description is based upon the sections of brain 3 7-82. In the walls of sulci there are from 1 to 3 small, lightly stained tangential fibers to the unit in layer I. There are no stained tangential fibers in layer I in the crown of the gyrus. A very small, very lightly stained horizontal fiber is seen occasionally in layer I V . N o stained horizontal fibers are present in layer I I or I I I . In lamina V a stained horizontal fibers number 1 or 2 to the unit, and they are small and light in color. In laminae V b and V i a (the inner band of Baillarger) stained horizontal fibers number 2 or 3 to the unit, and they are slightly larger and darker than those in lamina Va. Stained fibers number 3 or 4 to the unit in lamina V I b , and they are darker and larger than those in lamina V i a . T h e stained subcortical association fibers number 5 or 6 to the unit, and they are larger and darker than the stained horizontal fibers in lamina V I b . T h e stained horizontal and subcortical association fibers in this area are smaller and lighter in color than corresponding fibers in area PF. T h e stained vertical fibers are largest, darkest, and most numerous in the core of the gyrus. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 20 to 22 to the unit. T h e y are small, intermediate, and large in size, and all bear varicosities. T h e largest stained vertical fibers measure ι.ομ in size. A s the stained vertical fibers radiate toward the cortex they decrease in size and number, and become lighter in color. T h e stain ends on most of the ascending fibers in the lower part of lamina V I b , on some fewer it ends in the middle of lamina V I b , and on very few it ends in lamina V i a .

LOBUS

PARIETALIS

OCCIPITO-TEMPOi

THE CORTEX in this area has the same characteristics as in the three-month brain. In the crown of the gyri the cortex measures 2.282 mm. in width. It is illustrated in Figures 109, n o , h i , and 112. CELL-BODIES

Layer I. This layer measures 0.232 mm. in width, and contains 49 neurons to the unit. Layer II. The outer granular layer is prominent. It measures 0.121 mm. in width and contains 259 neurons to the unit. The cells measure 4/4-7/5//,. Layer III. The external pyramidal layer measures 0.685 m m · i n width. The layer does not have a granular appearance. The neurons number 64 to the unit in lamina I l i a , and measure 10/6-20/ιομ. In lamina I l l b the neurons number 48 to the unit, and measure 10/6-26/13^. In lamina IIIc the neurons number 53 to the unit, and they range in size from ιο/6μ to 30/15/*· Layer IV. The internal granular layer is more prominent than the external one. The layer measures 0.207 mm. in width, and contains 196 neurons to the unit. The cells measure 6/4—8/6/J.. Very few large cells of laminae IIIc and V a invade the layer. Layer V. The internal pyramidal layer measures 0.517 mm. in width. Small and large neurons are present in all parts of the layer. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb. The small neurons number 61 in lamina Va and 51 in lamina Vb. The large nerve cells number 16 per unit in each lamina. The extra-large pyramidal cells number 10 to the unit, and they range in size from 28/12^ to 33/12/n. The small neurons measure 8/6-13/ιομ., and the large ones measure 16/10-26/12/11. Layer VI. This layer measures 0.520 mm. in width. The neurons number 83 to the unit in lamina V i a and 50 in lamina VIb. The pyramidal cells measure 10/6-2 5/13/x, and the spindle cells measure 16/826/10/x. The chromophil substance in the large cells in layers IIIc, V, and V I in this area is not as well developed as that in corresponding cells in area P G in the cortex of the six-month infant, but it is more ad-

95

L ZONE, A R E A P H

vanced in development than that in corresponding cells in area P H in the three-month cortex. The extra-large cells in layer V and the large ordinary pyramidal and spindle cells in V and V I are more darkly stained than the small cells in these layers. The large pyramidal cells in lamina IIIc are stained more darkly than other cells in this layer except a few scattered large cells in lamina I l l b . The chromophil substance in the large cells in layers III, V, and V I is chiefly in the form of large clumps; small clumps and Nissl flakes are not as numerous as in corresponding cells in area PG. Differentiation of neurofibrils in the cells in this area is in approximately the same state as in corresponding cells in area PG. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are coarser and longer, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in layer V in this area are smaller and have fewer pedunculated bulbs than those of corresponding cells in area PG. The axons of the extra-large pyramidal cells are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of the extra-large pyramidal cells in layer V in area P H are smaller and have less myelin than those of corresponding cells in area PG. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of these cells are much smaller and have fewer pedunculated bulbs than those of the extra-large pyramidal cells in layer V.

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THE CORTEX OF THE SIX-MONTH INFANT

T h e apical dendrites of the large pyramidal cells in layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. T h e basal dendrites and the collateral branches of the apical dendrites are larger and more compact in structure, but no more numerous, than those of corresponding cells in this area in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer I I I than on the dendrites of corresponding cells in area P H in the cortex of the three-month infant. In the six-month cortex the dendrites of the large pyramidal cells in layer I I I in area P H are in general smaller and have fewer pedunculated bulbs than those of corresponding cells in area P G . T h e apical dendrites of the largest pyramidal cells in lamina I I I c are larger, but have fewer pedunculated bulbs than the apical dendrites of the large ordinary and special pyramidal cells in layer V .

form a thin mesh of fibers in lamina I H b . T h e mesh increases slightly in density in lamina I I I c , becomes more dense in layer I V , decreases in density in lamina Va, and increases slightly in density in lamina V b and in layer V I . T h e mesh is more dense in layer IV than elsewhere. T h e mesh in this area is of about the same density as that in area P G . N o fibers are contributed to this mesh b y the vertical exogenous fibers. N o terminal or collateral branches have been observed on any of the latter.

T h e axons of the large pyramidal cells in layer I I I are larger than those of corresponding cells in this area in the three-month cortex. T h e axons of these cells in the three-month cortex have no myelin, but in the six-month cortex the axons of the large pyramidal cells in lamina I I I c are slightly enlarged a short distance distal to the cell-body and bear a few globules on the surface. T h e axons of these cells are much smaller and have very much less myelin than those of the extra-large pyramidal cells in layer V . T h e axons of the large pyramidal cells in lamina I H b decrease in size as they descend. A few globules on their surfaces are the only indications of the presence of myelin. There are no evidences of myelin on the axons of any pyramidal cells in lamina I l i a .

N o horizontal exogenous fibers are present in layer I I or in lamina I l i a . These fibers number 2 or 3 to the unit in lamina I H b , 5 or 6 in the outer band of Baillarger, 5 or 6 in lamina Va, 8 to 10 in the inner band of Baillarger, and 8 to 10 in lamina V I b . T h e subcortical association fibers number 12 to 14 to the unit. T h e horizontal exogenous fibers are smallest in lamina I H b and largest in lamina V I b . T h e fibers are small, intermediate, and large in size. T h e largest fibers in lamina V I b measure ι.ομ in diameter. T h e horizontal exogenous and subcortical association fibers are coarser and more darkly stained than corresponding fibers in this area of the cortex of the three-month infant.

T h e dendrites of the cells of layers I I and I V are coarser, longer, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. Only one horizontal cell of Cajal has been observed in layer I in this area. Occasionally a short branch is seen on a tangential fiber in layer I, but no long branches have been observed on any of these fibers. T h e dendrites of Golgi type I I cells are in about the same state of development as those of pyramidal cells of comparable size. T h e axons of these cells

T h e tangential fibers in layer I are larger, more darkly stained, and more numerous in the walls of sulci than in the crowns of gyri. In the walls they number 16 to 18 to the unit. T h e fibers are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. T h e largest fibers measure ι.ομ in diameter. The tangential fibers are coarser and more darkly stained than the tangential fibers in layer I in this region of the cortex of the three-month infant.

T h e vertical exogenous fibers in the cores of the gyri immediately below the point where they begin to radiate toward the cortex number 30 to 35 to the unit. T h e y are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. T h e largest fibers measure ι.ομ in diameter. T h e vertical exogenous fibers are coarser and more darkly stained than the corresponding fibers in this area in the cortex of the infant at the age of three months. In the six-month cortex they are in approximately the same state of development as the vertical exogenous fibers in area P G . T h e vertical fibers decrease in number and size as they

LOBUS PARIETALIS radiate into the cortex. Most of them end in lamina VIb, some ascend to the inner band of Baillarger, and a very few end in layer IV. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 3 7 - 8 2 , (2) 38-35, (3) 37-49. The stain in the sections of brain 38-35 is distinctly lighter and less extensive than that in brain 37-82. The stain in the sections of brain 37-49 is very much lighter and less extensive than that in brain 38-35, but is very much darker and more extensive than the stain in the sections of brain 33-197, the three-month brain with the most myelin in this area. In each of the six-month brains the stain in the sections in area P H is but slightly lighter and less extensive than that in the sections in area PG. The following description is based upon the sections of brain 37-82. MYELINATION.

From ι to 3 small, short, very lightly stained tangential fibers to the unit are present in layer I in the walls of sulci, but there are no stained tangential fibers in this layer in the crowns of gyri. No stained horizontal fibers are present in the cortex external to layer V. A small, very lightly stained horizontal fiber is seen occasionally in lamina Va. In laminae Vb and V i a (the inner band of Baillarger) stained horizontal fibers number 1 to 3 to the

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unit, and they are larger and darker than those in lamina Va. Stained horizontal fibers number 3 or 4 to the unit in lamina VIb, and they are larger and darker than those in lamina V i a . The stained subcortical association fibers number 5 to 7 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal fibers in the cortex and the stained subcortical association fibers are about the same size and color as the corresponding fibers in area PG. They are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers are most numerous, largest, and darkest in the cores of gyri. They are slightly lighter in color than the stained vertical fibers in the core of gyrus angularis, area PG. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 20 to 22 to the unit. The stained fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure ι.ομ in diameter between varicosities. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number and become lighter in color. The stain on most of the ascending fibers ends in the lower part of lamina VIb, on a few it ends in the middle of lamina VIb, on a very few it ends in lamina V i a .

LOBUS OCCIPITALIS AREA PERISTRIATA, OA THE CORTEX in this area has the same characteristics and distribution as in the three-month brain. On the crown of the cuneus it measures 1.912 mm. in width. It is illustrated in Figures 113, 114, 115, and 116. CELL-BODIES

Layer I. The molecular layer measures 0.191 mm. in width and contains 50 neurons to the unit. Layer II. The outer granular layer is clearly visible macroscopically. The layer measures 0.102 mm. in width. The neurons number 304 to the unit, and measure 4/4-7/6μ. The external margin of the layer is fairly sharp, but internally the small cells of layer II intermingle freely with the larger cells of lamina I l i a . Layer III. This layer measures 0.587 mm. in width. In lamina I l i a the neurons number 74 to the unit, and measure 8/6-20/τομ. In lamina I l l b the neurons number 60 to the unit, and measure 10/623/ιομ. The neurons in lamina IIIc number 62 to the unit, and range in size from ιο/6μ to 29/16^. Layer IV. The internal granular layer is more prominent than the external one. The layer measures 0.200 mm. in width. The neurons number 187 to the unit, and measure 6/4-8/6/*. Only a few neurons of laminae IIIc and Va invade the granular layer. Layer V. This layer measures 0.382 mm. in width. Small and large neurons are scattered throughout the layer. The extra-large neurons are located chiefly in lamina Va, but some are present in lamina Vb. The small neurons measure 8 / 6 - 1 3 / ι ο μ , and number 68 to the unit in lamina V a and 51 in lamina Vb. The large neurons measure 16/10-23/12^, and number 17 in Va and 16 in Vb. The extra-large pyramidal cells number 6 to the unit, and range in size from 20/ΐ2μ, to 39/12/it. Layer VI. This layer measures 0.450 mm. in width. The neurons number 79 to the unit in lamina V i a and 47 in lamina VIb. The pyramidal cells measure 10/6-29/1 $μ, and the spindle cells measure 12/429/ιομ. The extra-large pyramidal cells in layer V, the large ordinary pyramidal cells in layers V and V I , the large spindle cells in V I , and the large pyramidal cells

in lamina IIIc are all stained about the same color, and are much darker than other cells except scattered large cells in lamina I l l b . Some of the largest cells in lamina IIIc are larger than the largest extra-large pyramidal cells in layer V. The chromophil substance in the largest cells in IIIc is greater in quantity and extends farther into the apical dendrites than that in the extra-large pyramidal cells in layer V . The state of development of the chromophil substance in the large ordinary pyramidal and spindle cells in layers V and V I is approximately the same as in corresponding cells in areas P G and PH. The chromophil substance is chiefly in the form of large clumps, but some small clumps and a few Nissl flakes are present. The chromophil substance in the extralarge pyramidal cells in layer V and in the large pyramidal cells in laminae IIIc and I l l b is greater in quantity and more advanced in development than in corresponding cells in areas P G and P H . In these cells the chromophil substance is predominantly in the form of large clumps, but small clumps and Nissl flakes are more abundant in the cell-bodies and in the apical dendrites than in corresponding cells in areas P G and PH. In the six-month cortex the chromophil substance is greater in quantity and more advanced in differentiation in all cells in area OA than in corresponding cells in this area in the three-month cortex. N o neurofibrils are present in any cells in the Cajal sections in this area. The axons and dendrites of the extra-large pyramidal cells in layer V are stained slightly more darkly than the cell-bodies. A few longitudinal rows of granules are present in the dendrites of these cells, but the granules in the cytoplasm are chiefly scattered irregularly throughout the cellbodies, axons, and dendrites. The cytoplasm in all other cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and processes. FIBERS. The dendrites of the extra-large pyramidal cells are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The axons of these cells are larger and have much more

LOBUS OCCIPITALIS myelin than those of corresponding cells in this area in the cortex of the three-month infant. In the sixmonth cortex the dendrites of the extra-large pyramidal cells in area OA have more pedunculated bulbs and the axons have more myelin than those of corresponding cells in the region of the head in area P C . The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are coarser, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the sixmonth cortex the apical dendrites are much smaller in caliber, and all the dendrites have fewer pedunculater bulbs than those of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells in layers V and V I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are much smaller and have much less myelin than those of the extra-large pyramidal cells in layer V. A slight enlargement and many small beads constitute the indications of myelin on the axons of these cells. The dendrites of these cells have more pedunculated bulbs and the axons have more myelin than those of corresponding cells in area P C in the region of the head. The dendrites of the pyramidal cells in layer I I I are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the sixmonth cortex the dendrites of the large pyramidal cells in lamina I I I c are larger, but have fewer pedunculated bulbs, than those of the large ordinary and special pyramidal cells in layer V. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer I I I in area OA than on the dendrites of corresponding cells in area PC. A slight enlargement distal to the cell-body and a few beads on the axons of the large pyramidal cells in lamina I I I c are the only indications of the presence of myelin. A few globules are the only indications of myelin on the axons of any of the pyramidal cells in lamina I l l b . The axons of the pyramidal cells in layer I I I are in about the same state of development as those of corresponding cells in area P C in the region of the head.

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The dendrites of the Golgi type I I cells are in about the same state of development as those of pyramidal cells of comparable size. The mesh of fibers formed by the axons of these cells is of approximately the same density as that in area P C in the region of the head. The tangential fibers in layer I are larger, more darkly stained, and slightly more numerous in the walls of sulci than in the crowns of the gyri. In the walls of sulci they number 14 to 16 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all the fibers bear varicosities, spines, and thorns. No branches have been observed on any of the tangential fibers. They are more numerous, larger, and more darkly stained in the outer one-half of the layer than in the inner one-half. The largest fibers in the Golgi sections measure 1.5/t in diameter between varicosities. Horizontal exogenous fibers number 3 or 4 to the unit in layer I I . They are small and faintly stained. The horizontal exogenous fibers gradually become larger and more darkly stained in the successively deeper strata of the cortex, and are largest and most darkly stained in lamina VIb. In lamina I l i a these fibers number 6 to 8 to the unit, and in lamina I l l b they average 8 to 10 per unit. The outer band of Baillarger contains 14 to 16 horizontal exogenous fibers to the unit. The horizontal exogenous fibers number 12 to 14 to the unit in lamina Va, 16 to 18 in the inner band of Baillarger in laminae Vb and V i a , and 12 to 14 in lamina VIb. The subcortical association fibers number 24 to 26 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns. They are not as large or as darkly stained as the corresponding fibers in area P B in the region of the head. They are larger and more darkly stained than these fibers in areas P G and P H . The largest fibers measure τ.$μ in diameter between varicosities. The majority of the large fibers are somewhat smaller than this. The vertical exogenous fibers in the cores of the gyri immediately below the place where the fibers begin to radiate toward the cortex number 35 to 40 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers

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measure 1.5/1, in diameter between varicosities. The vertical fibers are chiefly collected in bundles as they radiate toward the cortex. Many of the radiating vertical fibers end in the middle of layer VI, some end in the inner band of Baillarger, and a few end in the outer band. The vertical exogenous fibers number 10 to 12 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 4 or 5 in the lower part of layer IV. A few fibers end in the lower part of lamina IIIc. No collateral or terminal branches have been observed on any of the ascending vertical exogenous fibers.

No stained tangential fibers are present in layer I, and there are no stained horizontal fibers external to layer V. A very small, short, lightly stained fiber is seen occasionally in lamina Va. In laminae Vb and V i a (the inner band of Baillarger) the stained horizontal fibers number 3 or 4 to the unit, and they are larger, longer, and darker than the stained fibers in lamina Va. In lamina VIb the stained horizontal fibers number 4 to 6 to the unit, and they are larger, longer, and darker than those in lamina V i a . The stained subcortical association fibers number 6 to 8 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb.

MYELINATION. Brains 37-82 and 37-49 are the only two with Weigert sections in the occipital lobe. The stain in the sections in area OA in brain 37-82 is but slightly darker, and is no more extensive than that in the sections of brain 37-49. The stain in the sections of brain 37-49 is much darker and more extensive than that in the sections of brain 33-197, the threemonth brain with the most myelin in area OA. The stain in this area in both the six-month brains is darker than that in area PE, PF, PG, or PH. The stain in area OA in brain 37-82 is darker than that in the region of the head in areas P B and PC, but is lighter than the stain in the other regions of these areas. The following description is based upon the sections of brain 37-82.

The stained vertical fibers are largest, most numerous, and darkest in the cores of gyri. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 2 5 to 30 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained fibers measure 1.5/1 in diameter between varicosities. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number, and gradually become lighter in color. On most of the ascending fibers the stain ends in the region of the subcortical association fibers, on some it ends in the lower part of lamina VIb, and on a very few it ends in the inner band of Baillarger.

AREA PARASTRIATA, OB

THIS AREA has the same characteristics and distribution as in the cortex of the three-month infant. It measures 1.621 mm. in width, and it is illustrated in Figures 1 1 7 , 1 1 8 , 119, and 120. CELL-BODIES

Layer I. This layer measures 0.184 mm. in width, and contains 47 neurons to the unit. Layer II. The external granular layer is visible macroscopically. The layer measures 0.088 mm. in width. The neurons number 290 per unit, and measure 4/4-7/6/1. Layer III. The external pyramidal layer measures 0.548 mm. in width. The neurons in lamina I l i a number 98 to the unit, and measure 8/6-16/10/1. In lamina I H b the nerve cells number 81 to the unit, and

measure 10/6-23/12/*. In lamina IIIc the neurons number 84 per unit, and they range in size from ιο/6μ to 26/13/1. Layer IV. The internal granular layer is more prominent than the outer one. It measures 0.123 mm. in width and contains 174 neurons to the unit. The cells measure 6/4-8/6/1. Only a few neurons of laminae IIIc and Va invade the granular layer. Layer V. This layer measures 0.275 mm. in width. In lamina Va small and large neurons number 70 and 16 to the unit, respectively. In lamina Vb there are 50 small and 16 large cells to the unit. The extralarge pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb also. These cells number 8 to the unit, and they range in size from 26/12/1 to 49/13/1.

LOBUS OCCIPITALIS Layer VI. This layer measures 0.403 mm. in width. The neurons number 85 to the unit in lamina Via and 45 in lamina VIb. The pyramidal cells measure 10/6-23/ΐ2μ, and the spindle cells range in size from ΐ2/4μ to 26/τομ. The extra-large pyramidal cells in layer V are stained about the same color as the largest ordinary pyramidal cells in layers I I I and VI, and all these cells are stained more darkly than any other cells in this area. The chromophil substance in the extralarge pyramidal cells in layer V is greater in quantity and more advanced in differentiation than that in corresponding cells in area OA. In these cells in area OB the chromophil substance is principally in the form of small clumps and Nissl flakes, though granules and some large clumps are present. The apical and basal dendrites are stained for a greater distance and contain more chromophil substance than in area OA. The chromophil substance in the large ordinary pyramidal cells in layers V and VI and in the large spindle cells in V I is chiefly in the form of small clumps and Nissl flakes, but large clumps are present. The largest pyramidal cells in lamina IIIc contain more chromophil substance than the extra-large pyramidal cells in layer V, except the largest of the latter. The chromophil substance in the large pyramidal cells in lamina IIIc is chiefly in the form of small clumps and Nissl flakes, but large clumps are seen more commonly than in the extra-large pyramidal cells in layer V. Small clumps of chromophil substance and Nissl flakes predominate in number over large clumps in the large pyramidal cells in laminae I l l b and I l i a . In this respect the large pyramidal cells throughout layer I I I are more advanced in differentiation of chromophil substance than corresponding cells in this layer in area OA. The chromophil substance in all cells is greater in quantity and more advanced in differentiation than in corresponding cells in area OB in the three-month cortex. In the Cajal sections the cytoplasm in the extralarge pyramidal cells in layer V is filled with granules irregularly scattered throughout the cell-bodies and processes. Two or three longitudinal rows of granules are present in the apical dendrites of the largest cells, but there are no neurofibrils in any cells. The axons and dendrites are stained slightly darker than the cell-bodies.

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The cytoplasm in all other cells in this area is filled with granules which are scattered irregularly throughout the axons, dendrites, and cell-bodies. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are larger and more compact in structure than those of corresponding cells in this area in the three-month cortex. The terminal branches in layer I especially are more compact in structure than in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and longer, but no more numerous, than in the three-month cortex. They are distributed within the cortex in the same manner as in the three-month cortex. Pedunculated bulbs are but slightly, if any, more numerous on the dendrites of these cells than in the three-month cortex, except on the terminal branches of the apical dendrites in layer I, where the bulbs are definitely more numerous than in the three-month stage of development. The axons of the extra-large pyramidal cells in layer V are larger and have much more myelin than those of corresponding cells in the cortex of the threemonth infant. In the six-month cortex the dendrites of the extralarge pyramidal cells in layer V in this area are larger and have more pedunculated bulbs, and the axons are larger and have more myelin than those of corresponding cells in area OA. The dendrites of some of the large ordinary and special pyramidal cells in layers V and VI are larger and have more pedunculated bulbs than the dendrites of corresponding cells in this area in the three-month cortex. The apical dendrites of other cells of this type are extremely slender, as in area OA, and there is but little increase in size since the three-month stage of development. The dendrites of all the large ordinary and special pyramidal cells are more compact in structure than in the three-month cortex. Pedunculated bulbs appear to be but slightly more numerous than in the cortex of the three-month infant. The axons of the large ordinary and special pyramidal cells in layers V and VI are very much smaller and have much less myelin than those of the extralarge pyramidal cells in layer V. The axons are larger and have more myelin than the axons of the large ordinary and special pyramidal cells in layers V and

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VI in this area in the three-month cortex. In the sixmonth cortex the dendrites of these cells are larger and have more pedunculated bulbs, and the axons are larger and have more myelin than those of corresponding cells in area OA. The apical dendrites of the pyramidal cells in layer I I I are larger, and the basal dendrites and collateral branches of the apical dendrites are larger and longer, than those of corresponding cells in this area in the three-month cortex. Some of the pyramidal cells in IIIc are very large and have dendrites as large as some of the extra-large pyramidal cells in layer V. The dendrites of the pyramidal cells in layer I I I are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the sixmonth cortex the dendrites of the pyramidal cells in layer I I I are larger and have more pedunculated bulbs than those of corresponding cells in area OA. Large pyramidal cells are especially numerous in area ΟΒγ at the junction of areas OB and OC. Pedunculated bulbs are much less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the extra-large pyramidal cells in layer V. The axons of the pyramidal cells in layer I I I are larger than those of corresponding cells in this area in the three-month cortex. Myelin is present on the axons of the large pyramidal cells in laminae I l l b and IIIc, but not on the axons of the cells in lamina I l i a . The axons of the cells in I l l b have much less myelin than the axons of the cells in IIIc. The axons of the largest pyramidal cells in lamina IIIc have much less myelin than the axons of the extra-large pyramidal cells in layer V. The axons of the pyramidal cells in laminae I l l b and IIIc have more myelin than those of corresponding cells in this area in the three-month cortex, and more than the axons of corresponding cells in area OA in the six-month cortex. The dendrites and axons of the small pyramidal cells of the two granular layers are larger and longer than those of corresponding cells in this area in the three-month cortex. The dendrites of the cells of layer IV are larger and longer, and have more pedunculated bulbs than those of layer II. The bulbs are slightly more numerous on the dendrites of the pyramidal cells of layers I I and IV than on the dendrites

of corresponding cells in this area in the three-month cortex. No horizontal cells of Cajal have been observed in layer I. No long branches have been seen on any of the tangential fibers in this layer. The dendrites of Golgi type I I cells are in about the same state of development as those of pyramidal cells of comparable size. The axons of these cells form a mesh of fibers in layers IV, V, and VI which is slightly more dense than that in area OA. The mesh is definitely more dense in layer IV than that in this layer in area OA. The vertical exogenous fibers do not contribute any fibers to the mesh. No terminal or collateral branches have been observed on any of the vertical exogenous fibers in this area. All of these fibers end as single shafts vertically directed. The tangential fibers in layer I are more numerous, larger, and more darkly stained in the walls of sulci than in the crowns of gyri. In the walls they number 18 to 20 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all bear varicosities, spines, and thorns. They are more numerous, larger, and more darkly stained in the middle of layer I than in the outer and inner margins. The largest tangential fibers measure 1.5^ between varicosities. No branches have been observed on any tangential fibers. Horizontal exogenous fibers number 4 to 6 to the unit in layer II. They are very small and lightly stained. The horizontal exogenous fibers gradually become larger and more darkly stained in successively deeper layers of the cortex, and reach the maximum of size and depth of stain in lamina VIb and in the layer of subcortical association fibers. The fibers number 8 to 10 to the unit in lamina I l i a , 10 to 12 in lamina I l l b , and 16 to 18 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina Va the horizontal exogenous fibers number 14 to 16, and in the inner band of Baillarger in laminae Vb and Via these fibers average 18 to 20 to the unit. The horizontal fibers number 16 to 18 to the unit in lamina VIb, and the subcortical association fibers number 24 to 2 6 per unit. The horizontal exogenous fibers in all layers of the cortex and the subcortical association fibers are small, intermediate, and large in size, and in the Golgi preparations varicosities, spines, and thorns are present on all of them. The largest hori-

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zontal fibers in lamina V I b and the largest subcortical association fibers measure ι.ζμ in diameter between varicosities. No branches have been observed on any of these fibers.

darker than that in area OA or in the region of the head in area P C , but is lighter than that in the other regions of area P C . The following description is based upon the sections of brain 3 7-82.

The vertical exogenous fibers immediately subjacent to area O B and just before the fibers begin to radiate toward the cortex number 3 5 to 40 to the unit. They are small, intermediate, and large in size and in the Golgi preparations all bear varicosities, spines, and thorns. The vertical fibers are chiefly collected in bundles as they radiate toward the cortex. As many as 8 and as few as 4 fibers have been counted in a bundle, and a bundle usually contains small, intermediate, and large fibers. The fibers gradually decrease in size as they ascend in the cortex, and finally end as very small, very lightly stained, vertically directed fibers. Many of the fibers end in the middle of layer VI, some fewer end in the inner band of Baillarger, still fewer end in the outer band, a small number ascending as high as the lower part of lamina IIIc. The ascending vertical exogenous fibers number 10 to 12 to the unit in the lower part of lamina VIb, 6 to 8 in the lower part of lamina Vb, and 4 to 6 in the lower part of layer IV. N o terminal or collateral branches have been observed on any vertical exogenous fibers, either in the core of the gyrus or within the cortex.

No stained tangential fibers are present in layer I, and there are no stained horizontal fibers external to lamina IIIc. In lamina IIIc and layer IV (the outer band of Baillarger) stained horizontal fibers number from ι to 3 to the unit. They are small, short, and very lightly stained. In lamina Va the stained horizontal fibers number 3 or 4 to the unit, and they are larger, longer, and darker than those in layer IV. In laminae V b and V i a (the inner band of Baillarger) stained horizontal fibers number 4 or 5 to the unit, and they are larger and darker than those in lamina Va. In lamina VIb the stained horizontal fibers number 6 or 7 to the unit, and they are larger and darker than those in lamina V i a . The stained subcortical association fibers number 8 to 10 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal fibers in the cortex and the stained subcortical association fibers are small, intermediate, and large in size, and all bear varicosities.

MYELiNATioN. The brains examined fall into the following sequence with reference to the depth of stain in the Weigert sections: ( 1 ) 37-82, (2) 4 1 - 9 1 , (3) 37-49. The stain in the sections of brain 37-82 is but slightly darker than that in the sections of brain 4 1 - 9 1 . The stain in the sections of brain 37-49 is distinctly lighter and less extensive than that in the sections of brain 4 1 - 9 1 . The stain in the sections of brain 37-49 is much darker and more extensive than the stain in the sections of brain 33-197, the threemonth brain with the most myelin in this area. The stain in the sections in area O B in brain 37-82 is

The stained vertical fibers are largest, darkest, and most numerous in the cores of the gyri. They are stained more darkly than the vertical fibers in area OA. Immediately below the place where they begin to radiate toward the cortex they number 25 to 30 to the unit. The fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure χ in diameter between varicosities. Large fibers are more numerous in area O B than in area OA. The stained vertical fibers decrease in size and number and become lighter in color as they radiate into the cortex. On most of the ascending fibers the stain ends in the lower part of lamina VIb, on some fewer fibers the stain ends in the upper part of this lamina, and on a very few fibers it ends in lamina Va.

AREA STRIATA, OC THIS AREA has the same characteristics and distribution as in the three-month cortex. In the striate area the cortex is widest in the crown of the gyri and narrowest in the floor of sulci. The granulous cortex

extends on the medial surface of the cuneus and gyrus lingualis for approximately the same distance as it does in the three-month cortex. The place of transition between area O B and OC both in the cuneus and

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gyrus lingualis is characterized by the presence of an unusually large number of large neurons in layer III, as explained in Volume I. This place of transition is designated by Economo as area ΟΒγ. It is illustrated in Figures 121, 122, and 123. In the walls of gyri the granulous cortex of area OC measures x.967 mm. in width. It is illustrated in Figures 124, 125, 126, and 127. CELL-BODIES

Layer I. The molecular layer measures 0.262 mm. in width, and contains 53 neurons to the unit. Layer II. The external granular layer is clearly visible macroscopically. The outer margin of the layer is sharp, but the inner border is indistinct because the small cells of layer II mingle with the larger cells of lamina I l i a . The granular layer measures 0.093 mm. in width. The neurons number 322 to the unit and measure 4/4-7/6//,. Layer III. This layer measures 0.451 mm. in width, and has the same characteristics as in the threemonth cortex. The neurons are principally small, and the layer presents a granular appearance. The size of the large neurons gradually increases from the external to the internal margin of the layer. The neurons number 91 to the unit, and range in size from 8/6/u to 26/13/1. Layer IV. The internal granular layer is widest in the cuneus, in the crown on the medial surface of gyrus lingualis, and in the crown of the annectant gyrus which lies at the bottom of the calcarine sulcus. In the walls of the gyri the layer measures 0.665 m m · width. Three laminae, IVa, IVb, and IVc are distinctly visible under the microscope. Lamina IVa contains small neurons measuring 5/4-8/6/1 and numbering 177 to the unit. Lamina IVb, the stria of Gennari, is much lighter than the other two laminae. The nerve cells number 116 to the unit. The neurons in this lamina are chiefly small cells measuring 5/4το/6μ, but there are also large Meynert and Cajal cells which range in size from 14/ιομ to 26/13/*. Lamina IVc is wider, darker, and richer in cells than either of the other two laminae. The neurons in IVc are chiefly small cells measuring 5/4-10/6/1, but there are also a few large Meynert and Cajal cells. The cells number 238 to the unit. Layer V. This layer is narrow in all parts of area OC,

measuring only 0.173 mm. in width. The neurons are the small, large, and extra-large cells characteristic of this layer in all parts of the isocortex. The small cells number 78 to the unit, and measure 8/6-12/ιομ. The large cells number 15 to the unit, and measure 16/10-23/12^. The extra-large cells number 8 to the unit, and range in size from 29/16/x to 36/13/x. Layer VI. This layer measures 0.323 mm. in width. It has the same characteristics as in the brains of the younger infants. The layer is as distinctly characteristic of area OC as the inner granular layer. A t the boundaries between areas OB and OC there occurs an abrupt and large increase in the number of neurons in lamina Via. These cells are more darkly stained than the cells in lamina V i a in areas OB and OA, and they cause layer V I to be more prominent than any other layer in area OC. The neurons number 106 to the unit in lamina V i a and 54 in lamina VIb. The pyramidal cells measure 10/6-26/13^, and the spindle cells measure 12/4-2 6/1 ομ. The extra-large pyramidal cells in layer V are stained more darkly than any other cells in this area, and their chromophil substance is more advanced in development than that in any of the other cells. The chromophil substance is predominantly in the form of small clumps and Nissl bodies but some large clumps are usually present. In some of these cells the chromophil substance is composed entirely of small clumps and Nissl bodies. The chromophil substance is more advanced in differentiation than that in the extra-large pyramidal cells in layer V in area OB, but it is not as advanced in development as that in the giant pyramidal cells of Betz in area F A y in the region of the head. The large pyramidal and spindle cells in layer V I are darkly stained, and collectively constitute a conspicuous horizontal band of cells in lamina V i a . The chromophil substance in these cells is slightly greater in quantity and more advanced in differentiation than that in the corresponding cells in this area in the three-month cortex; the clumps of chromophil substance are larger and more darkly stained and Nissl bodies are more numerous in the six-month cortex. The chromophil substance in the larger Meynert and Cajal cells in layer IV is greater in quantity and more advanced in differentiation than that in the large pyramidal and spindle cells in layer VI. Small

LOBUS OCCIPITALIS clumps and Nissl bodies are more numerous, large clumps are less numerous, and the chromophil substance extends into the dendrites for a greater distance and in greater quantity than in the large cells in VI. The chromophil substance in the Meynert and Cajal cells in layer IV is greater in quantity and more advanced in differentiation than that in corresponding cells in the three-month cortex. The large pyramidal cells in lamina IIIc are as darkly stained as the large Meynert and Cajal cells. Because of their large size and dark color the large pyramidal cells in lamina IIIc contrast sharply with the small, lightly stained cells in lamina IVa. In this stage of development lamina IVa can be clearly seen as a distinct, fairly wide horizontal lamina of small cells. In the external part of this lamina a few large pyramidal cells are scattered at wide intervals. These large cells apparently belong to lamina IIIc. The large pyramidal cells in lamina IIIc are smaller and contain less chromophil substance in proportion to the size of the nuclei than corresponding cells in this lamina in areas OB and ÖA. The chromophil substance in these cells in area OC is in approximately the same state of development as that in the largest Meynert and Cajal cells in lamina IVb. It is chiefly in the form of small clumps and Nissl bodies, but a few large clumps are present in many of the cells. Some of the largest cells in lamina I l l b are stained almost as darkly as the large cells in lamina IIIc. The chromophil substance in the pyramidal cells in layer I I I is in approximately the same state of development as that in corresponding cells in area OB. The chromophil substance in the cells in layer I I I in area OC is greater in quantity and more advanced in development than that in corresponding cells in this area in the three-month cortex. The chromophil substance in the small cells in all three laminae of layer IV is greater in quantity than that in cells of comparable size in layer II. The chromophil substance in the cells in layers I I and IV, and in all small cells in all layers in area OC, is greater in quantity and more darkly stained than in corresponding cells in this area in the three-month cortex. In the Cajal preparations the extra-large pyramidal cells in layer V are more advanced in the development of neurofibrils than any other cells in this area.

105

The axons and dendrites are stained more darkly than the cell-bodies, but the contrast in color is not as great as in the giant pyramidal cells in area FAy. Neurofibrils are neither as numerous nor as advanced in development in the extra-large pyramidal cells in area OC as in the giant pyramidal cells in area FAy. In the extra-large pyramidal cells in area OC neurofibrils are more advanced in development in the axons and dendrites than in the cell-bodies. With low magnification the dendrites and axons appear to contain longitudinal rows of granules and a few neurofibrils. Under oil immersion, however, the neurofibrils have the appearance of granules which are connected by small strands, thus resembling strings of beads. In the dendrites of the giant pyramidal cells in area F A y the neurofibrils are definitely formed fibers which do not have a beaded structure. Irregularly scattered granules, longitudinal rows of granules, and strands of granules are present in the cell-bodies of the extralarge cells in area OC. The longitudinal rows and strands of granules in the cell-bodies are less numerous and more lightly stained than those in the dendrites of the cells. The giant stellate cells of Meynert in lamina IVb rank next to the extra-large pyramidal cells in layer V in regard to development of neurofibrils. The largest cells of Meynert are as large as the medium-sized extra-large pyramidal cells. No definitely formed neurofibrils are present in any of the Meynert cells. The granules in the dendrites are principally in longitudinal rows, but a few longitudinal strands of granules are present. The latter occur in less quantity than in the dendrites of the extra-large pyramidal cells in layer V. The dendrites and axons are stained more darkly than the cell-bodies. The large spindle and pyramidal cells in layer VI rank next to the Meynert cells as to the state of development of neurofibrils, but the gap between them is wide. The dendrites and axons of the large cells in layer VI are stained much more lightly than those of the Meynert cells. The processes of the large cells in layer VI are stained more darkly than the cell-bodies, but the contrast in color is not as great as in the Meynert cells. The cytoplasm in the cellbodies and dendrites contains many scattered granales. Longitudinal rows of granules are present in the dendrites of the large cells in VI but in less quan-

106

THE CORTEX OF THE SIX-MONTH INFANT

tity than in the dendrites of the Meynert cells. In the largest cells in VI two or three longitudinal strands of granules are present in the apical dendrites. The cytoplasm in the cell-bodies and processes of all other cells in area OC is filled with irregularly scattered granules. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are not appreciably larger or any more compact in structure than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites also are of about the same size and length as those in the three-month cortex. All of the dendrites, however, have more pedunculated bulbs than in the three-month cortex. The bulbs are more numerous on the apical dendrites than on the basal dendrites or collateral branches of the apical dendrites. The manner of distribution of the dendrites and of the bulbs on the dendrites is the same as in the three-month cortex. In the six-month cortex the basal dendrites and collateral branches of the apical dendrites of these cells are larger and longer and have many more pedunculated bulbs than those of corresponding cells in area OB. The cell-bodies and dendrites of the extra-large pyramidal cells in layer V in area OC are larger than those of corresponding cells in the region of the head in area PB„ but the dendrites of these cells in PB have more pedunculated bulbs than those of the cells in area OC. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells have more myelin than those of corresponding cells in area OB, but have less myelin than the axons of the extra-large pyramidal cells in layer V in the region of the head in area PB. The dendrites of the large ordinary and special pyramidal cells in layers V and VI are of about the same size and length as those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites than in the three-month cortex. In the six-month cortex the pedunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corre-

sponding cells in area OB. In area OC the bulbs are less numerous on the dendrites of the large ordinary and special pyramidal cells in layers V and VI than on the dendrites of the extra-large pyramidal cells in layer V. The dendrites of these cells in area OC are smaller and have fewer pedunculated bulbs than those of corresponding cells in the region of the head in area PB. The axons of the large ordinary and special pyramidal cells in layers V and VI are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells have more myelin than those of corresponding cells in area OB, but they have less myelin than the axons of these cells in area PB. There has been no apparent increase in the size of the dendrites of the pyramidal cells in layer III, nor in the quantity of pedunculated bulbs on the dendrites since the three-month satge of development. In the six-month cortex the cell-bodies and dendrites of the cells in layer III in area OC are smaller than those of corresponding cells in area OB, but the dendrites of the cells in area OC have more pedunculated bulbs than those of corresponding cells in area OB. The pyramidal cells in layer III in area OC are in general smaller than the corresponding cells in the region of the head in area PB, and the dendrites of the cells in area OC have fewer pedunculated bulbs than those in area PB. The axons of the large cells in layer III are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The axons of the large pyramidal cells in lamina IIIc are larger and have more myelin than those in lamina Illb. The axons of the cells in lamina IIIc in area OC have less myelin than those of corresponding cells in area PB in the region of the head. Some of the giant cells of Meynert are as large as the smaller extra-large pyramidal cells in layer V. The basal dendrites of these Meynert cells are as large and as long as those of the smaller extra-large pyramidal cells, but the apical dendrites of the former are more slender than those of the latter. Pedunculated bulbs are not as numerous on the dendrites of the giant cells of Meynert as those on the dendrites of the large ordinary and special pyramidal cells in layers V and VI. The dendrites of the giant cells of

LOBUS OCCIPITALIS Meynert are larger, longer, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The axons of the giant cells of Meynert are larger and have much more myelin than those of corresponding cells in the three-month cortex. In the sixmonth cortex the axons of these giant cells are almost as large and have almost as much myelin as the axons of the extra-large pyramidal cells in layer V. They are much larger and have much more myelin than the axons of the large ordinary and special pyramidal cells in layers V and VI. The mesh of fibers formed by the axons of Golgi type II cells is of about the same width and density as that in area OC in the three-month cortex. The mesh is much more dense in lamina IVc than in any of the other strata of the cortex. The mesh is of approximately uniform density throughout the length of area OC, and clearly delimits the boundaries of this area by abruptly ending at the junction of area OC and area Ο Β γ in gyrus lingualis and the cuneus, as is apparent in the photographs. The mesh does not receive any contribution of fibers from the vertical exogenous fibers in this area. No terminal or collateral branches have been observed on any of the vertical exogenous fibers. All of these fibers end as single shafts. No horizontal cells of Cajal have been observed in layer I. No inverted pyramidal cells have been observed in this area of the cortex. The tangential fibers are more numerous, larger, and more darkly stained in the external one-half than those in the internal one-half of layer I in both walls of the calcarine sulcus, in the floor of the sulcus, or in the lips and crowns of the cuneus and gyrus lingualis. In the outer one-half of the layer in each wall of the sulcus the fibers number 18 to 20 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure ι.&μ in diameter between varicosities, but most of the large fibers are somewhat smaller. No branches have been observed on any of the tangential fibers. Horizontal exogenous fibers are seen only occasionally in layer II and in the outer part of layer III,

107

and they are very small and faintly stained. In the middle of layer III horizontal fibers number from 1 to 3 to the unit, and they are slightly larger and more darkly stained than those in the outer part of layer III. In the inner part of layer III the horizontal fibers number 4 to 6 to the unit. The horizontal fibers number 8 to 10 to the unit in lamina IVa, 18 to 20 in lamina IVb and the upper one-half of IVc, and 10 to 12 in the lower one-half of lamina IVc. The fibers in laminae IVb and IVc are larger and more darkly stained than those in the lower part of layer III. The processes of the Cajal and Meynert cells together with the horizontal exogenous fibers form a dence neuropil in lamina IVb and the upper half of lamina IVc. The large fibers are larger and more numerous in the lower half of lamina IVc than in lamina IVa. In lamina Va the horizontal exogenous fibers number 8 to 10 to the unit. The horizontal exogenous fibers number 14 to 16 to the unit in the inner band of Baillarger in laminae Vb and V i a . In lamina VIb the horizontal fibers number 14 to 16 to the unit. The subcortical association fibers number 24 to 26 to the unit. The horizontal fibers in lamina VIb and the subcortical association fibers are of about the same size and the same depth of stain. The fibers in these strata are larger than those in lamina Va. The horizontal exogenous fibers in all layers of the cortex and the subcortical association fibers are small, intermediate, and large in size. In the Golgi preparations all the horizontal exogenous fibers and all the subcortical association fibers bear varicosities, spines, and thorns. The largest fibers in VIb and in the layer of subcortical association fibers measure ι ,&μ in diameter between varicosities, but most of the large fibers are somewhat smaller than this. The vertical exogenous fibers immediately subjacent to the cortex in area OC and just before the fibers begin to radiate toward the cortex number 40 to 45 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.8/u. in diameter between varicosities, but most of the large fibers are somewhat smaller. The vertical fibers are chiefly collected in bundles as they radiate toward the cortex. The bundles contain about 4 to 8 fibers, and small, intermediate, and large fibers are usually present in each

108

THE CORTEX OF THE SIX-MONTH INFANT

bundle. Some of the ascending vertical fibers end in the mid-region of lamina VIb, some end in the inner band of Baillarger in laminae Vb and Via, some end in lamina Va, a few end in the lower one-half of lamina IVc, but the majority of the ascending fibers can be traced directly through layers V I and V into the neuropil in lamina IVb, where they end. A few ascend still farther and end in lamina IVa, and occasionally a vertical fiber is seen in layer III, II, or I. No collateral or terminal branches have been observed on any of the vertical exogenous fibers. All of them decrease in size as they ascend and end as straight, vertical fibers. Weigert sections are available for only two brains, 37-82 and 37-49. The stain in the sections of brain 37-82 is distinctly darker than that in the sections of 37-49. The stain in brain 37-49 is much darker and more extensive than that in brain 33-197, the three-month brain with the most myelin in area OC. In both six-month brains the stain in area OC is distinctly darker than that in area OB. In each of the two brains the stain in the sections in area OC is darker than the stain in the region of the head in area PB, but it is lighter in area OC than it is in the other regions in area PB. Area OC extends into the crown of gyrus cuneus and gyrus lingualis on the medial surface of the occipital lobe for about the same distance as it does in the three-month cortex. The following description is based upon the sections of brain 37-82. MYELINATION.

From ι to 3 small, short, lightly stained tangential fibers are present in layer I in the cortex in the anterior limb of the calcarine sulcus. The stained fibers are larger and darker in the walls of the calcarine sulcus than in the crowns of gyrus cuneus and gyrus lingualis on the medial surface of the occipital lobe. Stained tangential fibers are seen but occasionally in layer I in the cortex in the posterior limb of the calcarine sulcus. In area OC in the anterior limb of the calcarine sulcus no stained horizontal fibers are present external to lamina IVb. A very small, short, lightly stained horizontal fiber is seen occasionally in this lamina. Stained horizontal fibers abruptly increase in number, size, and depth of color in lamina IVc. In the walls of the calcarine sulcus the stained horizontal

fibers in this lamina number 6 to 8 to the unit. In lamina Va the stained horizontal fibers number 4 or 5 to the unit in the walls of the calcarine sulcus. In laminae Vb and V i a (the inner band of Baillarger) the stained horizontal fibers number 6 to 8 to the unit, and they are larger and darker than those in lamina Va. In lamina VIb the stained horizontal fibers number 8 to 10 to the unit, and large fibers are more numerous than in lamina Via. The stained horizontal fibers in all laminae are slightly more numerous and darker in the walls of the calcarine sulcus than in the crown of the cuneus and gyrus lingualis on the medial surface of the occipital lobe. The stained horizontal fibers in all laminae in area OC are more numerous, larger, and more darkly stained in the anterior limb of the calcarine sulcus than in the posterior limb. The stained horizontal fibers are small, intermediate, and large in size, and all bear varicosities. The stained subcortical association fibers number 10 to 12 to the unit in the anterior limb of the calcarine sulcus. The stained fibers are larger and darker than the stained horizontal fibers in lamina VIb. They are small, intermediate, and large in size, and all bear varicosities. The stained horizontal and subcortical association fibers in area OC are larger and darker than the corresponding fibers in area OB. The stained vertical fibers subjacent to area OC are distinctly darker than the other stained vertical fibers in the cores of the cuneus and gyrus lingualis, and the extent of the koniocortex is clearly indicated by the contrast in color. The stain in the vertical fibers in area OC is darker and more extensive in the cortex in the anterior limb of the calcarine sulcus than in the posterior limb. The stained vertical fibers subjacent to area OC in the anterior limb of the sulcus number 25 to 30 to the unit. The stained fibers are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure χ.9ζμ in diameter between varicosities. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number and become lighter in color. On most of the fibers the stain ends in the lower part of lamina IVc; on many, however, it ends in the upper part of this lamina; and on some fewer the stain ends in lamina IVb. There are no stained vertical fibers external to this lamina. The stained vertical fibers which ascend in the cortex are

LOBUS OCCIPITALIS more numerous and darker in the anterior limb of the calcarine sulcus than in the posterior limb. The boundary between areas OC and OB in the crown of the cuneus and gyrus lingualis on the medial surface

109

of the occipital lobe is sharply delimited by the abrupt increase in the number of stained horizontal fibers in layer IV and stained vertical fibers which end in layer IV in area OC as compared with area OB.

LOBUS T E M P O R A L I S GYRUS TEMPORALIS

in the mid-region of gyrus temporalis superior measures 2.240 mm. in width. It is illustrated in Figures 128, 129, 130, and 131. The neurons in this area have a distinct arrangement in vertical columns, the columns including the neurons of all layers from II to VI, so that the cortex has an organlike appearance, as described by Economo. THE

CORTEX

CELL-BODIES

Layer I. The molecular layer measures 0.253 mm. in width, and contains 58 neurons to the unit. Layer II. The outer granular layer is visible macroscopically. The layer measures 0.128 mm. in width. The neurons number 297 to the unit, and measure 4/4-7/V· Layer III. This layer is wide, measuring 0.704 mm. in width. In lamina I l i a the neurons number 68 to the unit, and measure 8/6-20/12 μ. In lamina I l l b the neurons number 56 to the unit, and measure 10/6-2 9/15μ. The neurons in lamina IIIc number 53 to the unit, and measure 10/6-36/16^. Layer IV. The inner granular layer is more prominent than the outer one. It measures 0.237 mm. in width. The neurons number 160 to the unit, and measure 6/4-9/6/t. Layer V. The internal pyramidal layer measures 0.386 mm. in width. Small and large neurons are scattered throughout the layer. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb. The small cells number 65 to the unit in lamina Va and 53 in Vb. The large neurons number 16 in Va and 17 in Vb. The extra-large pyramidal cells number 10 to the unit, and they range in size from 29/12/t to 46/16/*. The small neurons measure 8/6-13/ιομ, and the large ones measure 16/10-23/12μ. Layer VI. This layer measures 0.532 mm. in width. The neurons number 76 to the unit in lamina V i a and 51 in VIb. The pyramidal cells measure 10/626/13/Λ, and spindle cells range in size from 12/4^ to 29/ιομ. Only a few of the extra-large pyramidal cells in layer V are darkly stained; most of them are light in color. Only the largest of these cells are larger

SUPERIOR, AREA

TA

than the largest pyramidal cells in layers V I and III. Most of the extra-large pyramidal cells in layer V are about the same size and contain approximately the same quantity of chromophil substance as the large cells in lamina IIIc and in layer VI. Most of the large cells in layers III and V I are stained more darkly than the extra-large pyramidal cells in layer V. The chromophil substance in the extra-large pyramidal cells in layer V, however, is more advanced in differentiation than that in the large cells in layer V I and lamina IIIc. In the extra-large cells in V the chromophil substance is chiefly in the form of Nissl bodies and small clumps; large clumps are seen only occasionally. Small clumps predominate in number in the large cells in layer V I and lamina IIIc, but large clumps are more numerous and Nissl bodies are less numerous than in the extra-large pyramidal cells in layer V. In almost all of these cells a darkly stained cone of chromophil substance rests upon the nuclear membrane immediately under the apical dendrite. The nuclear membrane is thick and darkly stained. The chromophil substance in the extra-large cells in layer V and in the large cells in layer V I and lamina IIIc is greater in quantity in proportion to the size of the nuclei and is more advanced in differentiation than that in corresponding cells in this area in the three-month cortex. Nissl bodies are more numerous and more sharply defined in the six-month cortex. The pyramidal cells in lamina IIIc are large and darkly stained, and constitute a conspicuous horizontal band of cells external to layer IV. A few large, darkly stained cells are present in lamina I l l b , but most of the cells in this lamina are much lighter in color than the large cells in lamina IIIc. As the cells become smaller toward layer II the chromophil substance decreases in quantity in proportion to the size of the nuclei and the cells become lighter in color. In all the cells in layer III the chromophil substance is greater in quantity and more darkly stained than that in corresponding cells in this area in the cortex of the three-month infant. The small cells of layer IV have more chromophil substance and are more darkly stained than the small cells of layer II. The chromophil substance in the

LOBUS TEMPORALIS cells of both these layers is greater in quantity and is more darkly stained than that in corresponding cells in this area in the three-month cortex. In the Cajal sections the dendrites and axons of the extra-large pyramidal cells in layer V are stained slightly more darkly than the cell-bodies. A few longitudinal rows of granules are present in the bodies and processes of these cells, but there are no neurofibrils. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are stained but slightly more darkly than the bodies of the cells. These cells are stained more lightly than the extra-large pyramidal cells in layer V. The granules are scattered irregularly throughout the cellbodies and processes. The dendrites and axons of the largest pyramidal cells in lamina IIIc are stained slightly more darkly than the bodies of the cells. These cells are stained about the same as the large ordinary and special pyramidal cells in layers V and VI. The granules are scattered irregularly throughout the cell-bodies and processes. All the other cells in layer III are stained more lightly than the largest pyramidal cells in lamina IIIc. In the Cajal sections the cells in this area are in about the same state of development as those in area PG. The large cells in layers IIIc, V, and VI are more darkly stained than the corresponding cells in area TA in the three-month cortex. The apical dendrites of the extra-large pyramidal cells in layer V are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the cortex of the three-month infant. The basal dendrites and collateral branches of the apical dendrites are coarser, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex pedunculated bulbs are more numerous on the dendrites of the extra-large pyramidal cells in area TA than on the dendrites of corresponding cells in area OA. The axons of the extra-large pyramidal cells in layer V are coarser and have much more myelin than those of corresponding cells in this area in the threeFIBERS.

111

month cortex. In the six-month cortex the axons of these cells in area TA have more myelin than those of corresponding cells in area OA. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI do not seem to be any larger, but they are more compact in structure and have more pedunculated bulbs than those of corresponding cells in this area in the threemonth cortex. In the six-month cortex the dendrites of these cells are very much more slender and have fewer pedunculated bulbs than the dendrites of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are much smaller and have much less myelin than those of the extra-large pyramidal cells in layer V. The axons of these cells in area TA have more myelin than those of corresponding cells in area OA. The apical dendrites of the pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous than in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites of all the cells in layer III than on corresponding cells in this area in the three-month cortex. The bulbs are more numerous on the dendrites of the large cells in lamina IIIc than on the dendrites of large cells in lamina Illb, and they are more numerous on the dendrites of large cells in lamina I l l b than on the dendrites of the large cells in lamina I l i a . Bulbs are rather sparsely scattered on the terminal branches of the apical dendrites of the large cells in Ilia. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the largest cells in lamina IIIc than on the dendrites of the largest ordinary and special pyramidal cells in layer V, but the bulbs are more numerous on the dendrites of the cells in IIIc than on the very slender apical dendrites of some of the ordinary and special pyramidal cells in layer V. The apical dendrites of the largest pyramidal cells in lamina IIIc are larger

112

THE CORTEX OF THE SIX-MONTH INFANT

than those of some of the extra-large pyramidal cells in layer V, but pedunculated bulbs are much less numerous on the dendrites of these cells in IIIc than on the extra-large cells in V. Pedunculated bulbs are present in approximately the same quantity on the dendrites of the pyramidal cells in layer I I I in area T A as on the dendrites of corresponding cells in area OA. The axons of the pyramidal cells in lamina IIIc become slightly enlarged a short distance distal to the cell-body and have a few small globules. The axons of the pyramidal cells in lamina I l l b become smaller as they descend and have a few globules on them. The enlargement and the globules probably indicate the presence of myelin. N o indications of myelin are present on the axons of the large pyramidal cells in lamina I l i a . The axons of the pyramidal cells in layer I I I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the cortex of the six-month infant the axons of the pyramidal cells in layer I I I in area T A are in approximately the same state of development as those of corresponding cells in area OA. The dendrites of the cells belonging to layers I V and I I are coarser, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the six-month cortex these cells in area T A are in approximately the same state of development as the corresponding cells in area OA. The dendrites of the Golgi type I I cells are in approximately the same state of development as those of pyramidal cells of comparable size. T h e axons of these cells form a thin mesh of fibers which is of approximately the same density as that in area OA. No horizontal cells of Cajal have been observed in layer I, and no inverted pyramidal cells have been observed in any layer of the cortex in this area. No long branches have been observed on any of the tangential fibers in layer I, but short branches are seen occasionally. A short branch is occasionally seen on a horizontal exogenous fiber, but no long branches have been observed. N o collateral or terminal branches have been observed on any of the vertical exogenous fibers. All of these fibers end as single, vertically directed shafts. In the crown of gyrus temporalis superior the tan-

gential fibers in layer I number 14 to 16 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The fibers in the outer one-half of layer I are more numerous, larger, and more darkly stained than those in the inner one-half. Horizontal exogenous fibers in layer I I number 6 to 8 to the unit. They are small and lightly stained. Proceeding internally the horizontal exogenous fibers gradually increase in size and in depth of stain, the fibers being largest and most darkly stained in lamina V I b and in the layer of subcortical association fibers. The horizontal fibers number 8 to 10 to the unit in lamina I l i a , 10 to 12 in lamina I l l b , and 14 to 16 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina V a the horizontal exogenous fibers number 12 to 14 to the unit, and in the inner band of Baillarger in laminae V b and V i a these fibers average 16 to 18 per unit. The horizontal fibers number 14 to 16 in lamina VIb, and the subcortical association fibers number 16 to 18 to the unit. The tangential fibers, the horizontal exogenous fibers, and the subcortical association fibers in this area are not as large or as darkly stained as the corresponding fibers in area OA. All the horizontal exogenous and subcortical association fibers in area T A are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The largest fibers measure 1 .ομ in diameter between varicosities. N o branches have been observed on any of these fibers. The vertical exogenous fibers subjacent to the crown of the gyrus and immediately below the place where the fibers begin to radiate toward the cortex number 35 to 40 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure τ.ομ in diameter between varicosities. As they radiate toward and into the cortex the vertical fibers are chiefly collected in bundles. Some of the ascending vertical exogenous fibers end in the middle of layer V I , some end in the neuropil in the inner band of Baillarger, and many end in the outer band of Baillarger, a few reaching the lower part of lamina IIIc. N o collateral or terminal branches have been observed on any of the vertical exogenous fibers.

LOBUS

TEMPORALIS

In making the Weigert sections for areas T A , T B , and T C the anterior transverse temporal gyrus is cut longitudinally and the adjoining portion of the superior temporal gyrus, area T A , is therefore cut transversely. The three brains examined fall into the following sequence with reference to the depth and extent of the stain in the sections: ( i ) 41-91, (2) 37-82, (3) 37-49· The stain in the sections of brain 41-91 is but slightly darker than that in the sections of brain 37-82. The stain in the sections of brain 37-49 is much lighter and less extensive than that in the sections of brain 37-82, but it is distinctly darker and more extensive than that in the sections of brain 38-128, the three-month brain with the most myelin in this area. In brains 37-82 and 37-49 the stain in area T A is lighter than that in area OA. The following description is based upon the sections of brain 37-82. MYELINATION.

No stained tangential fibers are present in layer I. There are no stained horizontal fibers in the cortex external to layer IV where a very short, delicate, lightly stained fiber is seen occasionally. In lamina Va the stained horizontal fibers number 2 or 3 to the unit, and they are small and light in color. In laminae

Vb and V i a (the inner band of Baillarger) the stained horizontal fibers number 3 or 4 to the unit, and they are slightly larger, longer, and darker than those in lamina Va. In lamina VIb the stained horizontal fibers number 5 or 6 to the unit, and they are larger and darker than those in lamina V i a . The stained subcortical association fibers number 6 or 7 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers in the core of gyrus temporalis superior immediately below the place where they begin to radiate toward the cortex number 18 to 20 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure τ.τ μ in diameter between varicosities. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number, and become lighter in color. On most of the ascending fibers the stain ends in the region of the subcortical association fibers, on some fewer it ends in the lower part of lamina VIb, and on a very few it ends in lamina Via.

GYRI TEMPORALES

in the anterior and posterior transverse temporal gyri has the same characteristics as described by Brodmann and Economo in the adult brain.

T H E CORTEX

AREA

SUPRA-TEMPORALIS

SIMPLEX,

TB

The cortex in this area measures 2.170 mm. in width. It is illustrated in Figures 132, 133, 134, and 135. The organ-pipe appearance of the cortex is present in this area also, but it is not as pronounced as in area T A . The cells in all the layers are included in the vertical columns. CELL-BODIES

Layer I. This layer measures 0.242 mm. in width, and contains 51 neurons to the unit. Layer II. The external granular layer is approximately the same as in area T A . The layer measures 0.147 m m · i n width. The neurons number 292 to the unit, and measure 4/4-7/6/i.

113

TRANSVERSI

Layer III. This layer measures 0.649 m m · width. The neurons number 62 to the unit in lamina I l i a , and measure 10/6-2 0/12 μ. In lamina I l l b the neurons number 53 to the unit, and measure 10/626/13^. The neurons in lamina IIIc number 51 to the unit, and range in size from io/βμ to 36/13/*. Layer IV. This layer measures 0.239 mm. in width. The neurons number 157 to the unit, and measure 6/4-1ο/βμ. Layer V. The inner pyramidal layer measures 0.389 mm. in width. Small and large neurons are scattered throughout the layer. The extra-large pyramidal cells are located chiefly in lamina Va, but some are present in lamina Vb also. The small neurons number 64 to the unit in lamina Va and 55 in lamina Vb. The large neurons number 15 to the unit in lamina Va and 16 in Vb. The extra-large pyramidal cells number 10 per unit, and range in size from 29/12/x to 36/13^. The small neurons measure 10/6-13/1 ομ, and the large ones measure 16/10-23/12μ.

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THE CORTEX OF THE SIX-MONTH INFANT

Layer VI. This layer measures 0.504 mm. in width. The neurons number 67 to the unit in lamina V i a and 41 in VIb. The pyramidal cells measure 10/62β/ΐ2μ, and the spindle cells measure 10/4-26/10^. The chromophil substance in the cells in this area is in approximately the same state of development as that in corresponding cells in area T A . The large pyramidal cells in lamina IIIc are more darkly stained than other cells in layer III and, therefore, are more conspicuous than in area T A . The chromophil substance in all cells is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the cortex of the threemonth infant. In the six-month cortex the chromophil substance in the large cells in lamina IIIc and in the extra-large pyramidal cells in layer V in area OB is greater in quantity than that in corresponding cells in area T B . In some of these cells in lamina IIIc and layer V in area OB the chromophil substance is more advanced in differentiation than that in corresponding cells in area T B . The chromophil substance in most of these cells, however, appears to be in approximately the same state of differentiation in both these areas. In the Cajal sections the cells in this area are in about the same state of development as to differentiation of neurofibrils as the cells in area T A . The apical dendrites of the extra-large pyramidal cells in layer V are more compact in structure and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. In the six-month cortex the dendrites of the extra-large pyramidal cells in layer V in this area are in approximately the same state of development as those of corresponding cells in area OB. These cells in area T B are, in general, larger and have larger dendrites than corresponding cells in area T A . FIBERS.

The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the cortex of the

three-month infant. In the six-month cortex the axons of these cells in area T B are larger and have more myelin than those of corresponding cells in area TA, but are in approximately the same state of development as to size and quantity of myelin as those of corresponding cells in area OB. Only one inverted pyramidal cell has been observed in this area and it is an extra-large pyramidal cell in lamina Vb. This cell is in the same state of development as to size and length of dendrites, quantity of pedunculated bulbs, size and condition of myelination of axon as the other extra-large pyramidal cells in layer V in this area. The apical dendrite descends vertically and ends abruptly in the lower part of lamina VIb without any terminal branches. The two collateral branches of the apical dendrite which are nearest the cell-body extend horizontally, but all the others are directed obliquely downward. The axon extends vertically upward and acquires a small myelin sheath in the upper part of lamina VIb. The sheath abruptly becomes about twofold larger in the middle of lamina Va and continues in this size to the upper part of this lamina where the sheath becomes much smaller as the axon enters the lower part of layer IV. Toward the middle of layer IV the sheath increases slightly in size, and ends abruptly in a clump of precipitate in the lower part of lamina IIIc. Only one branch is present on the axon. This collateral is very slender and emerges at a right angle immediately before the myelin sheath becomes quite large. The collateral gives off a secondary branch, and both extend horizontally among the horizontal exogenous fibers in lamina Va. The dendrites of the large ordinary and special pyramidal cells in layers V and V I are larger, longer, and more compact in structure than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites than in the three-month cortex. In the sixmonth cortex the dendrites of these cells in this area are in approximately the same state of development as those of corresponding cells in area OB. The axons of the large ordinary and special pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells in area T B are in approximately

LOBUS TEMPORALIS the same state of development as those of corresponding cells in area OB. The pyramidal cells in layer I I I are more advanced in development than corresponding cells in this area in the three-month cortex. The apical dendrites are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer I I I than on the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex some of the largest pyramidal cells in lamina IIIc in this area are larger than some of the extra-large pyramidal cells in layer V. The apical dendrites of these large cells in IIIc are as large as those of some of the extralarge pyramidal cells, but the basal dendrites are smaller. Pedunculated bulbs are much less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of even the smallest of the extra-large pyramidal cells in layer V. The cells in layer I I I in this area are larger than those in area TA. They are of about the same size and their dendrites are in approximately the same state of development as the corresponding cells in area OB. The axons of the large pyramidal cells in lamina IIIc are larger and have more myelin than those of the cells in lamina I l l b . The axons of the large pyramidal cells in lamina I l i a are smaller than those of the cells in lamina I l l b and they have no indications of myelin. The axons of the large cells in lamina IIIc are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The axons of the pyramidal cells in layer I I I are larger and have more myelin than the corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of the cells in layer I I I in area T B are in approximately the same state of development as to size and quantity of myelin as those of corresponding cells in area OB. The dendrites of the pyramidal cells of layers IV and I I are larger, longer, more compact in structure, and have more pedunculated bulbs than those of

IIS

corresponding cells in this area in the three-month cortex. No horizontal cells of Cajal have been observed in layer I in this area. The dendrites of the Golgi type I I cells are of about the same size and have approximately the same quantity of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size. The axons of these cells form a mesh of fibers which is more dense than that in area TA but is much less dense than that in area TC. The tangential fibers in layer I number 16 to 18 to the unit. They are more numerous, larger, and more darkly stained than those in area TA. The fibers are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The largest tangential fibers measure 1.3/x in diameter between varicosities. Horizontal exogenous fibers in layer I I number 6 to 8 to the unit. They are small and lightly stained. In lamina I l i a the horizontal fibers number 8 to 10, and in lamina I l l b they average 10 to 12 to the unit. The outer band of Baillarger in lamina IIIc and layer IV contains 14 to 16 horizontal exogenous fibers to the unit. These fibers number 12 to 14 to the unit in lamina Va, and 16 to 18 in the inner band of Baillarger in laminae Vb and Via. In lamina VIb the horizontal exogenous fibers number 16 to 18 to the unit. The subcortical association fibers number 18 to 20 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size. Proceeding inwardly the horizontal fibers of each category of size gradually become larger and more darkly stained, and are largest and darkest in lamina VIb. The subcortical association fibers closely resemble the horizontal fibers in lamina VIb in both size and density of stain. In the Golgi sections all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest fibers measure τ. ζ μ in diameter between varicosities. The horizontal exogenous and subcortical association fibers are larger and more darkly stained than those in area TA. The vertical exogenous fibers number 35 to 40 to the unit immediately before they begin to radiate toward the cortex in area T B . The fibers are small, intermediate, and large in size, and in the Golgi prep-

116

THE CORTEX OF THE SIX-MONTH INFANT

arations all of them bear varicosities, spines, and thorns. The largest fibers measure ι.ζμ in diameter between varicosities. The ascending vertical fibers end chiefly in the middle of lamina VIb and in the inner band of Baillarger. A few end in the outer band of Baillarger. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( i ) 4 1 91, (2) 37-82, (3) 37-49. There is very little difference in the stain in the sections of brains 41-91 and 37-82, but the stain in the sections of brain 37-49 is much lighter and less extensive than that in the sections of brain 37-82. The stain in the sections of brain 37-49 is distinctly darker and more extensive than that in the sections of brain 38-128, the threemonth brain with the most myelin in this area. In brains 37-82 and 37-49 the stain in area T B is lighter than that in area OB. In both these brains the stain in area T B is darker than the stain in area PC in the region of the head, but it is lighter than the stain in the other three regions of area PC. The following description is based upon the sections of brain 37-82. MYELINATION.

A very short, small, lightly stained tangential fiber is seen occasionally in layer I. No stained horizontal fibers are present in the cortex external to lamina IIIc. In this lamina and layer IV (the outer band of Baillarger) there are 2 or 3 small, lightly stained horizontal fibers to the unit. In lamina Va stained horizontal fibers number 3 or 4 to the unit, and they are larger, longer, and darker than those in layer IV. The stained horizontal fibers number 4 or 5 to the unit in the inner band of Baillarger, and they are larger and darker than those in lamina Va. In lamina VIb the stained horizontal fibers number 6 or 7 to the unit, and they are larger and darker than those in lamina V i a . The stained subcortical association fibers number 8 to 10 to the unit, and they are larger and darker than the stained horizontal fibers in lamina VIb. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities.

intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure 1.5/t in diameter between varicosities. The stained vertical fibers decrease in size and number, and become lighter in color as they radiate into the cortex. The stain on most of the fibers ends in the lower part of lamina VIb, on some fewer it ends in the middle of layer VI, and on a very few fibers the stain ends in layer V. AREA SUPRA-TEMPORALIS GRANULOSA, TC

This area of the cortex is the primary afferent area for sound. It has the same characteristics and distribution as in the three-month cortex. The cortex in the crown of gyrus temporalis transversus anterior measures 2.275 m m · i n width, and is illustrated in Figures 136, 137, 138, and 139. The neurons in this area are not arranged in distinct vertical columns, and the cortex here does not present the organlike appearance of areas Τ A and T B . CELL-BODIES

Layer I. The molecular layer measures 0.340 mm. in width, and contains 57 neurons to the unit. Layer II. The external granular layer is visible macroscopically. It measures 0.121 mm. in width. The neurons number 330 to the unit, and measure 4/47/6μ. The outer margin of the layer is fairly sharp, but the internal boundary is ill-defined because the small cells of the layer mingle freely with the neurons of lamina I l i a .

Layer III. Many small neurons are present in this layer and give it a granular appearance. The layer measures 0.715 mm. in width. In lamina I l i a the neurons number 79 to the unit, and measure 8/615/ιομ. The neurons number 62 to the unit in lamina I l l b , and measure 8/6-23/12^. In lamina IIIc the neurons number 63 per unit, and they range in size from ιο/6μ to 36/13^. Layer IV. The internal granular layer is more prominent than the external one. It is quite wide, measuring 0.244 nun, The neurons number 171 to the unit, and they measure 5/4-9/6/i. The layer is invaded by very few neurons of laminae IIIc and Va. The stained vertical fibers in the core of the an- Layer V. The internal pyramidal layer measures terior transverse temporal gyrus subjacent to area- 0.440 mm. in width. The small neurons measure T B number 18 to 20 to the unit. They are small, 8/6-13/io/x. They number 66 per unit in lamina Va

LOBUS TEMPORALIS

117

and 55 in lamina Vb. The large neurons measure the cells in layer III in this area are stained more 16/10-23/13/1, and number 20 per unit in lamina Va darkly than those of corresponding cells in area T A . and 16 in Vb. The extra-large pyramidal cells num- The bodies and processes of the cells in area T C are ber 10 to the unit, and they range in size from more advanced as to differentiation of neurofibrils 26/15/* to 33/13/4. than those of corresponding cells in this area in the Layer VI. This layer measures 0.415 mm. in width. three-month cortex. The neurons number 77 to the unit in lamina V i a and 42 in lamina VIb. The pyramidal cells measure FIBERS. The apical dendrites of the extra-large py10/6-2 6/13/1, and the spindle cells measure 10/4- ramidal cells in layer V are larger, more compact in 23/10/t. structure, and have more terminal branches in layer I The chromophil substance in the cells in this area than those of corresponding cells in this area in the is in approximately the same state of development as three-month cortex. The basal dendrites and the in area T B . Differentiation of the chromophil sub- collateral branches of the apical dendrites are larger, stance in the cells in area T C is more advanced than longer, and more compact in structure, but no more in corresponding cells in this area in the three-month numerous than those in the three-month cortex. Pecortex, but is not as advanced as in area OC in the dunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corresponding six-month cortex. In the Cajal sections the dendrites and axons of the cells in this area in the three-month cortex. In the extra-large pyramidal cells in layer V are stained cortex of the six-month infant the cell-bodies and more darkly than the bodies of the cells. The cell- dendrites of the extra-large pyramidal cells in layer bodies and processes of these cells are stained slightly V in area T C are approximately the same size as more darkly than the bodies and processes of the those of corresponding cells in area T B , but the denextra-large pyramidal cells in layer V in area T A . No drites of these cells in area T C have more peduncuneurofibrils are present in these cells in area T C , but lated bulbs than those of the cells in area T B . The longitudinal rows of granules are more numerous cell-bodies and dendrites of the extra-large pyramidal than in the corresponding cells in area T A . Portions cells in layer V in area T C are, in general, about the of some of the longitudinal rows of granules in apical same size as those of corresponding cells in area OC, dendrites look almost like fibrils. The bodies and but pedunculated bulbs are more numerous on the processes of the large ordinary and special pyramidal dendrites of the cells in area OC than on those of and spindle cells in layers V and V I are stained more the cells in area T C . darkly than those of corresponding cells in area T A . The axons of the extra-large pyramidal cells in No longitudinal rows of granules are present in the layer V are larger and have more myelin than those bodies or processes of these cells. of corresponding cells in this area in the three-month Some of the largest pyramidal cells in lamina IIIc cortex. The collateral branches of the axons are are as large as the smaller extra-large pyramidal cells very fine, but are larger than those in the three-month in layer V. The bodies and processes of the largest cortex. The primary collaterals are no more numerpyramidal cells in lamina IIIc are not stained as ous than in the three-month cortex, but one or two darkly as those of the extra-large pyramidal cells in secondary branches on the primary collaterals are layer V. The dendrites and axons of the largest cells seen more frequently than in the three-month cortex. in lamina IIIc are stained more darkly than the No indications of myelin are present on any of the bodies of the cells. No neurofibrils or longitudinal collateral branches of axons. In the six-month cortex rows of granules are present in any of the pyramidal the axons of the extra-large pyramidal cells in area cells in lamina IIIc. The pyramidal cells of laminae T C are of approximately the same size as those of I l l b and I l i a are stained more lightly than those of corresponding cells in area OC, but the axons of the the largest cells in lamina IIIc. No neurofibrils or cells in area OC have more myelin than those in longitudinal rows of granules are present in any cells area T C . in laminae I l l b or I l i a . The bodies and processes of

The dendrites of the large ordinary and special

118

THE CORTEX OF THE SIX-MONTH INFANT

pyramidal and spindle cells in layers V and VI are not any larger but are more compact in structure and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of these cells are much smaller and have fewer pedunculated bulbs than those of the extra-large pyramidal cells in layer V. The dendrites of the large ordinary and special pyramidal cells in area T C are of approximately the same size and length as those of corresponding cells in area OC, but pedunculated bulbs are more numerous on the dendrites of the cells in area OC. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger and have more myelin than those of corresponding cells in area T C in the three-month cortex. In the six-month cortex the axons of these cells have but a slight amount of myelin, much less than is present on the axons of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI in area T C have less myelin than the axons of corresponding cells in area OC. The dendrites of the cells in layer I I I in area T C are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the sixmonth cortex pedunculated bulbs are less numerous on the dendrites of the largest pyramidal cells in layer I I I (those in lamina IIIc) than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI. The cell-bodies and dendrites of the pyramidal cells in layer I I I are, in general, smaller than those of corresponding cells in area T B , but the dendrites of the cells in area T C have more pedunculated bulbs than those of the cells in area T B . The cell-bodies and dendrites of the pyramidal cells in layer I I I in area T C are larger than those of the corresponding cells in area OC, but the dendrites of the cells in area OC have more pedunculated bulbs than those of corresponding cells in area TC. The axons of the pyramidal cells in layer I I I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of the large pyramidal cells in lamina IIIc are larger and have more

myelin than those of the large pyramidal cells in lamina I l l b . The axons of the large pyramidal cells in lamina IIIc are smaller and have less myelin than those of the extra-large pyramidal cells in layer V. The axons of the large cells in layer I I I are smaller and have less myelin than those of corresponding cells in area PB. No small or giant cells of Meynert are present in layer IV in area TC. In this layer there are some pyramidal cells whose basal dendrites are directed downward and whose apical dendrites are very slender and have no branches. These cells may be the stellate cells of authors. They are about the same size and are approximately the same state of development as the large ordinary pyramidal cells in layer V. The dendrites of the small pyramidal cells of layer IV are larger, longer, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the small cells of layer I I are smaller and have fewer pedunculated bulbs than those of the small cells of layer IV. The dendrites of the cells of layer II are coarser and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the Golgi type I I cells are in approximately the same state of development as to size and number of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size. The axons of the Golgi type I I cells form a mesh of fibers which is more dense than that in this area in the threemonth cortex. The mesh is of about the same density as that in lamina IVc in area OC, but it is not as dense as the mesh in layer IV in the region of the head in area PB. The mesh of fibers in layer IV decreases considerably quite abruptly at the junction of area T C with area T B . The junction of these two areas is also characterized by the presence of an unusually large number of large pyramidal cells in layer I I I . The mesh of fibers formed by the axons of Golgi type I I cells does not receive any contributions from the vertical exogenous fibers. No terminal or collateral branches have been observed on any of the vertical exogenous fibers; all of them end as single, vertically directed shafts. No horizontal cells of Cajal have been observed in layer I in this area. The tangential fibers in layer I number 14 to 16 to

LOBUS TEMPORALIS the unit. The fibers are small, intermediate, and large in size. Those in the outer one-half of the molecular layer are larger, more numerous, and much more darkly stained than those in the inner one-half. In the Golgi preparations all the tangential fibers bear varicosities, spines, and thorns. The fibers are about the same size as the tangential fibers in area T B . The largest ones measure τ.ζμ in diameter between varicosities. No branches have been observed on any of the tangential fibers. Horizontal exogenous fibers number 6 to 8 to the unit in layer II. Proceeding inwardly the horizontal exogenous fibers gradually become larger and more darkly stained in the successively deeper laminae of the cortex. In lamina I l i a these fibers number io to i2 to the unit, and in lamina IHb they average 12 to 14 per unit. The outer band of Baillarger in lamina IIIc and layer IV contains 18 to 20 horizontal exogenous fibers to the unit. These fibers number 16 to 18 to the unit in lamina Va, and 18 to 20 in the inner band of Baillarger in laminae Vb and Via. In lamina VIb the horizontal fibers number 16 to 18 to the unit. The subcortical association fibers number 22 to 24 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The horizontal exogenous and subcortical association fibers are about the same size as the corresponding fibers in area TB. The largest fibers measure 1 .&μ in diameter between varicosities, but most of the large fibers are slightly smaller than this. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers subjacent to area T C just before they begin to radiate toward the cortex number 35 to 40 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.8μ in diameter between varicosities, but most of the large fibers are slightly smaller than this. The vertical exogenous fibers chiefly collect in bundles as they radiate toward the cortex, and each bundle usually contains small, intermediate, and large fibers. As the fibers ascend in the cortex they gradually decrease in size. Most of the ascending fibers end in the middle of layer VI, some

119

end in the neuropil in the inner band of Baillarger, and a few end in the lower part of layer IV. Occasionally a vertical fiber is seen ending in the lower part of lamina IIIc. No collateral or terminal branches have been observed on any of the vertical exogenous fibers. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: (1) 4 1 91, (2) 37-82, (3) 37-49. The stain in the sections of brain 41-91 is darker than that in the sections of brain 37-82. The stain in the sections of brain 37-49 is distinctly lighter and less extensive than that in the sections of brain 37-82, but is much darker and more extensive than that in brain 38-128, the three-month brain with the most myelin in this area. In brains 37-82 and 37-49 the stain in area T C is lighter than that in area OC. In both these brains the stain is slightly darker than that in the region of the head in area PB, but is distinctly lighter than that in the other three regions of area PB. The following description is based upon the sections of brain 37-82. MYELiNATiON.

Stained tangential fibers in layer I number 5 or 6 to the unit. No stained horizontal fibers are present in the cortex external to lamina IHb where a short, small, very light fiber is seen occasionally. In lamina IIIc and layer IV (the outer band of Baillarger) the stained horizontal fibers number 3 to 5 to the unit. In lamina Va the stained horizontal fibers number 3 or 4 to the unit. Stained horizontal fibers number 5 or 6 to the unit in laminae Vb and Via (the inner band of Baillarger), and they are larger, darker, and longer than those in lamina Va. In lamina VIb the stained horizontal fibers number 6 or 7 to the unit. They are larger and darker in color than those in lamina Via. The stained subcortical association fibers number 8 to 10 to the unit. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained vertical fibers in the core of the anterior transverse temporal gyrus subjacent to area T C are stained more darkly than those in any other part of the core except the fibers of the acoustic radiation which enter the medial end of the core. The vertical fibers subjacent to area T C are lighter in

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T H E CORTEX OF

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color than those subjacent to area OC in both the anterior and posterior limbs of the calcarine sulcus. The vertical fibers subjacent to area T C are stained more darkly than the vertical fibers subjacent to area P B in the region of the head, but they are much lighter in color than those subjacent to area P B in the other three regions. The stained vertical fibers number 20 to 25 to the unit. They are small, inter-

INFANT

mediate, and large in size, and all bear varicosities. The largest fibers measure 1.95/1 in diameter between varicosities. As they radiate into the cortex the stained vertical fibers decrease in size and number, and become lighter in color. The stain on most of the fibers ends in the middle of layer VI, on some fewer fibers it ends in the inner band of Baillarger, and on a very few fibers it ends in lamina Va.

GYRUS TEMPORALIS MEDIUS ET INFERIOR, AREA T E

THE CORTEX in the crown of the mid-region of the middle and inferior temporal gyri measures 2.206 mm. in width. It is illustrated in Figures 140, 141, 142, and 143. CELL-BODIES

Layer I. This layer measures 0.215 mm. in width, and contains 58 neurons to the unit. Layer II. The external granular layer is visible macroscopically. It measures 0.138 mm. in width, and contains 291 neurons to the unit. The cells range in size from 4/4/1 to 7/6/n. Layer III. This layer measures 0.650 mm. in width. In lamina I l i a the neurons number 57 to the unit, and measure 10/6-20/10/1. In lamina I l l b the neurons number 51 to the unit, and measure 10/623/12/t. The neurons in lamina IIIc number 50 to the unit, and they range in size from ιο/6μ to 36/13/1. Layer IV. The inner granular layer is more prominent than the outer one. It measures 0.200 mm. in width. The neurons number 163 to the unit, and measure 5/4-10/6/*,. Only a few neurons of laminae IIIc and Va invade the granular layer. Layer V. The internal granular layer measures 0.469 mm. in width. The small neurons number 61 to the unit in lamina Va and 53 in Vb. The large neurons number 15 per unit in lamina Va and 12 in Vb. The extra-large pyramidal cells number 9 to the unit, and they range in size from 26/13/1 to 36/12μ. The small cells measure 8/6-13/ιομ. and the large ones measure 16/10-23/12/*. Layer VI. This layer measures 0.534 mm. in width. The neurons number 76 to the unit in lamina V i a and 41 in lamina VIb. The pyramidal cells measure 10/6-26/15/4. The spindle cells range in size from 10/4/1 to 23/10/1.

Most of the extra-large pyramidal cells in layer V are of approximately the same size as the large pyramidal cells in lamina IIIc. Only occasionally is an extra-large pyramidal cell seen which exceeds in size the largest cells in IIIc. Some of the cells in layer V I are as large as the extra-large cells in layer V. The large cells in lamina IIIc, in layer V, and in layer V I are stained about the same color, and are darker than any other cells in this area. The chromophil substance in the extra-large pyramidal cells in layer V is chiefly in the form of Nissl flakes and small clumps; large clumps are few in number. In the large pyramidal and spindle cells in layer V I Nissl flakes and small clumps are present, but large clumps are proportionately more numerous than in the extra-large cells in layer V. The dark stain in the large pyramidal cells in lamina IIIc contrasts these cells sharply from the adjoining cells in layers I I I and IV. The chromophil substance in the large pyramidal cells in lamina IIIc is in the form of Nissl flakes, and small and large clumps. Nissl flakes and small clumps predominate, but large clumps constitute a greater proportion of the chromophil substance than they do in the large pyramidal and spindle cells in layer VI. As the cells in layer I I I become smaller in laminae I l l b and I l i a the chromophil substance decreases in quantity and gradually assumes the form of large clumps. The chromophil substance in the small cells in layer IV is greater in quantity and is more darkly stained than that in the small cells in layer II. Practically all the cells in all layers have a small cone or disc of darkly stained chromophil substance resting on the nuclear membrane immediately under the apical dendrite. The nuclear membrane in all cells is thick and darkly stained.

LOBUS TEMPORALIS The chromophil substance in all cells is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the three-month cortex. In the Cajal sections the extra-large pyramidal cells in layer V are stained more darkly than any other cells in this area, but they are not stained as darkly as the extra-large pyramidal cells in layer V in area TA. The apical and basal dendrites of these cells are stained but slightly more darkly than the cell-bodies. A few longitudinal rows of granules are present in the bodies and dendrites. The rows of granules are less in number than in the corresponding cells in area TA. The large ordinary and special pyramidal and spindle cells are stained more lightly than the extralarge pyramidal cells in layer V. No longitudinal rows of granules are present in these cells, but the granules are scattered irregularly throughout the cell-bodies, dendrites, and axons. The cell-bodies and processes are stained uniformly. The largest pyramidal cells in lamina IIIc are stained about the same as the large ordinary and special pyramidal and spindle cells in layers V and VI. No longitudinal rows of granules are present in any of these cells. The cells in lamina I l l b are stained more lightly than those in lamina IIIc, but are stained slightly more darkly than those in lamina I l i a . The granules in all cells in layer I I I are scattered irregularly throughout the cell-bodies and processes. All the cells in this area of the cortex are stained more darkly than corresponding cells in the threemonth cortex. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the cortex of the three-month infant. In the six-month cortex the dendrites of the extra-large pyramidal cells in layer V in this area are, FIBERS.

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in general, smaller and have fewer pedunculated bulbs than the dendrites of corresponding cells in areas TA and TB. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The collateral branches of the axons are larger but no more numerous than those in the threemonth cortex. The collateral branches are very fine and do not bear any indications of myelin. In the six-month cortex the axons of these cells in area T E have less myelin than those of corresponding cells in areas TA and TB. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of these cells are much smaller and have fewer pedunculated bulbs than those of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells have a very thin coat of myelin, whereas the axons of corresponding cells in this area in the three-month cortex show no indications of the presence of myelin. In the sixmonth cortex the axons of these cells in area T E have less myelin than those in area TA. Giant horizontal cells of Cajal are occasionally seen in layer VI. The dendrites of these cells are as large as the apical dendrites of the smallest of the extra-large pyramidal cells in layer V, but they have only about as many pedunculated bulbs as the basal dendrites of the latter cells. The axons of these cells are as large as those of the extra-large pyramidal cells, but they have less myelin. The apical dendrites of the large pyramidal cells in layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, more compact in structure, but no more numerous than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the cells in layer I I I than on the dendrites of corresponding cells in this area in the cortex of the three-month infant. Some of the largest

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THE CORTEX OF THE SIX-MONTH INFANT

cells in lamina IIIc have cell-bodies and apical dendrites as large as those of the smaller extra-large pyramidal cells in layer V, but pedunculated bulbs are much less in quantity on the dendrites of these large cells in IIIc than on the dendrites of the extralarge cells in layer V. The bulbs are less numerous on the dendrites of the large cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal cells in layers V and VI. The axons of all pyramidal cells in layer I I I in the six-month cortex are larger than those of corresponding cells in the cortex of the three-month infant. No myelin is present on the axons of any pyramidal cells in layer I I I in the three-month cortex. In the cortex of the six-month infant the axons of the large pyramidal cells in lamina IIIc have a small amount of myelin, less than is present on the large ordinary and special pyramidal cells in layers V and VI. The axons of the pyramidal cells in laminae I l l b and I l i a bear no indications of myelin. The dendrites of the small pyramidal cells belonging to layer IV are coarser, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The dendrites of the granular and small pyramidal cells of layer I I are smaller and have fewer pedunculated bulbs than those of the small pyramidal cells of layer IV, but they are coarser, more compact in structure, and have more pedunculated bulbs than the cells of layer I I in this area in the three-month cortex. The dendrites of the Golgi type I I cells are in approximately the same state of development as those of pyramidal cells of comparable size. The axons of the Golgi type I I cells form a very thin mesh of fibers in lamina IIIc and in layers IV, V, and VI. The mesh is more dense in layer IV and in laminae Vb and V i a than in laminae IIIc, Va, or VIb. The mesh is not any more dense than that in this area in the three-month cortex. In the sixmonth cortex the mesh in area T E is less dense than that in area TA. No horizontal cells of Cajal have been observed in layer I in this area. A short branch is seen occasionally on a tangential fiber in layer I, but no long branches have been observed on any of these fibers. No collateral or terminal branches have been ob-

served on any of the vertical exogenous fibers in the cortex in this area. In the Golgi sections the cell-bodies of many cells of all types in this area are rounded. The tangential fibers in layer I are slightly more numerous, larger, and more darkly stained in the walls of sulci than in the crowns of the gyri. In the walls of sulci the fibers number 14 to 16 to the unit. The fibers in the external one-half of the layer are larger and more darkly stained than those in the internal one-half. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.3/t in diameter between varicosities. No horizontal exogenous fibers are present in layer II. In lamina I l i a the horizontal fibers number 2 or 3 to the unit. These fibers number 5 or 6 to the unit in lamina I l l b and 8 to 10 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina Va the horizontal exogenous fibers number 6 to 8 to the unit. The horizontal fibers number 10 to 12 to the unit in the inner band of Baillarger in laminae Vb and Via, and 8 to 10 in lamina VIb. The subcortical association fibers number 14 to 16 to the unit. The horizontal exogenous fibers are generally smaller and more lightly stained in this area than corresponding fibers in regions TA, T B , and TC. The fibers in lamina I l i a are smallest and most lightly stained, and they gradually increase in size and density of stain in the successively deeper layers. They are largest and most darkly stained in lamina VIb and in the layer of subcortical association fibers. In the Golgi preparations all the horizontal exogenous fibers bear varicosities, spines, and thorns. The horizontal exogenous fibers and the subcortical association fibers are small, intermediate, and large in size. The horizontal exogenous fibers are not as numerous or as darkly stained as the corresponding fibers in areas TA, T B , and TC. The largest fibers in lamina VIb measure 1.3/1 in diameter between varicosities. The vertical exogenous fibers in the core of the gyrus number 30 to 35 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.3/4 in diameter between varicosities. As the vertical exogenous fibers

LOBUS

TEMPORALIS

radiate toward and into the cortex they are chiefly assembled in bundles. The fibers decrease in number, in size, and in density of stain as they ascend in the cortex. The majority of them end in the middle of layer VI, some end in the neuropil in the inner band of Baillarger, and a few end in layer IV where they are very lightly stained. Occasionally a vertical fiber is seen in lamina IIIc. No collateral or terminal branches have been observed on any of the vertical exogenous fibers. The tangential, subcortical association, and horizontal and vertical exogenous fibers in the six-month cortex are larger, more numerous, and more darkly stained than the corresponding fibers in this area in the cortex of the three-month infant. MYELINATION. The cortex of the middle and inferior temporal gyri is included in this area. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( i ) 37-82, (2) 38-35, (3) 37-49· The stain in the sections of brain 37-49 is much lighter than that in the sections of brain 37-82. In each of the three brains the stain is much lighter in color and less extensive in area T E than that in areas T A , T B , and T C . The stain in the sections of the middle temporal gyrus is slightly darker in color and more extensive than that in the sections of the inferior temporal gyrus. The stain in the sections of both gyri of brain 37-49 is much darker and more extensive than that in the corresponding gyri in brain 34-188, the three-month brain with the most myelin in area T E . The following description is based upon the sections of brain 37-82. Stained tangential fibers in layer I number 4 or 5 to the unit in both the middle and inferior gyri, but

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they are slightly larger and darker in color in the middle gyrus. No stained horizontal fibers are present in the cortex in the middle temporal gyrus external to lamina Va, where a very small, short, lightly stained fiber is seen occasionally. In laminae Vb and V i a (the inner band of Baillarger) the stained horizontal fibers number 2 or 3 to the unit, and they are larger, longer, and darker than those in Va. In lamina VIb the stained horizontal fibers number 3 to 4 to the unit, and they are larger, longer, and darker than those in lamina Via. Stained subcortical association fibers number 5 or 6 to the unit. The stained horizontal fibers in the cortex and the stained subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. The stained horizontal and subcortical fibers are smaller, less in number, and lighter in color in the inferior temporal gyrus than in the middle gyrus. The stained vertical fibers are largest, most numerous, and darkest in the cores of the gyri. The vertical fibers in the core of the middle temporal gyrus are stained more darkly than those in the core of the inferior gyrus. The vertical fibers in the core of the middle gyrus are stained more lightly than those in the core of the superior temporal gyrus. In the middle gyrus the stained vertical fibers number 16 to 18 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure 1 in diameter between varicosities. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number, and become lighter in color. On most of the ascending fibers the stain ends in the region of the subcortical association fibers, on a few it ends in the lower part of lamina VIb, and on very few it ends in the upper part of this lamina.

GYRUS FUSIFORMIS, AREA TF

THE CORTEX in this area has the same characteristics as in the three-month brain. The neurons are arranged in thin columns which include the cells of all the layers, except layer I. In the crown of the gyrus the cortex measures 2.141 mm. in width. It is illustrated in Figures 144, 145, 146, and 147. CELL-BODIES

Layer I. This layer measures 0.198 mm. in width, and contains 52 neurons to the unit.

Layer II. The external granular layer is visible macroscopically. The layer measures 0.109 mm. in width. The neurons number 312 to the unit, and measure 4/4-7/6/4. Layer III. The external pyramidal layer measures 0.629 mm. in width. In lamina I l i a the neurons number 64 to the unit, and measure 10/6-2ο/ιομ. In lamina I l l b the neurons number 55 per unit, and measure 10/6-23/12/*. The neurons in lamina IIIc

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THE CORTEX OF THE SIX-MONTH INFANT

number 51 to the unit and measure 10/6-36/13//.. Layer IV. The internal granular layer is more prominent than the external one. It measures 0.208 mm. in width. The neurons number 170 to the unit, and they measure 6/4-10/6/i. Only a few neurons of laminae I I I c and Va invade the granular layer. Layer V. The internal pyramidal layer measures 0.480 mm. in width. The small neurons number 64 in lamina Va and 53 in Vb, and they measure 8 / 6 13/iOjx. The large neurons measure 16/10-23/12^, and they number 19 per unit in lamina Va and 17 in Vb. The extra-large pyramidal cells number 8 to the unit, and range in size from 26/12/*. to 36/13/i. Layer VI. This layer measures 0.517 mm. in width. The neurons number 73 to the unit in lamina Via and 45 in lamina VIb. The pyramidal cells measure 10/6-26/13/4, and the spindle cells measure 1 0 / 4 23/ιομ. The chromophil substance in the cells in this area is in approximately the same state of development as that in corresponding cells in area TE, except that large clumps are more frequently present in the large cells in layers I I I , V, and VI in area T F than in area T E . The chromophil substance is greater in quantity and more advanced in differentiation in all cells than that in corresponding cells in this area in the threemonth cortex. In the Cajal sections the bodies and processes of the cells in this area are stained more lightly than those in area T E , but are stained more darkly than corresponding cells in area T F in the three-month cortex. The cell-bodies, dendrites, and axons of any one cell are stained to a uniform degree. The extralarge pyramidal cells in layer V are stained only slightly more darkly than the large ordinary and special pyramidal and spindle cells in layers V and VI. A few longitudinal rows of granules are present in the extra-large pyramidal cells in layer V, but the rows are less numerous than in these cells in area T E . No longitudinal rows of granules are present in the large ordinary and special cells in layers V and VI. The largest pyramidal cells in lamina I I I c are stained almost as deeply as the extra-large pyramidal cells in layer V. The cells in lamina I l l b are stained more lightly than those in lamina IIIc, but are more deeply stained than the cells in lamina I l i a . No

longitudinal rows of granules are present in any cells in layer I I I , not even the largest pyramidal cells in lamina IIIc. The granules are scattered diffusely throughout the cell-bodies, dendrites, and axons. The apical dendrites of the extra-large pyramidal cells in layer V are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than on the dendrites of corresponding cells in the three-month brain. FIBERS.

The axons of the extra-large pyramidal cells in layer V are coarser and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the dendrites of the extralarge pyramidal cells in layer V in area T F are about the same size and have approximately the same quantity of pedunculated bulbs as the dendrites of corresponding cells in area T E . The pedunculated bulbs in area T F are very small and delicate. The axons of these cells in area T F are approximately of the same size and have about the same quantity of myelin as the axons of the corresponding cells in area T E . The dendrites and axons of these cells in area T F are more advanced in development than those of the corresponding cells in area P H in the posterior region of gyrus fusiformis and the middle and inferior temporal gyri. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are coarser, longer, and more compact in structure than those of corresponding cells in this area in the threemonth cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the threemonth cortex. In the six-month cortex the dendrites of these cells are smaller and have fewer pedunculated bulbs than those of the extra-large pyramidal cells in layer V. The axons of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger and have more myelin than those of corresponding

LOBUS TEMPORALIS cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are in about the same state of development as those of corresponding cells in area TE. A giant horizontal cell of Cajal is occasionally seen in layer VI. The dendrites of these cells are about as large as the apical dendrites of the extra-large pyramidal cells in layer V, and have pedunculated bulbs in about the same quantity as the basal dendrites of the extra-large cells. One of these giant cells is present in layer III. The apical dendrites of the pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the pyramidal cells in layer III than on the dendrites of corresponding cells in the cortex of the three-month infant. In the six-month cortex the dendrites of the large pyramidal cells in lamina IIIc are smaller than those of the extra-large pyramidal cells in layer V. Pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI.

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The axons of the large pyramidal cells in lamina IIIc are larger and have more myelin than those of corresponding cells in this area of the cortex of the three-month infant. The axons of the pyramidal cells in laminae I l l b and I l i a are larger than those of corresponding cells in the three-month cortex, and do not have any myelin. In the six-month cortex the axons of the large pyramidal cells in lamina IIIc are as large as those of the large ordinary and special pyramidal cells in layers V and VI, but have less myelin. The dendrites and axons of the cells in layer

corresponding cells in this area in the three-month cortex. In the six-month cortex the cells of the two granular layers are in approximately the same state of development as the corresponding cells in area TE. Four inverted pyramidal cells have been observed in this area. One of these is in lamina Va and the others are all in layer VI. The dendrites of the Golgi type II cells are in approximately the same state of development as to size and length and as to quantity of pedunculated bulbs as the dendrites of pyramidal cells of comparable size. The axons of the Golgi type cells form a thin mesh of fibers which is of about the same width and density as that in area T E . Portions of the dendrites only of one horizontal cell of Cajal are present in layer I in this area. A short branch is occasionally seen on a tangential fiber in layer I, but no long branches have been observed. The cell-bodies of many of the cells of all types and in all layers in this area are rounded. Cells with rounded bodies are not as numerous as in area TE. The tangential fibers in layer I are larger, more darkly stained, and slightly more numerous in the walls of sulci than in the crown of gyrus fusiformis. Those in the wall of the gyrus in the inferior temporal sulcus are larger and more darkly stained than those in the wall of gyrus fusiformis in the collateral fissure. The tangential fibers in this area are smaller and more lightly stained than those in area T E . In the wall of the gyrus within the inferior temporal sulcus the tangential fibers number 14 to 16 to the unit. The fibers in the outer one-half of the layer throughout gyrus fusiformis are much larger and more darkly stained than those in the inner one-half. The tangential fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers in the Golgi sections measure χ .ομ in diameter between varicosities. No branches have been observed on any of the tangential fibers.

III in area T F are in about the same state of development as those of corresponding cells in area T E , but they are more advanced in development than the cells in layer III in area PH. The dendrites of the small cells belonging to layers IV and II are larger, longer, more compact in structure, and have more pedunculated bulbs than those of

No horizontal exogenous fibers are present in layer II. These fibers number 2 or 3 to the unit in lamina I l i a , 4 or 5 in lamina I l l b , and 6 to 8 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina Va the horizontal fibers number 5 or 6 to the unit, and average 8 to 10 per unit in the inner band of Baillarger in laminae Vb and Via. Horizontal

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T H E C O R T E X OF T H E SIX-MONTH I N F A N T

exogenous fibers in lamina VIb number 6 to 8 to the unit, and the subcortical association fibers number 8 to ι ο per unit. In lamina I l i a the horizontal exogenous fibers are very small and lightly stained. These fibers gradually increase in size and density of stain in successively deeper strata and are largest and most darkly stained in layer V I and in the subcortical association layer. The fibers are small, intermediate, and large in size, and all bear varicosities, spines, and thorns in the Golgi preparations. The largest horizontal exogenous and subcortical association fibers measure 0.9/n in diameter between varicosities. No branches have been observed on any horizontal exogenous or subcortical association fibers. The horizontal exogenous and subcortical association fibers in this area are smaller, less numerous, and more lightly stained than corresponding fibers in area T E , but they are larger, more numerous, more compact in structure, and more darkly stained than these fibers in area T F in the cortex of the infant at the age of three months. The vertical exogenous fibers in the core of gyrus fusiformis number 30 to 35 to the unit. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers in the Golgi sections measure o.g/i in diameter between varicosities. The radiating vertical fibers end chiefly in the lower one-half of layer VI, a few fibers end in the inner band of Baillarger, and still fewer end in layer IV. The vertical fibers gradually decrease in size as they ascend, and those ending in layer IV are very small and very lightly stained. No collateral or terminal branches have been observed on any of the POLUS TEMPO

THE CORTEX in this area has the same characteristics as it has in the three-month brain. It measures 2.180 mm. in width, and is illustrated in Figures 148, 149, 150, and 151. CELL-BODIES

Layer I. This layer measures 0.238 mm. in width, and contains 57 neurons to the unit. Layer II. The external granular layer measures o.roo mm. in width. The neurons number 296 to the unit, and they measure 4/4-7/6^.

vertical exogenous fibers. The vertical exogenous fibers in area T F are smaller and more lightly stained than the vertical fibers in area T E in the six-month cortex, but they are larger and more darkly stained than the corresponding fibers in area T F in the cortex of the three-month infant. MYELINATION. T h e W e i g e r t sections of b r a i n 3 8 - 3 5

only are available for study of this region. The stain in the sections in this area is lighter in color and less extensive than that in the sections of the inferior temporal gyrus. The stain in the sections of the fusiform gyrus is much darker and more extensive in the six-month than in the three-month brain. No stained tangential fibers are present in layer I, and there are no stained horizontal fibers in the cortex. Stained subcortical association fibers number 1 to 3 to the unit, and they are very small and lightly stained. The stained vertical fibers in the core of gyrus fusiformis are lighter in color than those in the core of gyrus temporalis inferior. Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers number 14 to 16 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest stained vertical fibers measure τ.ομ in diameter between varicosities. The stained vertical fibers decrease in size and number and become lighter in color as they radiate toward and into the cortex. On most of the radiating fibers the stain ends in the region of the subcortical association fibers, on some fewer fibers it ends in the lower part of lamina VIb, and on a very few fibers the stain ends in the middle of this lamina. N.LIS, AREA TG

Layer III. The external pyramidal layer measures 0.695 m m · i n width. In lamina I l i a the neurons number 62 to the unit and measure 10/6-20/10^. In lamina 111b the neurons number 49 to the unit and measure 10/6-23/12^. The neurons in lamina IIIc number 51 per unit and measure 10/6-2 6/13/*. Giant horizontal neurons, slightly larger than the largest extra-large pyramidal cells in layer V, are occasionally seen in lamina IIIc. These giant cells also are present, but more rarely, in layers V and VI. Layer IV. The internal granular layer is more prom-

LOBUS TEMPORALIS inent than the external one. It measures 0.150 mm. in width. The neurons number 150 to the unit, and measure 6/4-10/βμ. A few neurons of laminae IIIc and Va invade the granular layer. Layer V. The internal pyramidal layer measures 0.480 mm. in width. The small neurons number 56 to the unit in lamina Va and 46 in lamina Vb. The large neurons number 16 per unit in each lamina. The extra-large pyramidal cells number 8 to the unit, and measure 26/12-39/13/*. The small neurons measure 8/6-13/10/t, and the large ones range in size from 16/ιομ to 23/12/1. Layer VI. This layer measures 0.517 mm. in width. The neurons number 81 to the unit in lamina V i a and 45 in VIb. The pyramidal cells measure 10/626/13/*, and the spindle cells measure 10/4-26/10/1. The state of development of the chromophil substance in the neurons in this area as revealed by the cresyl violet sections is not as advanced as it is in area TE, but it is more advanced than in area T G in the three-month cortex. In the Cajal sections the cells in this area are more advanced as to differentiation of neurofibrils than corresponding cells in the three-month cortex. The extra-large pyramidal cells in layer V are stained more darkly than any other cells in area TG. The apical and basal dendrites of the extra-large pyramidal cells are stained slightly more darkly than the cell-bodies. No longitudinal rows of granules are present in the cell-bodies or the processes. The large ordinary and special pyramidal and spindle cells in layers V and VI are stained slightly more lightly than the extra-large pyramidal cells in layer V. The axons and dendrites are stained slightly more darkly than the cell-bodies. No longitudinal rows of granules are present in any of these cells. The largest pyramidal cells in lamina IIIc are stained about the same color as the large ordinary and special pyramidal and spindle cells in layers V and VI. The axons and dendrites are stained slightly more darkly than the cell-bodies. No longitudinal rows of granules are present in any of the cells in lamina IIIc. Proceeding externally the cells in layer III become more lightly stained. The axons and dendrites are the same color as the cell-bodies. The granules are scattered diffusely through the cellbodies and the processes.

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In the six-month cortex differentiation of neurofibrils is less advanced in area T G than in area T E . The apical dendrites of the pyramidal cells in layers III and V are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites of these cells are larger, longer, and more compact in structure, but no more numerous than those of corresponding cells in the three-month cortex. The dendrites of all other cells in this area are larger, longer, and more compact in structure than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all cells than on those of corresponding cells in this area in the three-month cortex. FIBERS.

The axons of the cells in this area are larger than those of corresponding cells in the cortex of the threemonth infant. The axons of the extra-large pyramidal cells in layer V, the large ordinary and special pyramidal and spindle cells in layers V and VI, and the large pyramidal cells in lamina IIIc have more myelin than those of corresponding cells in the cortex of the three-month infant. The axons of the extra-large pyramidal cells in layer V have more myelin than those of the large ordinary and special pyramidal and spindle cells, and the axons of the latter cells have more myelin than the axons of the large pyramidal cells in lamina IIIc. Myelin is not present on the axons of any other cells in this area, except the axons of the large Golgi type II cells. The cells in this area are in about the same state of development as those in area TE. Many of the cells in this area have rounded cellbodies. A giant horizontal cell of Cajal is occasionally seen in layer VI. One of these cells is present in lamina IIIc in one preparation. These horizontal cells, large and small, occur more frequently in this area than in either area T E or T F . Only three inverted pyramidal cells have been observed in this area, and all of them are located in layer VI. The tangential fibers in layer I number 10 to 12 to the unit. They are small, intermediate, and large in size, and in the Golgi preparations all of them bear

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THE CORTEX OF THE SIX-MONTH INFANT

varicosities, spines, and thorns. The largest fibers measure 1.3m in diameter between varicosities. No horizontal exogenous fibers are present in layer II. These fibers number 2 or 3 to the unit in lamina I l i a , 4 or 5 in lamina I l l b , and 6 to 8 in the outer band of Baillarger in lamina IIIc and layer IV. In lamina Va the horizontal fibers number 5 or 6 to the unit, and in the inner band of Baillarger they average 8 to 10 per unit. The horizontal fibers number 6 to 8 to the unit in VIb, and the subcortical association fibers number 8 to 10. In the Golgi preparations all the horizontal exogenous fibers bear varicosities, spines, and thorns. No branches have been observed on any of these fibers. The fibers are small, intermediate, and large in size. The fibers in lamina VIb and the subcortical association fibers are larger and more darkly stained than those in layers III, IV, or V. The largest subcortical association fibers measure 1.3/1 in diameter. The vertical exogenous fibers are more numerous in the core of the gyrus than elsewhere, and in this location number 35 to 40 to the unit. They are small, intermediate, and large in size, and in the Golgi sections all of them bear spines, varicosities, and thorns. The largest fibers measure 1.3/1 in diameter between varicosities. The vertical exogenous fibers decrease in size, number, and density of stain as they radiate toward the cortex. Most of them end in lamina VIb. Very few vertical exogenous fibers ascend as high as layer IV and they are very small. The tangential, horizontal exogenous, subcortical, and vertical exogenous fibers are coarser, more compact in structure, and more darkly stained than corresponding fibers in this area in the cortex of the threemonth infant. MYELiNATiON. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: (1)41-91, (2) 37-82, (3) 38-35, (4) 37-49· The stain in the sections of brain 41-91 is distinctly darker and more extensive than that in the sections of brain 37-82. The stain in the sections of brain 38-35 is clearly lighter in color and less extensive than that in brain 37-82. The stain in brain 37-49 is about the same in color and extent as that in brain 38-35. The stain in brain 37-49 is distinctly darker and more extensive

than that in the sections of brain 38-128, the threemonth brain with the most myelin in this area. In each brain the sections are cut transversely and include the extreme anterior ends of the three temporal gyri and the fusiform gyrus. The stain in each of these gyri in area TG in each brain is distinctly lighter in color than that in the sections in the midregion of the corresponding gyri. The stain in the tip of the temporal pole is also lighter than that in the mid-regions of these gyri. At the anterior ends of the gyri the stain is darkest in the superior temporal, is slightly lighter in the middle temporal, still lighter in the inferior temporal, and is lightest in the fusiform gyrus. The following description is based upon the sections of brain 37-82. In the floor of the sulcus between the superior and middle temporal gyri there are 4 or 5 very small, lightly stained tangential fibers to the unit in layer I. In the floor of the sulcus between the middle and inferior temporal gyri there are 3 or 4 stained tangential fibers to the unit, and they are lighter in color than those in the other two gyri. In the floor of the sulcus between the inferior temporal and fusiform gyri stained tangential fibers in layer I number 2 or 3 to the unit. Stained tangential fibers are present in the crowns of the gyri in slightly smaller number and are slightly lighter in color than those in the floors of the sulci. In layer I at the tip of the temporal pole the stained tangential fibers number 5 or 6 to the unit, and they are larger and darker than any of those in the anterior ends of the temporal gyri. Stained horizontal fibers are slightly more numerous, larger, and darker in the anterior end of the superior temporal gyrus than in the anterior ends of the other three gyri or in the tip of the temporal pole. No stained horizontal fibers are present in the cortex external to layer V. In lamina Va there are 1 or 2 small, lightly stained horizontal fibers to the unit. In laminae Vb and Via (the inner band of Baillarger) the stained horizontal fibers number 2 or 3 to the unit. In lamina VIb the stained horizontal fibers number 3 or 4 to the unit, and they are slightly larger and darker than those in lamina Via. The subcortical association fibers number 5 or 6 to the unit, and they are slightly larger and darker than the stained horizontal fibers in lamina VIb. The stained vertical fibers are slightly darker in

LOBUS TEMPORALIS the superior temporal gyrus than in the middle gyrus, those in the inferior gyrus are still lighter, and those in the fusiform gyrus are the lightest in color. In the core of the superior temporal gyrus just posterior to the tip of the temporal pole the stained vertical fibers number 16 to 18 to the unit. The fibers are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1.3/x in diameter

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between varicosities. As they radiate toward and into the cortex the stained vertical fibers decrease in number and size, and become lighter in color. On most of the fibers the stain ends in the region of the subcortical association fibers, on some fewer fibers it ends in the lower part of lamina VIb, and on a very few fibers it ends in the middle of lamina VIb.

LOBUS INSULAE THE CORTEX in the gyri of the insula has the same characteristics as in the adult brain as described by Brodmann and Economo. The cortex in the anterior AREA PRECENTf

IN THE crowns of the anterior central gyri the cortex measures 2.150 mm. in width. It is illustrated in Figures 152, 153, 154, and 155. CELL-BODIES

Layer I. This layer measures 0.241 mm. in width, and contains 65 neurons to the unit. Layer II. The external granular layer is visible macroscopically. It measures 0.099 m m · i n width in the widest parts. The neurons number 275 to the unit, and measure 4/4-8/6/*. Layer III. The external pyramidal layer measures 0.672 mm. in width. In lamina I l i a the neurons number 61 to the unit, and measure 10/6-2 ο/τομ. In lamina I l l b the neurons number 52 to the unit, and measure 10/6-2 0/10/x. The neurons in lamina IIIc number 53 to the unit, and they range in size from ιο/6μ to 26/12/x. Layer IV. The inner granular layer is not any more prominent than the outer one. It measures 0.182 mm. in width, and contains 118 neurons to the unit. The cells range in size from 5/4//, to 10/6μ. The layer is invaded by many large neurons belonging to laminae IIIc and Va. Layer V. The internal pyramidal layer measures 0.500 mm. in width. Small and large neurons are scattered throughout the layer. The small cells measure 8/6-13/io/x, and number 54 to the unit in lamina Va and 63 in Vb. The large cells measure 16/1023/12/n, and number 18 to the unit in lamina Va and 20 in Vb. The extra-large neurons are exceptionally numerous in both areas IA and IB. They are located chiefly in lamina Va, but some are present in lamina Vb also. These cells number 30 to the unit, and they range in size from 26/12/i to 39/13/1. Layer VI. This layer measures 0.454 mm. in width. The neurons number 74 to the unit in lamina V i a and 44 in lamina VIb. The pyramidal cells measure

gyri resembles the granular frontal type, and that in the posterior gyri resembles the parietal type.

LIS INSULAE, IA

10/6-2 6/13μ, and the spindle cells range in size from ιο/4μ to 23/ιομ. The extra-large pyramidal cells in layer V contain more chromophil substance in proportion to the size of the nucleus and are more darkly stained than any other cells in this area. The chromophil substance is in the form of small and large clumps and small Nissl flakes. The small clumps predominate in number. In almost every cell a small cone-shaped clump of chromophil substance is resting upon the nuclear membrane immediately below the base of the apical dendrite. Small clumps and elongated Nissl flakes extend into the apical dendrites for a distance equal to about twice the length of the cell-body. The clumps and Nissl flakes gradually decrease in size and number distally. Basal dendrites are stained for a short distance, but they do not contain any chromophil substance. The nuclear membrane is thick and darkly stained, but the karyoplasm is light in color. The chromophil substance is greater in quantity, small clumps and Nissl flakes are more numerous and extend into the apical dendrite in greater quantity and for a greater distance than in corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the chromophil substance in the extra-large pyramidal cells in layer V is not as advanced in development as that in the corresponding cells in areas F D and F C B m in the opercular part of the frontal lobe. A few pyramidal and spindle cells in layer V I are as large as the extra-large pyramidal cells in layer V. They contain as much chromophil substance and are stained as darkly as the extra-large cells in V. All the other large ordinary pyramidal cells in layers V and V I and the other spindle cells in V I have less chromophil substance and are stained more lightly than the extra-large pyramidal cells. The large pyramidal cells in lamina IIIc have less

LOBUS INSULAE chromophil substance and are stained more lightly than the extra-large pyramidal cells in layer V. A few large cells in lamina I l l b are stained as darkly as the large cells in IIIc. The chromophil substance in the large cells in IIIc is in the form of small Nissl flakes and small and large clumps, the small clumps predominating in number. Small, elongated clumps and small Nissl flakes extend into the apical dendrites in less quantity and for a shorter distance than in the extra-large pyramidal cells in layer V. In almost every cell a small cone of darkly stained chromophil substance rests upon the nuclear membrane immediately below the base of the apical dendrite. The nuclear membrane is thick and darkly stained. Basal dendrites are lightly stained for a short distance, but do not contain any chromophil substance. As the cells in layer I I I become smaller toward layer II the chromophil substance decreases in quantity, Nissl flakes decrease in number, and large clumps increase in number. The chromophil substance in all cells in layer I I I is greater in quantity and more advanced in differentiation than in corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the chromophil substance in the cells in layer III, particularly in lamina IIIc, in this area is less in quantity than that in corresponding cells in areas F D and F C B m in the operculum of the frontal lobe. The small granular and pyramidal cells in layer I I have less chromophil substance and are more lightly stained than the small cells in layer IV. In the smallest cells in layer I I the chromophil substance is in the form of a single layer of small beads which are clinging to the external surface of the nuclear membrane. The apical dendrite is stained for a distance about equal to the length of the nucleus. Basal dendrites are not indicated. The karyoplasm is homogeneous in structure throughout the nucleus and is darkly stained, but it is not as dark as the nuclear membrane or the nucleolus. The chromophil substance in the small cells in layers II and IV is greater in quantity, more advanced in differentiation, and more darkly stained than that in corresponding cells in this area in the cortex of the three-month infant. The cells in this area are more advanced as to differentiation of neurofibrils than corresponding cells in the three-month cortex. The extra-large py-

131

ramidal cells in layer V are more darkly stained than any other cells in area IA. The apical and basal dendrites of these cells are stained slightly more darkly than the cell-bodies. The apical dendrites of many of the extra-large cells subdivide in the upper part of layer IV into two apical dendrites of equal size. A few longitudinal rows of granules are present in the apical dendrites. In many of the apical dendrites short intervals of some of the longitudinal rows of granules are condensed into short pieces of neurofibrils and are stained more darkly than the other granules. The large ordinary and special pyramidal and spindle cells in layers V and VI are stained more lightly than the extra-large pyramidal cells in layer V, but more darkly than other cells in layers V and VI. The axons and dendrites of these cells are not stained more darkly than the bodies. No longitudinal rows of granules are present in the bodies or processes. The bodies and processes of the large pyramidal cells in lamina IIIc are stained to about the same degree as the large ordinary and special pyramidal and spindle cells in layers V and VI. No longitudinal rows of granules are present in any cells in layer III. Proceeding externally the pyramidal cells in layer I I I gradually become lighter in color. The granules are scattered diffusely throughout the cell-bodies and processes of all cells in layer III. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the cortex of the three-month infant. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than in the three-month cortex. Pedunculated bulbs are more numerous on all the dendrites than in the three-month cortex. The bulbs are most numerous on the apical dendrites, less numerous on the basal dendrites and collateral branches of the apical dendrites, and least in quantity on the terminal branches of the apical dendrites in layer I. The bulbs are present in maximum quantity on the apical dendrites as they are traversing layer IV and the inner one-half of layer III. In the six-month cortex the dendrites of the extra-large cells in layer V are larger

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THE CORTEX OF THE SIX-MONTH INFANT

and have more pedunculated bulbs than the dendrites of any other cells in this area. The axons of the extra-large pyramidal cells in layer V descend in a more or less tortuous course, and become lost among the vertical exogenous fibers in the core of the gyrus. The axons become slightly enlarged in about the middle of layer VI, and maintain this size until they reach the subjacent white substance where they become about twice the size they are in lamina VIb. The axons are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. From one to three collateral branches are present on some of the axons. The branches are small, but are larger than those in the three-month cortex. A few very small globules scattered along the surfaces are the only indications of myelin on the branches.

month cortex. The dendrites of the large cells in lamina IIIc have more pedunculated bulbs than those of the large pyramidal cells in I l l b , and the dendrites of the latter cells have more bulbs than those of the large pyramidal cells in lamina I l i a . On all pyramidal cells in layer I I I the bulbs are more numerous on the apical dendrites than on the basal dendrites or the collateral branches of the apical dendrites, and the bulbs are least in quantity on the terminal branches of the apical dendrites. Pedunculated bulbs are more numerous on the dendrites of all cells in layer I I I than on the dendrites of corresponding cells in this area in the three-month cortex. The bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI.

The dendrites and axons of the extra-large pyramidal cells in this area in the six-month cortex are in about the same state of development as those of corresponding cells in the deep portions of the opercular part of the inferior frontal gyrus. The dendrites and axons of the extra-large pyramidal cells in layer V in area IA are not as advanced in development as the corresponding cells in area F C B m in the lateral surface of the opercular part of the inferior frontal gyrus. The dendrites of the large ordinary and special pyramidal and spindle cells in layer V are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The axons of these cells are larger and have more myelin than those of corresponding cells in the cortex of the threemonth infant. In the six-month cortex the dendrites and axons of these cells are smaller, and the axons have much less myelin than those of the extra-large pyramidal cells in layer V. The apical dendrites of the large pyramidal cells in all three laminae of layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in the threemonth cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those of corresponding cells in the three-

The axons of the large pyramidal cells in all three laminae of layer I I I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The axons of the large pyramidal cells of lamina I l i a decrease in size as they descend and bear no indications of myelin. The axons of the large pyramidal cells in lamina I l l b decrease in size and have a few small globules widely scattered along their surfaces. The axons of the large pyramidal cells in lamina IIIc are larger and bear more indications of myelin than those of the large pyramidal cells in I l l b . The axons of the large pyramidal cells in IIIc show a slight increase in size as they traverse layers IV and V. The axons of these cells are much smaller and have much less myelin than those of the extra-large pyramidal cells in layer V. The dendrites and axons of the pyramidal cells in layer I I I in this area are in about the same state of development as those of corresponding cells in the deep surface of the opercular portion of the inferior frontal gyrus. The dendrites of the small pyramidal cells of layer IV are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the cortex of the threemonth infant. The dendrites of the small pyramidal cells in layer IV are larger, longer, and have more pedunculated bulbs than those of the small cells belonging to layer II. The dendrites of the granule and

LOBUS

small pyramidal cells of layer II are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs are present on the dendrites of the cells of layer II in less quantity than on the dendrites of any other cells in this area of the cortex. The bulbs are sparsely scattered along the dendrites of the small granular, pyramidal, and Golgi type I I cells belonging to the outer granular layer. The dendrites of the Golgi type II cells in this area are in about the same state of development as those of pyramidal cells of comparable size. The axons of the Golgi type I I cells form a very thin mesh of fibers in layers IV, V, and VI. The mesh is thinnest in layer V. The vertical exogenous fibers do not contribute any fibers to this mesh. These fibers end as single shafts, and have no terminal or collateral branches. No horizontal cells of Cajal have been observed in layer I in this area. The dendrites and axons of cells in area IA are more advanced in development than those of corresponding cells in the anterior regions of the frontal lobe. The tangential fibers in layer I number 16 to 18 to the unit. They are small, intermediate, and large in size. The largest fibers measure τ.ζμ in diameter. The fibers are not only more numerous but they are larger, more compact in structure, and more darkly stained than the tangential fibers in layer I in this area in the cortex of the three-month infant. N o

133

INSULAE

branches have been observed on any of the tangential fibers. No horizontal exogenous fibers are present in layer II. They number ι or 2 to the unit in lamina I l i a , 2 or 3 in I l l b , 4 or 5 in lamina IIIc and layer IV (the outer band of Baillarger), 5 or 6 in lamina Va, 8 to 10 in the inner band of Baillarger, and 10 to 12 in lamina VIb. The horizontal exogenous fibers are small, intermediate, and large in size, and in the Golgi sections all bear varicosities, spines, and thorns. The largest fibers measure τ.ζμ between varicosities. The subcortical association fibers number 10 to 12 to the unit, and they are about the same size as the horizontal exogenous fibers in lamina VIb. N o branches have been observed on any of the horizontal exogenous or subcortical association fibers. At the place where they begin to radiate toward the cortex the vertical exogenous fibers number 2 5 to 30 to the unit. The vertical fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure τ.ζμ in diameter between varicosities. The radiating vertical fibers are chiefly collected in bundles as they radiate toward the cortex. Most of them end in lamina V I b , some end in the inner band of Baillarger, and a few ascend as high as lamina Va. N o branches, either collateral or terminal, have been observed on any of the vertical exogenous fibers. The state of development of myelin in area IA is discussed under area I B .

AREA POSTCENTRALIS INSULAE, IB THIS AREA includes all the cortex in the posterior central gyri. The cortex has the same characteristics as in the three-month brain. The two granular layers are more prominent than those in area IA, and layer IV is more prominent than layer II. The cortex in area I B has a more granular appearance than that in area IA. In area I B the cortex measures 2.063 m m · in width. It is illustrated in Figures 156, 157, 158, and 159. CELL-BODIES

Layer I. The molecular layer measures 0.237 mm. in with and contains 58 neurons to the unit. Layer II. The external granular layer measures 0.105

mm. in width. The neurons number 324 to the unit and measure 4/4-8/7/n. The outer margin of the layer is fairly sharp, but the internal boundary is illdefined because the small cells of the granular layer mingle freely with the cells in lamina I l i a . Layer III. The external pyramidal layer measures 0.586 mm. in width. Small neurons are fairly numerous and give the layer a granular appearance. In lamina I l i a the neurons number 54 to the unit, and measure 10/6-20/12^. In lamina I l l b the neurons number 42 to the unit, and measure 10/6-23/12/*. The neurons in lamina IIIc number 46 per unit, and measure 10/6-26/12^. Layer IV.

The internal granular layer measures

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THE CORTEX OF THE SIX-MONTH INFANT

0.230 mm. in width. The neurons number 169 to the unit, and measure 5/4-^8/6 μ. Layer V. This layer measures 0.441 mm. in width. The small neurons number 50 to the unit in lamina Va and 52 in lamina Vb. The large neurons number 20 to the unit in each lamina. The extra-large pyramidal cells number 2 8 to the unit, and range in size from 26/12/1 to 39/ 13/x. The small neurons measure 7/6-12/8/*, and the large cells measure 15/1023Λ2/*. Layer VI. This layer measures 0.464 mm. in width. The neurons number 61 to the unit in lamina V i a and 42 in lamina VIb. The pyramidal cells measure 10/6-2 6/12 μ, and the spindle cells measure 12/623/10/1. The chromophil substance in the cells in this area is in approximately the same state of development as that in corresponding cells in area IA. The cells in this area are in the same state of development as to the differentiation of neurofibrils as corresponding cells in area IA, and are more advanced in development in this respect than the cells in area IB in the three-month cortex. The dendrites of all cells in this area are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The apical dendrites of the extra-large pyramidal cells in layer V and of the large pyramidal cells in all three laminae of layer III have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The axons of the pyramidal cells in layers III, V, and V I are larger and have more myelin than those of corresponding cells in the three-month cortex. In the six-month cortex the cells in area IB are in about the same state of development as corresponding cells in area IA. In general the cells in layer III in area I B are smaller than those in area IA. Golgi type II cells are much more numerous in area I B than in area IA, and the mesh formed by their axons is wider and more dense than that in area IA. The mesh is sufficiently dense in layer IV to be visible macroscopically. The vertical exogenous fibers do not contribute any fibers to this mesh. All of the vertical fibers end as single shafts, and have no terminal or collateral branches. FIBERS.

No horizontal cells of Cajal have been observed in layer I. No long branches have been seen on any tangential fibers in the molecular layer. The tangential fibers in layer I number 16 to 18 to the unit. They are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The largest fibers measure i.S/i in the Golgi sections. The tangential fibers are coarser and more darkly stained than the corresponding fibers in this area in the cortex of the threemonth infant. No branches have been observed on any of the tangential fibers. No horizontal exogenous fibers are present in layer II or in lamina I l i a . These fibers number 2 or 3 in lamina I l l b , 6 to 8 in the outer band of Baillarger, 5 or 6 in lamina Va, 8 to 10 in the inner band of Baillarger, and 10 to 12 to the unit in lamina VIb. The subcortical association fibers number 10 to 12 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The horizontal fibers in lamina I l l b are very small and lightly stained. They gradually increase in size and depth of stain toward lamina VIb where they are largest and darkest in color. The subcortical association fibers are of approximately the same size and color as the horizontal fibers in lamina VIb. The largest horizontal exogenous and subcortical association fibers measure 1.5/1 in diameter between varicosities. The horizontal exogenous and subcortical association fibers are larger, more compact in structure, and more darkly stained than corresponding fibers in this area in the cortex of the three-month infant. The vertical exogenous fibers number 25 to 30 to the unit immediately below the place where they begin to radiate toward the cortex. The fibers are small, intermediate, and large in size, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.5/1 in diameter between varicosities. As the vertical fibers radiate into the cortex they are chiefly collected in bundles, each bundle usually containing small, intermediate, and large fibers. Some fibers of each size proceed singly into the cortex. Most of the radiating fibers end in lamina VIb, some end in the inner band of Baillarger, and a few ascend to the lower part of

LOBUS INSULAE layer IV. No branches, either collateral or terminal, have been observed on any of the vertical exogenous fibers. These fibers are coarser, more compact in structure, and more darkly stained than the corresponding fibers in this area in the cortex of the threemonth infant. The Weigert sections of brains 37-82 and 37-49 are the only ones available for study of the myelination in areas IA and IB. The stain in the sections of brain 37-82 is but slightly darker than that in the sections of brain 37-49. The stain in the sections in the insula of each of these brains is slightly lighter in color than the stain in the sections in area T G in the tip of the temporal pole. The stain in the sections of brain 37-49 is but slightly darker and more extensive than that in brain 33-197, the threemonth brain with the most myelin in the cortex of the insula. The stain in the cortex of the insula in each brain is much lighter than that in the cortex in the frontal and parietal portions of the operculum in the corresponding brain. The following description is based upon the sections of both brains. MYELINATION.

A small, lightly stained tangential fiber is occasionally seen in layer I in the crowns of the gyri and in the walls of sulci. Widely scattered clusters of 2 or 3 small stained fibers are present in the walls of the sulci. No stained horizontal fibers are present external to lamina V i a . A small, short, lightly stained horizontal fiber is occasionally seen in this lamina.

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In lamina V l b stained horizontal fibers number 2 or 3 to the unit, and they are slightly larger and darker than those in V i a . T h e stained subcortical association fibers number 3 or 4 to the unit, and they are larger and darker than the stained horizontal fibers in lamina V l b . The stained vertical fibers are largest, darkest in color, and most numerous in the gyrus immediately anterior to the central sulcus, and those in the gyrus immediately posterior to the sulcus are slightly smaller, lighter in color, and less in number. Proceeding in both directions from the central sulcus the stained vertical fibers under the cortex and in the cores of the gyri decrease in size and number, and become lighter in color. The stained vertical fibers in the core of the anterior central gyrus immediately anterior to the central sulcus number 12 to 14 to the unit. In the core of the posterior central gyrus immediately posterior to the central sulcus the stained vertical fibers number 10 to 12 to the unit. The stained fibers are small, intermediate, and large in size, and all bear varicosities. The largest fibers in the anterior central gyrus measure τ.τ,μ in diameter between varicosities. Large fibers are more numerous in the anterior than in the posterior gyrus. A s they radiate toward the cortex the stained vertical fibers decrease in size and number, and become lighter in color. On most of the fibers the stain ends before they reach the cortex, on some fewer it ends in the lower part of lamina V l b , and on very few fibers the stain ends in lamina V i a .

RHINENCEPHALON THE SAME areas are selected for description in the cortex of the six-month infant for the rhinencephalon as have been described in Volumes I, II, and I I I of this series of monographs. These areas are parts of the cortex which Brodmann and Economo assigned to

the rhinencephalon in the adult brain. T h e cortex in each of the following areas has the same characteristics in the brain of the six-month infant as it has in the corresponding area in the brain of the adult.

GYRUS CINGULI BRODMANN AND ECONOMO recognized differences in

the anterior, posterior, and retro-splenial portions of this gyrus. Economo designated the cortex in the anterior part of the gyrus as area L A , that in the posterior part as area L C , and in the retro-splenial part he distinguished two areas, L D and L E . FORMATIO LIMBICA SUPERIOR ANTERIOR AGRANULARIS, AREA LA This area of the cortex in gyrus cinguli lies adjacent to the medial surface of the frontal lobe, being separated from the latter by sulcus cinguli. The cortex in this area is of the agranular, heterotypical type. The two granular layers are present, but are poorly developed. In the crown of the gyrus the cortex measures 1.995 mm. in width, and it is illustrated in Figures 1 6 0 , 1 6 1 , 1 6 2 , and 163. CELL-BODIES

Layer I. This layer measures 0.266 mm. in width and contains 59 neurons to the unit. Layer II. The external granular layer measures 0.087 mm. in width. The neurons number 241 to the unit, and measure 4/4-7/6/*. The layer is invaded by many large neurons belonging to lamina I l i a . Layer III. The external pyramidal layer measures 0.571 mm. in width. In lamina I l i a the neurons number 57 to the unit, and measure 10/6-20/12/*. In lamina I l l b the neurons number 45 per unit, and measure 10/6-23/12/*. The nerve cells in lamina IIIc number 51 to the unit, and measure 10/6-29/13/*. Layer IV. The internal granular layer is visible under the microscope, but it is not prominent. The layer is invaded by many large neurons belonging to laminae IIIc and Va. The granular layer measures 0.171 mm. in width, and contains 104 neurons to the unit. The neurons measure 5/4-8/6/*.

Layer V. The internal pyramidal layer measures 0.401 mm. in width. The layer is characterized by the presence of extra-large cells of the "corkscrew" type described by Economo. Small neurons number 53 to the unit in lamina V a and 50 in lamina Vb, and they measure 7/6-12/10/*. The large neurons number 16 to the unit in lamina Va and 1 7 m V b , and they measure 16/10-23/12/*. The extra-large neurons number 12 to the unit, and range in size from 26/10/* to 36/13/·«·· Layer VI. This layer measures 0.499 m m · i n width. The neurons number 75 to the unit in lamina V i a and 50 in lamina VIb. The pyramidal neurons measure 10/6-26/12/*, and the spindle cells measure 10/426/6/*. The extra-large pyramidal cells have more chromophil substance and are more darkly stained than any other cells in this area. The chromophil substance is in the form of NissI flakes and small and large clumps. The small clumps predominate in number, but large clumps are more numerous than in areas IA and I B . Small clumps of chromophil substance extend into the apical dendrite for a distance equal to about one and one-half times the length of the cellbody. A small cone or disc of darkly stained chromophil substance rests upon the nuclear membrane immediately under the base of the apical dendrite. The membrane of the nucleus is thick and darkly stained. The chromophil substance in the extra-large pyramidal cells is greater in quantity and more advanced in development than that in corresponding cells in this area in the cortex of the three-month infant. In the six-month infant the chromophil substance in the extra-large pyramidal cells in this area of the cortex is in approximately the same state of development as that in corresponding cells in areas F C L and F D L on the medial surface of the superior frontal gyrus.

RHINENCEPHALON The chromophil substance in the peculiar "corkscrew" cells is stained more lightly than that in the extra-large pyramidal cells. These cells are long, narrow spindle cells with a single dendrite at the superior and inferior ends. Both dendrites are more or less in the shape of a corkscrew. The nucleus is elongated, and there is only an extremely thin layer of cytoplasm along the sides of the nucleus. The nuclear membrane is thick and darkly stained. A dark cone of chromophil substance rests upon the nuclear membrane immediately under the base of each dendrite. The chromophil substance in the cytoplasm is entirely confined to the dendrites except a few slender, elongated strands alongside the nucleus. The chromophil substance extends into each of the two dendrites for a distance equal to about three times the length of the nucleus. It is in the form of granules, slender, elongated strands, and small amorphous clumps. The chromophil substance is greatest in quantity at the proximal end of the dendrite, and decreases rapidly in amount as it extends distally. The substance is greater in quantity at the periphery than in the center of the dendrite. Distally the chromophil substance is almost entirely confined to the periphery. The place of origin of each of two or three of the proximal collaterals is indicated by a small, darkly stained clump of chromophil substance on the internal surface of the wall of the dendrite. A very delicate, short, and extremely faintly stained collateral is occasionally seen on a dendrite. The chromophil substance in these cells is greater in quantity and more advanced in development than that in corresponding cells in this area of the cortex in the threemonth infant. A few of the large spindle and ordinary pyramidal cells in layer VI are as large and as darkly stained as the extra-large pyramidal cells in layer V. The chromophil material in these large cells in layer VI is of approximately the same quantity and in the same condition as that in the extra-large pyramidal cells in layer V. The large ordinary pyramidal cells in layers V and VI and the large spindle cells in VI are more lightly stained than the extra-large pyramidal cells in layer V. The chromophil substance is in the form of Nissl flakes and small and large amorphous clumps. The small clumps predominate in number, but the large

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clumps are more numerous in proportion to the total amount of chromophil substance than in the extralarge pyramidal cells. A smaller proportion of the chromophil substance is in the form of Nissl flakes than is the case in the extra-large pyramidal cells. In almost every cell a small, darkly stained cone of chromophil substance rests upon the nuclear membrane immediately below the base of the apical dendrite. Small clumps and flakes of chromophil substance extend into the apical dendrite for a distance equal to about one and one-half times the length of the cell-body, diminishing rapidly in quantity distally. The chromophil substance in the large pyramidal cells in lamina IIIc is of about the same quantity and in the same state of development as that in the large ordinary pyramidal cells in layers V and VI. As the cells become smaller toward the external boundary of layer III the chromophil substance gradually decreases in quantity and in depth of stain. The chromophil substance in the large ordinary pyramidal cells in layers V and VI and in all the cells in layer III is greater in quantity and more advanced in development than that in corresponding cells in this area in the cortex of the three-month infant. In the sixmonth cortex the chromophil substance in the cells in layer III in area LA is less in quantity and less advanced in development than that in corresponding cells in areas FCL and FDL on the medial surface of the superior frontal gyrus. This difference is particularly noticeable in the large cells in laminae IIIc and Illb. The cells in layer I I I in areas FCL and FDL contain more Nissl bodies and small clumps in proportion to large clumps than corresponding cells in area LA. The small cells in layer II contain less chromophil substance than the small cells in layer IV or in layers III, V, and VI. The chromophil substance in all small cells is greater in quantity and more darkly stained than that in corresponding cells in this area of the cortex in the three-month infant. In the Cajal sections the extra-large pyramidal cells in layer V are stained slightly more darkly than any other cells in this area. The bodies of the extralarge pyramidal cells are stained the same color as the processes. The cytoplasm of all cells in this area is filled with granules which are diffusely scattered

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throughout the bodies and processes, except that there are a few short bits of longitudinal rows in the apical dendrites of the largest extra-large pyramidal cells in layer V. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are present on the dendrites of these cells in about the same quantity as in the threemonth cortex. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in the three-month cortex. The apical dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are coarser and more compact in structure than those of corresponding cells in this area in the three-month cortex. The basal dendrites are larger, longer, and more compact in structure than those in the threemonth cortex. The dendrites of these cells have approximately the same quantity of pedunculated bulbs as the dendrites of corresponding cells in this area in the three-month cortex. In the six-month cortex the apical dendrites of these cells are much more slender and have much fewer pedunculated bulbs than the apical dendrites of the extra-large pyramidal cells in this area. The axons of the large ordinary and special pyramidal and spindle cells in this area are larger and have more myelin than those of corresponding cells in the three-month cortex. In the six-month cortex the axons of these cells are much smaller and have much less myelin than the axons of the extra-large pyramidal cells in layer V. The apical dendrites of the large pyramidal cells in layer III are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser, longer, FIBERS.

and more compact in structure, but no more numerous, than those in the three-month cortex. The dendrites of the cells in lamina IIIc are larger, longer, and have more pedunculated bulbs than those of the cells in lamina Illb. The dendrites of the cells in lamina I l l b are larger, longer, and have more pedunculated bulbs than those of the cells in lamina Ilia. Pedunculated bulbs are present on the dendrites of all cells in layer III in approximately the same quantity as on the dendrites of corresponding cells in this area in the three-month cortex. The bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI. The pattern of distribution of the pedunculated bulbs on the dendrites of all cells is the same as in the three-month cortex. The axons of the large pyramidal cells in lamina IIIc have but slightly more myelin than those of corresponding cells in this area in the three-month cortex. No indications of myelin are present on the axons of the large pyramidal cells in laminae I l l b and Ilia. The dendrites of the small cells belonging to the inner and outer granular layers are larger and more compact in structure than those of corresponding cells in this area in the three-month cortex. Pedunculated bulbs do not appear to be any more numerous than in the three-month cortex. The dendrites of Golgi type II cells are in approximately the same state of development as those of pyramidal cells of comparable size. The axons of these cells form a very thin plexus in layers IV, V, and VI. The plexus is thinner in layer V than in the other two layers. Only one inverted pyramidal cell has been observed in this area, and it is in layer VI. In the crown of gyrus cinguli at the anterior end of this area the tangential fibers number 16 to 18 to the unit. They are small, intermediate, and large in size, and in the Golgi sections all of these fibers bear varicosities, spines, and thorns. The largest fibers measure ι.ο μ in diameter between varicosities. The tangential fibers are larger, more numerous, and more darkly stained in the external one-half of layer I than those in the internal one-half. The tangential fibers

RHINENCEPHALON are coarser, more compact in structure, and more darkly stained than corresponding fibers in this area in the cortex of the three-month infant. Horizontal exogenous fibers number ι to 3 to the unit in layer II. They are very small and faintly stained. The horizontal fibers gradually increase in size and depth of stain in the successively deeper layers of the cortex, and are largest and darkest in lamina VIb. These fibers number 4 to 6 to the unit in lamina I l i a , 8 to 10 in IHb, 14 to 16 in the outer band of Baillarger in lamina IIIc and layer IV, 12 to 14 in lamina Va, 14 to 16 in the inner band of Baillarger in laminae V b and V i a , and 12 to 14 in lamina VIb. The subcortical association fibers number 16 to 18 to the unit; they are approximately of the same size and depth of stain as the horizontal fibers in lamina VIb. The horizontal exogenous and subcortical association fibers are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The largest fibers in lamina V I b measure 1 .ομ in diameter between varicosities. N o branches have been observed on any horizontal exogenous or subcortical association fibers. These fibers are coarser and more darkly stained than corresponding fibers in area L A in the cortex of the three-month infant. The vertical exogenous fibers in the core of the gyrus number 25 to 30 to the unit just before they begin to radiate toward the cortex. The vertical fibers are small, intermediate, and large in size, and in the Golgi-Cox sections all the fibers bear spines, thorns, and varicosities. The largest fibers measure 1.3/i in diameter between varicosities. As the vertical fibers radiate toward the cortex they are chiefly assembled in bundles. Most of the radiating vertical fibers end in lamina VIb, a few end in the inner band of Baillarger, and an occasional vertical fiber ascends to layer IV. The fibers decrease in size and depth of stain as they ascend in the cortex, and those which end in layer IV are very small and very lightly stained. No collateral or terminal branches have been observed on any vertical exogenous fibers. The vertical exogenous fibers in the core of the gyrus are more compact in structure and more darkly stained than the corresponding fibers in this area in the cortex of the three-month infant.

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MYELiNATiON. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 4 1 91, (2) 37-82, (3) 38-35, (4) 37-49· The stain in the sections of each brain is distinctly darker and more extensive than that in the sections of the next following brain. The stain in the sections of brain 3 7-49 is much lighter and less extensive than that in the sections of brain 4 1 - 9 1 , but it is much darker and more extensive than that in the sections of brain 34188, the three-month brain with the most myelin in area L A . In each brain the stain is but slightly lighter in color than that in areas F B or F C on the medial surface of the superior frontal gyrus. The following description is based upon the sections of brain 37-82. Stained tangential fibers number from 1 to 3 to the unit in the cortex forming the superior wall of the callosal sulcus. The stained fibers are small and light in color. Small, very lightly stained tangential fibers are present in about the same number along the entire length of the crown of the gyrus. In the superior wall of the gyrus in sulcus cinguli the stained tangential fibers number 1 to 4 to the unit, and are larger and darker than those in the crown. N o stained horizontal fibers are present in the cortex external to layer IV. A very small, short, lightly stained horizontal fiber is seen occasionally in layer IV. In lamina V a stained horizontal fibers number 2 or 3 to the unit. In laminae V b and V i a (the inner band of Baillarger) the stained horizontal fibers number 3 or 4 to the unit, and they are larger and darker than those in lamina Va. Stained horizontal fibers are yet larger and darker in lamina VIb, and they number 3 to 5 to the unit. Stained subcortical association fibers number 4 or 5 to the unit, and they are larger and darker in color than the stained horizontal fibers in lamina VIb. The stained horizontal and subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. Stained vertical fibers in the core of gyrus cinguli immediately below the place where they begin to radiate toward the cortex number 16 to 18 to the unit. They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1.3/x in diameter between varicosities. The stained vertical fibers decrease in size and number, and become

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lighter in color as they radiate toward and ascend in the cortex. On most of the ascending fibers the stain ends in the region of the subcortical association fibers. On some fewer fibers it ends in the lower part of lamina VIb, and on a very few it ends in lamina V i a . Stained fibers are much larger, more numerous, and darker in the cingulum, longitudinal striae, and corpus callosum than those in the three-month cortex. FORMATIO LIMBICA SUPERIOR POSTERIOR GRANULARIS, AREA LC The cortex in this area resembles the granular parietal type. The granular layers are prominent. In the crown of the gyrus the cortex measures 2.143 mm. in width. It is illustrated in Figures 164, 165, 166, and 167. CELL-BODIES

Layer I. This layer measures 0.238 mm. in width and contains 56 neurons to the unit. Layer II. The external granular layer is faintly visible macroscopically. It measures 0.116 mm. in width. The neurons number 275 to the unit, and measure 4/4-7/6/*. Layer III. This layer measures 0.660 mm. in width. The neurons number 60 to the unit in lamina I l i a , and measure 10/6-20/10/*. In lamina I l l b the neurons number 57 to the unit, and measure 10/623/12/*. In lamina IIIc the nerve cells number 53 per unit, and they range in size from 10/6 μ to 26/13/1. Layer IV. The internal granular layer is more prominent than the external one. It measures 0.216 mm. in width in the widest places. The neurons number 157 to the unit, and measure 5/4-9/6/*. The layer is invaded by a few cells of laminae IIIc and Va. Layer V. This layer measures 0.446 mm. in width. The small neurons number 61 to the unit in lamina Va and 51 in lamina Vb. The large neurons number 18 in lamina Va and 16 in Vb. The extra-large pyramidal cells number 8 to the unit, and they range in size from 26/10/* to 36/15/*. The small neurons measure 8/6-13/10/*, and the large ones measure 16/10-23/12/*. Layer VI. This layer measures 0.467 mm. in width. The neurons number 83 to the unit in lamina V i a and

49 in lamina VIb. The pyramidal cells measure 10/6-26/12/*, and the spindle cells measure 10/426/τομ. The chromophil substance in the cells in this area, in both the middle and the posterior regions of gyrus cinguli, is in approximately the same condition as that in corresponding cells in area LA. The chromophil substance in all cells in area L C is greater in quantity and more advanced in development than that in corresponding cells in this area of the cortex in the three-month infant. The cells in this area are in the same state of development as to differentiation of neurofibrils as corresponding cells in area LA. FIBERS. The apical dendrites of the extra-large pyramidal cells in layer V are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs appear to be slightly more numerous on the dendrites of these cells than on the dendrites of corresponding cells in the cortex of the three-month infant. In the six-month cortex the dendrites of the extra-large pyramidal cells are in approximately the same state of development as those of corresponding cells in area LA. The dendrites of these cells in area L C are slightly larger and have more pedunculated bulbs than those of corresponding cells in area P E in the precuneus.

The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells have more myelin than the axons of corresponding cells in area P E in the precuneus. The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The axons of the large ordinary and special pyramidal and spindle cells in layers V and V I have more myelin than those of corresponding cells in this area in the three-month

RHINENCEPHALON

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brain. In the six-month cortex the dendrites and axons of these cells are in approximately the same state of development as those of corresponding cells in area LA, but they are more advanced in development than these cells in area P E in the precuneus. The apical dendrites of the large pyramidal cells in layer III are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. In the six-month cortex the dendrites of the large pyramidal cells in lamina IIIc are larger and have more pedunculated bulbs than those of the large pyramidal cells in lamina I l l b , and the dendrites of the large pyramidal cells in lamina I l l b are larger and have more bulbs than those of the large pyramidal cells in lamina I l i a . Pedunculated bulbs are less numerous on the dendrites of the large pyramidal cells in lamina IIIc than on the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI. The dendrites of the pyramidal cells in layer I I I in area L C are in approximately the same state of development as those of corresponding cells in area LA. The dendrites of these cells in area L C are approximately the same size and length as those of corresponding cells in area PE in the precuneus, but they have more pedunculated bulbs than the cells in area PE.

No horizontal cells of Cajal have been seen in layer I. The tangential fibers in layer I number 16 to 18 to the unit in the crown of the gyrus. The fibers are more numerous and more darkly stained in the outer one-half of the layer than in the inner one-half. They are small, intermediate, and large, and in the Golgi preparations all of them bear varicosities, spines, and thorns. The largest fibers measure 1.3/j in diameter between varicosities. No branches have been observed on any tangential fibers. The tangential fibers are larger and more darkly stained than corresponding fibers in this area in the cortex of the three-month infant.

The axons of the large pyramidal cells in layer III are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of the large pyramidal cells in lamina I l i a do not have any indications of myelin. A few small globules sparsely scattered along the surface are the only indications of myelin on the axons of the large pyramidal cells in lamina I l l b . A slight enlargement a short distance distal to the cell-body and several globules scattered along the surface indicate a greater amount of myelin on the axons of the large pyramidal cells in lamina IIIc. The axons of these cells have less myelin than the axons of the large ordinary and special pyramidal and spindle cells in layers V and VI. The axons of the large pyramidal cells in laminae I l l b and IIIc have slightly

Horizontal exogenous fibers in layer II number 1 or 2 to the unit. They are very small and faintly stained. The horizontal fibers gradually increase in number, size, and depth of stain in the successively deeper layers. They number 3 or 4 to the unit in lamina I l i a , 6 to 8 in lamina I l l b , 8 to 10 in the outer band of Baillarger, 6 to 8 in lamina Va, 8 to 10 in the inner band of Baillarger, and 6 to 8 in lamina VIb. The subcortical association fibers number 10 to 12 to the unit. The horizontal exogenous and subcortical association fibers are small, intermediate, and large. In the Golgi sections all the horizontal exogenous and subcortical association fibers bear varicosities, spines, and thorns. The largest fibers are found in lamina VIb and in the subcortical association layer, and they measure 1 .ομ in diameter between varicosi-

more myelin than those of corresponding cells in area PE in the precuneus. The dendrites of the small cells belonging to layers IV and II are larger, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. Golgi type II cells are more numerous in this area than in area LA. The dendrites of these cells are of about the same size and have approximately the same quantity of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size. The axons of the Golgi type II cells form a mesh of fibers in layers IV, V, and VI which is rather thin but is more dense than that in area LA. The vertical exogenous fibers do not contribute any fibers to the mesh. No terminal or collateral branches have been observed on any of the vertical exogenous fibers.

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T H E C O R T E X OF T H E SIX-MONTH INFANT

ties. No branches have been observed on any of the horizontal exogenous or subcortical association fibers. The horizontal exogenous and subcortical association fibers are larger and more darkly stained than corresponding fibers in this area in the cortex of the threemonth infant. The vertical exogenous fibers in the core of the gyrus immediately before they begin to radiate toward the cortex number 25 to 30 to the unit. They are small, intermediate, and large in size, and in the Golgi sections all of them bear varicosities, spines, and thorns. The largest fibers in the Golgi preparations measure 1.3/1 in diameter between varicosities. As the vertical exogenous fibers radiate toward the cortex they are chiefly assembled in bundles. The fibers decrease in size and depth of stain as they ascend. Most of them end in lamina VIb, a few end in the inner band of Baillarger, and occasionally a very small, faintly stained vertical fiber reaches layer IV. No branches, either collateral or terminal, have been observed on any of the vertical exogenous fibers. The vertical exogenous fibers are larger, more compact in structure, and more darkly stained than the corresponding fibers in this area in the cortex of the three-month infant. The brains examined fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 4 1 - 9 1 , (2) 37-82, (3) 38-35, (4) 37-49· In each of these brains the stain in the sections in area L C is slightly lighter than that in the sections in area L A . The stain in the sections in area L C in brains 37-82, 38-35, and 37-49 is slightly lighter than that in the sections in area P E on the medial surface of the superior parietal lobule. The stain in the sections in area L C is much darker and more extensive than that in this area in the three-month cortex. The following description is based upon the sections of brain 37-82. MYELiNATiON.

In the walls of the sulci stained tangential fibers in layer I number 1 to 3 to the unit, but in the crown of the gyrus single stained fibers are scattered at wide intervals. The stained tangential fibers are smaller and lighter in color than those in area L A . N o stained horizontal fibers are present in the cortex external to layer IV. A very small, short, lightly stained fiber is seen occasionally in this layer. In

lamina Va there are 1 or 2 small, lightly stained horizontal fibers to the unit. In laminae V b and V i a (the inner band of Baillarger) stained horizontal fibers number 2 or 3 to the unit, and they are larger, longer, and darker than those in lamina Va. In lamina V I b the stained horizontal fibers number 3 or 4 to the unit, and they are slightly larger, longer, and darker than those in lamina V i a . The stained subcortical association fibers number 4 or 5 to the unit, and they are larger and darker in color than the stained horizontal fibers in lamina VIb. The stained horizontal fibers in the cortex and the stained subcortical association fibers are small, intermediate, and large in size, and all bear varicosities. They are more lightly stained than corresponding fibers in area L A . Immediately below the place where they begin to radiate toward the cortex the stained vertical fibers in gyrus cinguli number 18 to 20 to the unit. The vertical fibers are stained more lightly than those in area L A . They are small, intermediate, and large in size, and all bear varicosities. The largest fibers measure 1.3/A in diameter between varicosities, but fibers of this size are few in number. As the stained vertical fibers radiate toward and into the cortex they decrease in size and number and become lighter in color. On most of the fibers the stain ends in the region of the subcortical association fibers, on some fewer it ends in the lower part of lamina VIb, and on very few it ends in lamina V i a . FORMATIO RETRO-SPLENIALIS AGRANULARIS, LD This area has the same characteristics and distribution as in the three-month cortex. It measures 2.192 mm. in width, and is illustrated in Figures 168, 169, 170, and 171. CELL-BODIES

Layer I. The molecular layer measures 0.255 mm. in width and contains 58 neurons to the unit. Layer II. The external granular layer measures 0.118 mm. in width. The neurons number 236 to the unit, and measure 4/4-7/6/1. Many large neurons of lamina I l i a invade the granular layer. Layer III. This layer measures 0.789 mm. in width. In lamina I l i a the neurons number 65 to the unit, and measure 10/6-2ο/ιομ. In lamina I l l b the neu-

RHINENCEPHALON rons number 57 per unit, and measure 10/6-23/10^. The nerve cells in lamina IIIc number 53 to the unit, and they range in size from ιο/6/χ to 26/13/*. Throughout the layer large neurons are more numerous than small ones. Layer IV. The inner granular layer measures 0.240 mm. in width. It is not prominent, and is richly invaded by the neurons of laminae IIIc and Va. The neurons of the granular layer number 140 to the unit, and measure 6/4—8/6/i. Layer V. The internal pyramidal layer measures 0.388 mm. in width. Small neurons number 58 to the unit in lamina Va and 54 in lamina Vb. Large neurons number 20 per unit in lamina Va and 18 in lamina Vb. The small neurons measure 7/6-13/ιομ, and the large ones measure 16/10-23/12^. The extra-large pyramidal cells are located chiefly in lamina Va but some are present in lamina Vb also. These cells number 8 to the unit, and range in size from 26/ΐΟμ. to 36/12/*. Layer VI. This layer measures 0.402 mm. in width. The neurons number 75 in lamina V i a and 49 in VIb. The pyramidal cells measure 10/6-2 3/12 μ, and the spindle cells measure 10/4-23/ιομ. The chromophil substance in all the cells in this area is present in approximately the same quantity and condition of development as that in corresponding cells in areas LA and LC. In all cells the chromophil substance is greater in quantity and more advanced in development than that in corresponding cells in area L D in the cortex of the three-month infant. In the Cajal sections the extra-large pyramidal cells in layer V are stained more darkly than any other cells in this area. The cell-bodies are stained about the same color as the processes. Several longitudinal rows of granules are present in the apical dendrites and cell-bodies, but there are no neurofibrils. The large ordinary and special pyramidal and spindle cells in layer V are more lightly stained than the extra-large pyramidal cells in layer V, but are darker in color than the small cells in layers V and VI. The cytoplasm of these cells is filled with granules which are diffusely scattered throughout the bodies and processes. The largest pyramidal cells in lamina IIIc are stained approximately the same color as the large

143

ordinary and special pyramidal and spindle cells in layers V and VI. The large pyramidal cells in lamina I l l b are stained more lightly than those in lamina IIIc, and the large pyramidal cells in I l i a are lighter in color than those in I l l b . The bodies of all cells in layer III are stained the same color as the axons and dendrites. The granules in all cells are diffusely scattered throughout the bodies and processes. The cells in area L D are more advanced as to differentiation of neurofibrils than corresponding cells in this area in the three-month cortex. In the six-month cortex the cells in area L D are slightly more advanced as to differentiation of neurofibrils than corresponding cells in areas LA and LC. fibers. The apical dendrites of the extra-large pyramidal cells in layer V and of the large pyramidal cells in layer III are coarser, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the threemonth cortex. The basal dendrites and the collateral branches of the apical dendrites are coarser, longer, and more compact in structure, but no more numerous than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all these cells than on corresponding cells in the three-month cortex. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The axons of the large pyramidal cells in laminae IIIc and I l l b have more myelin than those of corresponding cells in the cortex of the three-month infant. In the six-month cortex the axons of the large pyramidal cells in lamina IIIc have more myelin than those of the large pyramidal cells in lamina I l l b . The axons of the large pyramidal cells in lamina I l i a have no indications of myelin. The large pyramidal cells in lamina IIIc have much less myelin than the axons of the extra-large pyramidal cells in layer V. The dendrites and axons of the extra-large pyramidal cells in layer V and of the large pyramidal cells in layer III are in approximately the same state of development as those of corresponding cells in area LC. The dendrites of the small cells belonging to layers II and IV are coarser, longer, more compact in structure, and have more pedunculated bulbs than those of

144

T H E C O R T E X OF T H E SIX-MONTH INFANT

corresponding cells in this area in the three-month cortex. In the six-month cortex these cells are in approximately the same state of development as corresponding cells in area L C . Large and small stellate cells, like those which are characteristic of layer I I in area H A , are occasionally seen in layer II. The dendrites of the large stellate cells are as large and have as many pedunculated bulbs as the basal dendrites of the extra-large pyramidal cells in layer V. The axons of these stellate cells descend vertically into layer III. The axon of one of the large stellate cells descends into lamina I H b . It has as much myelin as the axons of the extra-large pyramidal cells in layer V. Two slender collaterals emerge from the axon at right angles and extend horizontally, one in lamina I l i a and the other in lamina I H b . Each of these collaterals has a very short and delicate secondary branch. The dendrites of the Golgi type II cells are in approximately the same state of development as the basal dendrites of pyramidal cells of comparable size. The axons of these cells form a very thin mesh of fibers in layers I V , V, and V I . The mesh is much thinner than that in area L C . No horizontal cells of Cajal have been observed in layer I. The tangential fibers in layer I number 20 to 25 to the unit. They are small, intermediate, and large in size. The largest fibers measure 1.3ft in diameter. The tangential fibers are larger and more darkly stained than those in area L D in the three-month cortex. Horizontal exogenous fibers number 2 or 3 to the unit in layer II, 3 or 4 in lamina I l i a , 4 or 5 in lamina IHb, 8 to 10 in the outer band of Baillarger and in lamina Va, 10 to 12 in the inner band of Baillarger, and 12 to 14 in lamina VIb. All the horizontal exogenous fibers are stained more lightly than the tangential fibers in layer I except those in lamina Vb, and they are stained about the same color as the tangential fibers. The subcortical association fibers number 12 to 14 to the unit, and they are stained slightly more darkly than the horizontal fibers in lamina VIb. The horizontal and subcortical fibers are small, intermediate, and large in size. They are larger and more darkly stained than corresponding fibers in this area in the three-month cortex. The horizontal exogenous

fibers in lamina VIb are larger than any other horizontal fibers in the cortex. T h e largest fibers in lamina VIb measure τ.ομ in diameter. The subcortical association fibers are larger than the horizontal exogenous fibers in lamina VIb. The largest subcortical association fibers measure 1.4/i in diameter, but fibers of this size are few in number. N o branches have been observed on any of the horizontal exogenous or subcortical association fibers. The vertical exogenous fibers in the core of gyrus cinguli immediately before they begin to radiate toward the cortex number 25 to 30. They are small, intermediate, and large in size. The largest fibers measure 1.3/t in diameter. The vertical exogenous fibers are larger and are stained more darkly than those in this area in the cortex of the three-month infant. MYELINATioN. The state of development of myelin in this area is discussed with that in area L E . FORMATIO RETRO-SPLENIALIS GRANULOSA, LE This area has the same characteristics and distribution as in the adult brain. The two granular layers are prominent, and small cells are so numerous in layer I I I as to give it a granular appearance. The cortex in this area measures 1.716 mm. in width. It is illustrated in Figures 172, 173, 174, and 175. CELL-BODIES

Layer I. This layer measures 0.207 m m · i n width and contains 59 neurons to the unit. Layer II. The external granular layer is clearly visible macroscopically. It measures 0.116 mm. in width. The neurons number 256 to the unit, and measure 4/4-8/6/*. Layer III. The external pyramidal layer measures 0.362 mm. in width. Small cells are numerous throughout the layer. In lamina I l i a the neurons number 71 to the unit, and measure 10/6-20/10^. In lamina I H b the neurons number 59 to the unit, and measure 10/6-23/12/*. The neurons number 53 per unit in lamina IIIc, and range in size from ιο/6μ to 23/12/*. Layer IV. The internal granular layer is more prominent than the external one. It measures 0.172 mm.

RHINENCEPHALON in width. T h e neurons number 123 to the unit, and measure 4/4-8/6/*. Layer V. T h e internal pyramidal layer measures 0.316 mm. in width. T h e small neurons number 55 to the unit in lamina Va and 53 in Vb. The large neurons number 22 per unit in lamina V a and 20 in lamina Vb. T h e extra-large pyramidal cells number 10 to the unit, and range in size from 26/ιομ, to 36/12/Λ. T h e small neurons measure 9 / 6 - 1 3 / ι ο μ , and the large ones measure 16/10-23/12^. Layer VI. This layer measures 0.543 mm. in width. The neurons number 81 to the unit in lamina V i a and 45 in V I b . T h e pyramidal cells measure 1 0 / 6 26/ι^μ, and the spindle cells measure 10/4-26/10^. T h e chromophil substance in all the cells in this area is present in approximately the same quantity and condition of development as that in corresponding cells in areas L A and L C . In all cells the chromophil substance is greater in quantity and more advanced in development than that in corresponding cells in area L E in the cortex of the three-month infant. T h e cells are more advanced as to differentiation of neurofibrils than corresponding cells in this area in the three-month cortex. In the six-month cortex the cells in area L E are in about the same state of development as to differentiation of neurofibrils as corresponding cells in area L D . T h e apical dendrites of the extra-large pyramidal cells in layer V are more compact in structure and have more terminal branches in layer I, but they are no larger than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are about the same size and length as those of corresponding cells in the three-month cortex, but they are more compact in structure than at the threemonth stage. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. FIBERS.

The larger, myelin month

axons of the extra-large cells in layer V are more compact in structure, and have more than those of corresponding cells in the threecortex.

T h e apical and basal dendrites of the large ordinary and special pyramidal and spindle cells in layers

145

V and V I are about the same size as those of corresponding cells in this area in the three-month cortex, but they are more compact in structure and have more pedunculated bulbs in the six-month cortex. The axons of these cells are larger, more compact in structure, and have more myelin than in the cortex of the three-month infant. In the six-month cortex the dendrites of these cells are smaller, shorter, and have fewer pedunculated bulbs than those of the extralarge pyramidal cells in layer V. T h e axons of the ordinary pyramidal and spindle cells are smaller and have much less myelin than those of the extra-large pyramidal cells. T h e apical dendrites of the large pyramidal cells in layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. T h e basal dendrites and the branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than in the cortex of the three-month infant. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. T h e axons of the large pyramidal cells in layer I I I are coarser than those of corresponding cells in this area in the three-month cortex. N o indications of myelin are present on the axons of the large pyramidal cells in lamina I l i a . A few small, widely scattered globules constitute the only indications of myelin on the axons of the large pyramidal cells in lamina I l l b . The axons of the large pyramidal cells in lamina I I I c increase slightly in size a short distance distal to the cell-bodies and have numerous globules on their surfaces, all of which probably indicate the presence of myelin. T h e axons of the large pyramidal cells in lamina I I I c have less myelin than those of the large ordinary and special pyramidal and spindle cells in layers V and V I . T w o or three collateral branches are present on the axons of some of the large pyramidal cells in laminae I l l b and IIIc. Each collateral emerges from the axon at a right angle. T h e collaterals are but slightly larger than those on the axons of corresponding cells in the three-month cortex, and they bear no indications of myelin. T h e dendrites of the small pyramidal cells of layer I V are coarser, longer, more compact in structure, and have more pedunculated bulbs than those of

146

THE

CORTEX

OF

THE SIX-MONTH

INFANT

corresponding cells in this area in the three-month

of brain 37-82 is distinctly darker than that in the

cortex.

sections of brain 38-35. T h e stain in brain 38-35 is

T h e dendrites of the small pyramidal and granular

much darker and more extensive than that in brain

cells of layer I I are coarser, longer, more compact in

34-188, the three-month brain with the most myelin

structure and have more pedunculated bulbs than

in these two areas.

those of corresponding cells in this area in the three-

brains the stain in areas L D and L E is about the same

month cortex.

color as that in area L C . T h e following description

T h e dendrites of the small cells in

layer I I are smaller, shorter, and have fewer pedun-

In each of the two six-month

is based upon the sections of brain 3 7 - 8 2 .

culated bulbs than those of the small cells of layer I V .

There are no stained tangential fibers in layer I

T h e dendrites of the Golgi type I I cells are in ap-

and no stained horizontal fibers external to lamina

proximately the same state of development as those

V i a . In this lamina stained horizontal fibers number

of pyramidal cells of comparable size. T h e axons of

ι or 2 to the unit. In lamina V I b stained horizontal

the Golgi type I I cells form a mesh of fibers which is

fibers number 4 to 6 to the unit. Stained subcortical

of approximately the same extent and density as that

association fibers number 6 to 8 to the unit.

in this area in the cortex of the three-month infant.

stained horizontal and subcortical fibers are small,

The

In the six-month cortex the dendrites and axons of

intermediate, and large in size. T h e stained subcorti-

the cells in area L E are in approximately the same

cal association fibers are larger and darker in color

state of development as those of corresponding cells

than the stained horizontal fibers in lamina V I b .

in area L D .

Stained vertical fibers number 18 to 20 to the unit

N o horizontal cells of C a j a l have been observed in

immediately below the place where they begin to

layer I. N o inverted pyramidal cells have been seen

radiate toward the cortex. T h e stained vertical fibers are small, intermediate, and large in size, and all bear

in the cortex in this area. T h e tangential, horizontal exogenous, subcortical

varicosities. T h e largest stained vertical fibers meas-

association, and vertical exogenous fibers in this area

ure ΐ · 3 μ in diameter between varicosities.

are of approximately the same number and in the

stained vertical fibers radiate toward and into the

A s the

same state of development as those in area L D .

cortex they decrease in size and number, and become lighter in color. On most of the fibers the stain ends

T h e Weigert sections of brains 37-82

in the region of the subcortical association fibers, on

and 38-35 only are available for the study of myeli-

some fewer it ends in the lower part of lamina V I b ,

nation in areas L D and L E . T h e stain in the sections

and on a very few it ends in lamina V i a .

MYELINATION.

GYRUS T H E MAJOR

HIPPOCAMPI

part of the cortex in the hippocampal re-

AREA UNCINATA,

HA

gion is usually assigned to the allocortex, but the six

T h i s area is characterized b y the presence of

layers typical of the isocortex are clearly recognizable

glomeruli of large cells in layer I I . T h e cortex in the

in the uncus and in the crown and external wall of

pole of the uncus measures 2.762 mm. in width. It is

gyrus hippocampi.

illustrated in Figures 176, 177, 178, and 179.

In the hippocampus and gyrus

dentatus, however, the cortex is considerably modified, and while six layers can not be identified, the

CELL-BODIES

largest neurons are located in the deepest strata and

Layer I. T h i s layer measures 0.365 mm. in width and

the small ones are external to these, the smallest being

contains 53 neurons to the unit.

the most superficial, which is the order of arrange-

Layer II.

ment in the isocortex. T h e same areas are selected for

mm. in width.

description as have been described in the three-month

having clumps, or glomeruli, of large neurons in layer

cortex. T h e cortex in each area has the same char-

I I . Small granular and pyramidal cells are scattered

acteristics in the six-month brain as it has in the brain

among the large cells and between the glomeruli. T h e

of the three-month infant.

neurons number 46 to the unit. T h e small neurons

T h e external granular layer measures 0.402 In this area this layer is unique in

147

RHINENCEPHALON measure 4/4-8/6/11. T h e large neurons range in size

the apical dendrite is indicated b y a small, d a r k l y

f r o m 2 θ / ΐ 2 μ to 53/16/*.

stained clump of chromophil substance, but no col-

Layer

laterals are stained.

III.

T h e external p y r a m i d a l l a y e r measures

T w o or three basal dendrites

0.950 mm. in w i d t h . T w o laminae can be identified.

on each cell are stained for a v e r y short distance

In lamina I l i a the neurons n u m b e r 50 to the unit,

a n d contain a f e w small particles of

a n d measure 1 0 / 6 - 2 9 / 1 2 μ.

substance.

T h e outer p a r t of the

chromophil

lamina is f r e q u e n t l y invaded b y the large neurons of

T h e large cells in l a y e r V I contain less chromophil

l a y e r I I . I n lamina I l l b the neurons n u m b e r 40 to

substance a n d are stained m o r e lightly than the large

the unit, and measure 1 0 / 6 - 3 3 / 1 2 μ .

cells in layer V . T h e largest p y r a m i d a l and spindle

Layer IV.

T h e inner granular layer is represented b y

cells in l a y e r V I are smaller, contain less chromophil

a small n u m b e r of small cells scattered a m o n g large

substance in proportion to the size of the nuclei, and

cells. T h e large cells belong to lamina I l l b and the

are stained more lightly than the largest cells in l a y e r

outer p a r t of l a y e r V . T h e granular l a y e r measures

V I in areas L A and L C .

0.146 m m . in width. T h e small neurons n u m b e r o n l y 46 to the unit, a n d measure 8 / 6 - 1 6 / i o / n . Layer 0.219

V. mm

T h e internal p y r a m i d a l l a y e r

measures

- i n w i d t h . T h e usual two laminae can be

identified.

T h e cells in lamina I l i a are slightly larger, contain more chromophil substance, a n d are more

Small and large neurons are

stained than those in lamina I l l b .

darkly

T h e cells in the

outer one-half of l a y e r I I I are of about the same size

scattered

and color and h a v e a p p r o x i m a t e l y the same q u a n t i t y

throughout the layer, b u t the large cells are more

of chromophil substance as the large cells in l a y e r V .

n u m e r o u s in lamina V b than in V a .

The

L a m i n a V a is

m u c h lighter than V b .

T h e neurons n u m b e r 50 to

the unit in lamina V b .

T h e small neurons measure

10/6-13/10^.,

substance

in

all the

large

cells

small a n d large clumps. T h e small clumps predomi-

16/10-

nate in number, but large clumps are present in al-

23/12/x, and the extra-large cells range in size f r o m

most e v e r y cell. T h e chromophil substance extends

2 9 / ι ο μ . to 3 6 / 1 2 μ .

into the apical dendrite in less q u a n t i t y and f o r a

Layer

T h i s layer measures 0.680 mm. in the

shorter distance than in the large p y r a m i d a l cells in

widest parts. T h e neurons n u m b e r 48 to the unit in

l a y e r V . A d a r k cone or disc of chromophil substance

VI.

the large neurons measure

chromophil

throughout l a y e r I I I is in the f o r m of N i s s l flakes and

The pyramidal

rests upon the nuclear m e m b r a n e immediately under

1 0 / 6 - 2 3 / 1 2 μ , and the spindle cells

the base of the apical dendrite. T h e m e m b r a n e of the

lamina V i a and 35 in lamina V I b . cells measure

nucleus is thick and d a r k l y stained.

measure 1 0 / 4 - 3 9 / 1 ζ μ . I n the c r e s y l violet sections the large cells in l a y e r

T h e stellate and p y r a m i d a l cells in the glomeruli

V contain more chromophil substance a n d are stained

in layer I I are larger than a n y other cells in this area.

more d a r k l y than a n y cells in l a y e r s I I I , I V , and V I ,

T h e y contain more chromophil substance in propor-

b u t t h e y are not stained as d a r k l y as the large cells in

tion to the size of the nucleus and are more d a r k l y

layer I I .

stained than the large cells in l a y e r V or in lamina

T h e chromophil substance is present in

a p p r o x i m a t e l y the same q u a n t i t y in proportion to the

Ilia.

size of the nucleus, and is in a b o u t the same state of

the same condition of d e v e l o p m e n t as that in the large

d e v e l o p m e n t as t h a t in the extra-large

pyramidal

cells in lamina I l i a a n d l a y e r V . I t extends into the

cells in l a y e r V in area L A . I t is in the f o r m of small

dendrites in greater q u a n t i t y a n d for a greater dis-

and large c l u m p s a n d N i s s l

tance than that in the large cells in l a y e r V

flakes.

N i s s l flakes and

small clumps extend into the apical dendrite for a

T h e chromophil substance is in a p p r o x i m a t e l y

and

lamina I l i a .

distance equal to about one and one-half times the

T h e chromophil substance in all cells is greater in

length of the cell-body. I n almost e v e r y cell a d a r k l y

q u a n t i t y and more a d v a n c e d in differentiation than

stained disc or cone of chromophil substance rests

that in corresponding cells in this a r e a in the cortex

upon the nuclear m e m b r a n e immediately under the

of the three-month infant.

base of the apical dendrite.

T h e place of origin of

f r o m one to three of the most p r o x i m a l collaterals of

N o neurofibrils are present in a n y cells in this area.

I n all the cells the c y t o p l a s m is

filled

with

148

THE CORTEX OF THE SIX-MONTH INFANT

granules which are scattered irregularly throughout the cell-bodies and processes. The dendrites of the extra-large pyramidal cells in layer V are larger, longer, and have more pedunculated bulbs than those of any other cells in this area except the giant horizontal cells. The latter cells are seen occasionally in layers III, V, or VI. The apical dendrites of the extra-large pyramidal cells are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on all the dendrites of these cells than on the dendrites of corresponding cells in the cortex of the three-month infant. FIBERS.

The axons of the extra-large pyramidal cells in layer V descend vertically, pursuing a more or less tortuous course, into the white matter in the core of the gyrus where they mingle with the vertical exogenous fibers. As they traverse lamina V i a the axons become slightly larger and have small globules clinging to their surfaces. In lamina VIb the axons increase slightly in size and the globules on the surfaces increase in size and become more numerous, giving the axons a beaded appearance. The axons of the extra-large pyramidal cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are larger and have more myelin than those of any other cells in this area. The dendrites and axons of the extra-large pyramidal cells in layer V are slightly more advanced in development than those of corresponding cells in areas L D and T G . The dendrites of the large ordinary and special pyramidal and spindle cells in layers V and VI are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the cortex of the three-month infant. In the six-month cortex the dendrites of these cells are smaller and have fewer pedunculated bulbs than those of the extra-large pyramidal cells. The axons of these cells are larger and have more myelin

than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are smaller and have less myelin than the axons of the extra-large pyramidal cells in layer V. The dendrites and axons of the large ordinary and special pyramidal and spindle cells in layers V and V I in this area are more advanced in development than those of corresponding cells in areas L D and T G . Two inverted large ordinary pyramidal cells have been observed in layer VI. The apical dendrites of these cells are directed vertically downward and end in the lower part of layer V I by terminal arborizations. The collateral branches of the apical dendrites are directed obliquely downward. Some of the basal dendrites are directed horizontally in lamina Vb and others are directed upward into lamina Va. Pedunculated bulbs are present on the dendrites of the inverted cells in approximately the same quantity and are distributed in the same manner as on the dendrites of other large ordinary pyramidal cells in layer VI. The axons extend vertically upward into lamina Va, then form a loop, and descend vertically. The axons are in approximately the same condition of myelination as the axons of other large ordinary pyramidal cells in layer VI. The dendrites of the giant horizontal cells in layer V I are in approximately the same state of development as the basal dendrites of the extra-large pyramidal cells in layer V as to size and as to the quantity of pedunculated bulbs. The proximal end of at least one dendrite is about the same size as the proximal end of the apical dendrite of an extra-large pyramidal cell, but it quickly tapers to a smaller size. The axons of the giant horizontal cells have less myelin than those of the extra-large pyramidal cells. The dendrites and axons of the giant horizontal cells are more advanced in development than those of corresponding cells in this area in the three-month cortex. The apical dendrites of the large pyramidal cells in layer I I I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those of corresponding cells in the three-

RHINENCEPHALON month cortex. Pedunculated bulbs on the dendrites of the pyramidal cells in layer I I I are more numerous than in the cortex of the three-month infant. In the six-month cortex the dendrites of the largest cells in lamina I I I c are smaller, shorter, and have fewer pedunculated bulbs than the dendrites of the extralarge pyramidal cells in layer V . T h e dendrites of the largest pyramidal cells in lamina I l l b are larger, but have fewer pedunculated bulbs, than the dendrites of the large ordinary and special pyramidal and spindle cells in layers V and V I . T h e dendrites of the large cells in lamina I l l b are larger, longer, and have more pedunculated bulbs than the large pyramidal cells in lamina I l i a . T h e presence of myelin on the axons of the large pyramidal cells in lamina I l l b is indicated b y a slight increase in caliber a short distance distal to the cellbodies and b y a few small globules scattered along the surfaces of the axons. T h e axons of these cells are as large as those of the large ordinary and special pyramidal and spindle cells in layers V and V I , but have less myelin. N o indications of myelin are present on the axons of the large pyramidal cells in lamina I l i a . The dendrites and axons of the pyramidal cells in layer I I I appear to be in approximately the same state of development as those of corresponding cells in area T G . Giant horizontal cells are seen occasionally in layer I I I . T h e y occur principally in lamina I l l b . Some of them are as large as the giant horizontal cells in layer V I , but they are usually slightly smaller. Small pyramidal cells are few in number in layer I V . Their dendrites are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the threemonth cortex. Granular and small pyramidal cells in layer I I are located chiefly between the glomeruli, but a few of these cells are present in the glomeruli. T h e dendrites of the granular and small pyramidal cells are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in this area in the three-month cortex. In the sixmonth cortex the dendrites of these cells are smaller, shorter, and have fewer pedunculated bulbs than those of any other cells in this area of the cortex. T h e stellate cells in layer I I occur chiefly in the

149

glomeruli, but a few are scattered between the glomeruli. T h e cell-bodies of these cells are of about the same size as the cell-bodies of the large pyramidal cells in laminae I l i a and I l l b . T h e apical dendrites of the stellate cells in layer I I are larger and longer than the basal dendrites, but not as large or as long as the apical dendrites of the pyramidal cells of comparable size in layer I I I . T h e cell-bodies of the stellate cells are larger than those of corresponding cells in this area in the three-month cortex. T h e dendrites are larger, longer, more compact in structure, and have more pedunculated bulbs than in the threemonth cortex. T h e apical dendrites of the stellate cells have more pedunculated bulbs than the basal dendrites, but not as many as the apical dendrites of the pyramidal cells of comparable size in layer I I I . T h e axons of the stellate cells descend vertically downward into layer I I I . T h e y are of about the same size and are in approximately the same state of myelination as the axons of the large pyramidal cells of comparable size in layer I I I . T h e mesh of fibers formed b y the axons of Golgi type I I cells is slightly more dense than that in this area in the three-month cortex. T h e mesh is thinnest in laminae I l l b and V a and is most dense in layers I V and V I , having about the same density in these two layers. T h e tangential fibers are larger, more darkly stained, and more numerous in the middle of layer I than in the outer or inner regions. In the mid-region the fibers number 14 to 16 to the unit. T h e y are small, intermediate, and large in size, and in the Golgi sections all the fibers bear varicosities, spines, and thorns. T h e largest fibers measure τ.ομ in diameter between varicosities. N o branches have been observed on any tangential fibers. Horizontal exogenous fibers number 1 or 2 to the unit in layer II, 3 or 4 in lamina I l i a , 5 or 6 in lamina I l l b , 8 to 10 in layer I V , 6 to 8 in lamina Va, 8 to 10 in laminae V b and V i a , and 6 to 8 in lamina V I b . T h e subcortical association fibers number 8 to 10 to the unit. T h e horizontal fibers in layer I I are very small and lightly stained, but the horizontal fibers gradually become larger and more darkly stained in the successively deeper strata and are largest and darkest in color in lamina V I b and in the zone of subcortical association fibers. T h e largest horizontal

T H E C O R T E X OF T H E S I X - M O N T H I N F A N T

ISO

exogenous fibers in lamina V I b measure τ.ομ.

No

number 6 to 8 to the unit, and they are larger and

branches have been observed on any of the horizontal

darker than the stained horizontal

exogenous or subcortical association fibers.

ina V I b .

T h e vertical exogenous fibers number 25 to 30 to

fibers

in lam-

Stained vertical fibers number 18 to 20 to the unit

T h e y are

in the core of the gyrus. T h e y decrease in size and

small, intermediate, and large in size, and in the Golgi

number, and become lighter in color, as they radiate

the unit immediately below lamina V I b .

preparations all of them bear varicosities, spines, and

toward and into the cortex. T h e stain ends on most

thorns. T h e largest fibers measure ι.ομ in diameter

of the fibers in the lower part of lamina V I b , on some

between varicosities.

T h e vertical exogenous

fibers

decrease in size and number and become lighter in

fewer fibers it ends in lamina V i a , and on a very few fibers the stain ends in the lower part of layer I V .

color as they ascend in the cortex. M o s t of them end

AREA PRESUBICULARIS GRANULOSA, HD

in lamina V i a , m a n y end in layer I V , and some ascend as high as the middle of layer I I I . N o branches,

T h e cortex in this area has the same characteristics

either collateral or terminal, have been observed on

and distribution as in the three-month brain, and the

any vertical exogenous fibers.

same as in the adult as described b y Economo.

T h e tangential, horizontal exogenous, subcortical association, and vertical exogenous fibers are coarser, more compact in structure, and more d a r k l y stained

The

cortex in area H D measures 2.120 mm. in width. I t is illustrated in Figures 180, 181, 182, and 183.

than corresponding fibers in this area in the cortex

CELL-BODIES

of the three-month infant.

Layer I. T h e molecular layer measures 0.460 mm. in width and contains 57 neurons to the unit.

MYELiNATioN.

T h e brains examined fall into the

Layer

//-///.

T h e external granular and external

following sequence with reference to the depth and

pyramidal layers are combined as one layer which

extent of the stain in the Weigert sections: ( 1 ) 3 7-82,

has a granular appearance. T h e layer measures 0.657

(2) 38-35, ( 3 ) 3 7 - 4 9 . T h e stain in the sections of

mm. in width in the widest places. Both the external

brain 37-82 is slightly darker and more extensive

and internal margins are fairly sharp. T h e neurons

than that in the sections of brain 38-35. T h e stain in

number 2 60 to the unit in the outer one-half of

brain 3 7 - 4 9 is distinctly lighter and less extensive

layer and 61 in the inner one-half. Small and large

the

than that in brain 38-35. T h e stain in the sections of

cells are scattered throughout the layer, but the large

brain 3 7 - 4 9 is slightly darker, but is more extensive,

ones are larger in the inner one-half of the layer. In

than that in the sections of brain 38-128, the three-

the outer one-half of the layer the neurons measure

month brain with the most myelin in this area.

The

5/4-16/io/x. In the inner one-half they range in size

following description is based upon the sections of

from 10/6μ to 23/12^.

brain 37-82. T h e stain in area H A is of about the

Layer IV.

same color as that in the temporal pole, area T G .

II—III and layer V - V I there is a light band in which

L y i n g between the inner margin of layer

Stained tangential fibers in layer I number 6 to 8

a few small cells are scattered. T h i s light band m a y

to the unit. T h e y are small, intermediate, and large

represent an internal granular layer. T h e neurons are

in size, and are lightly stained. N o stained horizontal

small pyramidal cells and small Golgi type I I cells

fibers are present in the cortex external to layer V . In

numbering 45 to the unit and measuring 8 / 4 - 1 6 / ι ο μ .

lamina V a there are from 1 to 3 small, short, lightly

T h e layer measures 0.200 mm. in width.

stained horizontal fibers to the unit. In laminae V b

Layer V-VI.

and V i a (the inner band of Baillarger) stained hori-

there is one broad layer which represents a fusion of

zontal fibers number 3 or 4 to the unit, and they are

layers V and V I .

larger, longer, and darker than those in lamina V a .

0.803

In lamina V I b stained horizontal fibers number 4 to 6

dle, and Golgi type I I cells are scattered throughout

to the unit, and they are larger and darker than those

the layer, but large cells predominate in number in

in lamina V i a . Stained subcortical association fibers

the outer part of the layer. These large cells probably

m m

Internal to the light band of layer I V T h i s combined layer measures

· i n width. Small and large pyramidal, spin-

RHINENCEPHALON represent the extra-large cells of layer V in other areas. The neurons number 50 to the unit. T h e small cells measure 10/6-13/10^, and the large ones range in size from 1 6 / ι ο μ . to 29/ 13/x. T h e large pyramidal and spindle cells in layer V V I contain more chromophil substance and are more darkly stained than the cells in layer II—III. In the large cells in V - V I the chromophil substance is chiefly in the form of large clumps. Small clumps and a few Nissl flakes are also present. T h e nuclear membrane is thick and darkly stained. A heavily stained disc or cone of chromophil substance rests upon the nuclear membrane immediately below the apical dendrite of the pyramidal cells, and below both polar dendrites of the spindle cells. Small clumps and particles of chromophil substance invade these dendrites for a distance approximately equal to the length of the cell-body. Basal dendrites are faintly stained for a short distance, but do not contain any Nissl bodies or clumps of chromophil substance. T h e cells in layer II—III contain much less chromophil substance in proportion to the size of the nucleus and are more lightly stained than the large cells in layer V - V I . T h e chromophil substance is chiefly in the form of large clumps, but small clumps and a few Nissl bodies are also present. T h e chromophil substance in all cells is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the cortex of the three-month infant. In the C a j a l sections all cells are filled with granules which are scattered irregularly throughout the cell-bodies and processes. N o neurofibrils or rows of granules are present in any cells. T h e bodies of the cells in this area are smaller and their dendrites and axons are smaller than those of corresponding cells in area H A . T h e apical dendrites of the extra-large pyramidal cells in layer V - V I are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the

FIBERS.

151

dendrites of these cells than in the three-month cortex. T h e apical dendrites of the extra-large spindle cells in layer V - V I are smaller than the apical dendrites of the large pyramidal cells in this layer, but the basal dendrites of the spindle cells are fully as large as those of the extra-large pyramidal cells. Pedunculated bulbs are present on the dendrites of the pyramidal and spindle cells in layer V - V I in slightly less quantity than on the dendrites of the extra-large pyramidal cells in layer V in area H A . T h e axons of the extra-large pyramidal and spindle cells in layer V - V I are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. In the six-month cortex the axons of these cells are larger and have more myelin than the axons of any other cells in this area. T h e axons of these cells are smaller and have less myelin than the axons of the extra-large pyramidal cells in area H A . T h e cells in the internal part of layer II—III are pyramidal and Golgi type I I cells. Proceeding externally in the layer the cells of both these types gradually become smaller, and are smallest in the external part of the layer. Small granule cells are present in the external part only. The apical dendrites of the larger pyramidal cells extend into the molecular layer. T h e apical dendrites of all the cells are larger, and more compact in structure, and those which reach layer I have more terminal branches, than the apical dendrites of corresponding cells in this area in the three-month cortex. T h e basal dendrites and collateral branches of the apical dendrites are coarser, longer, and more compact in structure, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of these cells than in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the cells in layer I I - I I I in this area than on the dendrites of corresponding cells in layers I I and I I I in area H A . Golgi type I I cells are much more numerous in this area than in area H A . T h e dendrites of these cells are in approximately the same state of development as the basal dendrites of pyramidal cells of corresponding size. T h e axons of the Golgi type I I cells form a mesh of fibers which extends from the most ex-

THE CORTEX OF THE SIX-MONTH INFANT

152

ternal part of layer II—III to the innermost part of layer V - V I . The mesh is of maximum density in the inner part of layer II—III. In length the mesh extends from area H C to area H E l a and is of about the same density throughout the length of area HD. This dense mesh begins rather abruptly at the junction of area H D with H C and ends abruptly at the junction of H D with area H E l a . The mesh is more dense than that in area H D in the three-month cortex. In the cortex of the six-month infant the mesh in area H D is of about the same density as that in area T C . The state of myelination is discussed at the end of the description of area HF.

MYELINATION.

AREA PYRAMIDALIS,

HE

The area of pyramidal cells has the same characteristics, distribution, and divisions as described by Economo in the adult cortex. AREA PYRAMIDALIS PRESUBICULI, H E L A

The cortex in this area measures 2.191 mm. in width. It is illustrated in Figures 184, 185, 186, and 187. CELL-BODIES

Layer I. The molecular layer measures 0.658 mm. in width and contains 50 neurons to the unit. Layer II. A few small neurons scattered among the large neurons at the external margin of layer III constitute the only trace of an external granular layer. Layer III-IV. This layer consists almost entirely of large pyramidal cells. A few small cells scattered along the inner part of the layer suggest an internal granular layer. The layer measures 1.022 mm. in width. The neurons are smallest externally, gradually increase in size internally, and are largest at the junction of this layer with layer V. In the external part of the layer the neurons number 45 to the unit, and measure 13/10-33/12^.. In the internal part of the layer they number 36 per unit, and range in size from 16/ιομ, to 39/13/i. Most of the neurons are elongated. Layer V. This layer represents a combination of layers V and VI. It measures 0.511 mm. in width in the widest parts. There is no distinct boundary line between this layer and layer I I I - I V . The neurons are more numerous in layer V than they are in the inner

part of layer I I I - I V , and they are scattered rather uniformly throughout the width and length of the layer. In the middle of the layer, where the cells are most numerous, they number 65 to the unit. The neurons are pyramidal, spindle, and Golgi type II cells. They range in size from 10/4/n to 39/13/x. The small cells are very few in number. The chromophil substance in the large pyramidal cells in this area is present in approximately the same amount and state of development as that in the large pyramidal and spindle cells in layer V - V I in area HD. The cells in the glomeruli in layer II contain approximately the same quantity of chromophil substance in proportion to the size of the nuclei as cells of comparable size in layers I I I - I V and V. In all the cells in area H E l a the chromophil substance is greater in quantity and more advanced in differentiation than that in corresponding cells in this area in the cortex of the three-month infant. In the Cajal sections the apical dendrites of the largest pyramidal cells have several longitudinal rows of granules as well as scattered granules. Longitudinal rows of granules are present also in the cell-bodies and basal dendrites, but in smaller quantity than in the apical dendrites. The apical dendrites of the very largest pyramidal cells also have one or two longitudinal strands of granules. No well-defined neurofibrils are present in any cells. In all the smaller pyramidal cells in this area the granules in the cytoplasm are scattered irregularly throughout the cellbodies and processes. The dendrites of all the cells are larger, more compact in structure, and have more pedunculated bulbs than the dendrites of corresponding cells in this area in the three-month cortex. The apical dendrites of all large pyramidal and spindle cells in layers I I I IV and V end in layer I and have more terminal branches than in the three-month cortex. The dendrites of the large pyramidal and spindle cells in layer V are larger, longer, and have more pedunculated bulbs than the dendrites of any other cells in this area. Proceeding externally in layer III-IV the size of the dendrites of the cells and the number of pedunculated bulbs on them gradually decrease. The dendrites of the cells in layer II are the smallest and have the least quantity of pedunculated bulbs of all FIBERS.

RHINENCEPHALON

153

cells in this area. The distribution of pedunculated bulbs on the dendrites is the same as in other areas of the cortex. The axons of the large pyramidal and spindle cells in layer V are larger and have more myelin than those of corresponding cells in this area in the three-month cortex. The axons of the large pyramidal and spindle cells in the lower part of layer I I I - I V are smaller and have less myelin than the axons of the pyramidal and spindle cells in layer V, but they are larger and have more myelin than the cells in the lower part of layer I I I - I V in the three-month cortex. The axons of the cells in the mid-region of layer I I I - I V have less myelin than those of the cells in the lower part of the layer, but they have more myelin than the axons of the cells in the mid-region of layer I I I - I V in the cortex of the three-month infant. The axons of cells in the upper part of layer I I I - I V have a few small widely scattered globules on the surface as the only indications of the presence of myelin.

are intermingled with the small cells of layer II. The layer measures o . n o mm. in width. The neurons number 70 to the unit, including the large ones, and range in size from 6/4^ to 23/ιομ. Layer III-IV. This layer measures 1.100 mm. in width in the widest part adjoining the presubicular area, and only 0.365 mm. in the narrowest part adjoining area HE. Small and large neurons are scattered throughout the length of the area, but the small cells predominate in number. The large cells are more numerous near areas H E l a and HE 2 than they are in the mid-region of area HE l b . The neurons are smaller in the external part of the layer than in the internal part. The neurons number 50 to the unit, and range in size from ιο/6μ to 33/13^. Layer V. This layer is a combination of layers V and VI. It measures 0.146 mm. in width, and contains 30 neurons to the unit. The cells are small and large, but the large ones predominate in number. The neurons range in size from 12/6μ to 28/15/x.

The dendrites of the cells in this area have more pedunculated bulbs and the axons have more myelin than those of corresponding cells in area HA. The axons of the Golgi type II cells form a mesh of fibers in layer V and in the inner part of layer I I I - I V . The mesh is much less dense than that in area HD. This area has the same characteristics as other areas of the cortex which are immediately adjacent to afferent areas, viz., areas PC, OB, and T B .

Economo describes this area as belonging to the pyramidal area, but the cells in H E l b are very much smaller than the pyramidal cells of H E l a and HE 2 . Area H E l b is fairly sharply delimited from H E l a and HE 2 . Area H E l b lies in the subiculum, and HE 2 begins at the borderline between the subiculum and the cornu ammonis. Area H E l b is quite broad where it adjoins area HD 3 , but abruptly narrows considerably in the subiculum.

AREA PYRAMIDALIS SUBICULI, H E L B

The cortex in the subiculum has the same characteristics in the six-month brain as in the brain of the three-month infant, and the same as described in the adult by Economo. The same layers are recognizable as are present in the presubiculum. The transition from the large-celled presubicular area to the smallcelled subicular area is gradual. The cortex in area H E l b measures 1.757 m m · i n width, and it is illustrated in Figures 188, 189, 190, and 191. CELL-BODIES

Layer I. This layer measures 0.511 mm. in width in the widest places. The neurons number 45 to the unit. Layer II. An outer granular layer can be identified, but it is less prominent than in the three-month cortex. Many large neurons belonging to layer I I I - I V

The cells are chiefly pyramidals, and they are of fairly uniform size throughout the entire length and width of area HE l b . Small cells are scattered among the pyramidal cells in all parts of the area, but more small cells are present among the pyramidal cells adjoining layer I than elsewhere, and they may be interpreted as constituting layer II. The pyramidal cells form a dense layer which is interpreted as consisting of layers III and IV, the latter being indicated by an increased number of small granular cells along the internal margin of the pyramidal layer. Layer V is represented by a few more or less widely scattered large cells which lie internal to the inner margin of layer I I I - I V . The chromophil substance in all cells in area H E l b is present in approximately the same quantity and condition as that in corresponding cells in area H D . It is greater in quantity and more advanced in differ-

THE CORTEX OF THE SIX-MONTH INFANT

154

FIBERS.

sponding cells in this area of the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all cells in layer I I I - I V than on the dendrites of corresponding cells in the three-month cortex.

Proceeding externally from layer V the bodies of the cells in layer I I I - I V gradually decrease in size, and the dendrites gradually decrease in size, length, and quantity of pedunculated bulbs which they bear. The apical dendrites of all cells in layer I I I - I V are larger, more compact in structure, and have more terminal branches in layer I than those of corre-

The dendrites of the small cells of layer II are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. The axons of all cells in this area are larger than those of corresponding cells in the three-month cortex. The axons of the large cells in layers V and I I I - I V have more myelin than those of corresponding cells in the cortex of the three-month infant. The axons of the cells in layer II do not have any indications of myelin. The dendrites and axons of the cells in this area are in about the same state of development as those of corresponding cells in area HEi a . The dendrites of Golgi type II cells are of about the same size and have approximately the same quantity of pedunculated bulbs as the basal dendrites of pyramidal cells of comparable size. The axons of the Golgi type II cells form a mesh of fibers in layer V and in the lowermost part of layer I I I - I V . The mesh is less dense than that in area H E l a and gradually decreases in density toward the boundary between area H E l b and HE 2 .

entiation than that in corresponding cells in area HE l b in the cortex of the three-month infant. No neurofibrils or longitudinal rows of granules are present in any cells in this area. The cytoplasm of all cells is filled with granules which are scattered irregularly throughout the cell-bodies and processes. Extra-large and large ordinary and special pyramidal and spindle cells are present in layer V. The dendrites of the extra-large cells are larger and have more pedunculated bulbs than those of the large, ordinary, and special pyramidal and spindle cells. The apical dendrites of all these cells end in layer I. The apical dendrites of these cells are larger, more compact in structure, and have more terminal branches in layer I than those of corresponding cells in this area in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of all cells in layer V than on the dendrites of corresponding cells in the cortex of the three-month infant.

HIPPOCAMPUS

(CORNU

C O R T E X in the entire cornu ammonis has the same characteristics and distribution in the sixmonth brain as in the brain of the adult as described by Economo. The transition from the small-celled cortex of area H E l b to the large-celled pyramidal area H E is fairly abrupt. Large neurons predominate in number throughout the length of area HE.

AMMONIS)

THE

CELL-BODIES

The cortex in this area measures 1.168 mm. in width. It is illustrated in Figures 192, 193, 194, and 195 together with the cortex in area dentata, HF.

Layers /, II, III. These combined layers, which probably represent the molecular layer of the other areas, measure 0.657 m m - i n width. The neurons number 50 to the unit. Layer IV. The stratum pyramidale measures 0.326 mm. in width near the junction of area H E with area HE l b , and 0.237 mm. in the mid-region of the layer where it is narrowest. The neurons are chiefly large pyramidal cells, but scattered among these are some small Golgi type II cells. The neurons number 62 to the unit, and they range in size from 16/ιομ. to 40/2Ομ. Layer V. The stratum oriens measures 0.146 mm. in

AREA

PYRAMIDALIS AMMONICA,

HE2

RHINENCEPHALON

width. The neurons number only 20 to the unit, and are more numerous in the upper part of the layer than in the lower part. Most of the cells in the lower part of the layer have their long axes in a horizontal plane. The neurons are small and large, the small cells measuring 6/4-8/6^, and the large ones ranging in size from 20/τομ to 30/2ζμ. The larger cells in the pyramidal layer in area HE 2 contain more chromophil substance and are stained more darkly than any other cells in areas H D or HE. The smaller cells in HE 2 have less chromophil substance and are stained more lightly than the larger cells, but they have more chromophil substance in proportion to the size of the nucleus than the large cells in area HE l a . In the larger cells the chromophil substance is greater in quantity at the upper and lower poles than around the nucleus. The chromophil substance is in the form of small and large darkly stained clumps and a few Nissl flakes, the small clumps predominating in number. A few small clumps and particles of chromophil substance invade the apical dendrite for a distance equal to approximately the length of the cell-body. The nuclear membrane is thick, and a small disc or cone of chromophil substance rests upon it immediately below the base of the apical dendrite. Basal dendrites of the pyramidal cells are stained for a short distance and have a few particles of chromophil substance in the proximal end. The dendrites of a stellate cell are all stained approximately alike and contain about the same amount of chromophil substance. The cells in the stratum oriens have approximately the same quantity of chromophil substance and are stained about the same density as the large cells in area HE l a . The chromophil substance in all cells is greater in quantity and more advanced in differentiation than that in corresponding cells in area HE 2 in the cortex of the three-month infant. GYRUS T H E C O R T E X in the gyrus dentatus (fascia dentata) has the same characteristics and distribution as in the three-month brain and the same as in the adult brain as described by Economo. AREA

DENTATA,

HF

The cortex in this area measures 1.059 mm. in

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The cytoplasm of all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and processes. No neurofibrils or longitudinal rows of granules are present in any cells in this area in the Cajal sections. The cells in the three subdivisions of this area, viz., C 1 ( C 2 , and C 3 , have the same general appearance and characteristics as in the three-month cortex. The apical dendrites of the cells in all three subdivisions are larger and more compact in structure than those of corresponding cells in the three-month cortex. The subdivisions of the apical dendrites are no more numerous than in the three-month cortex. The basal dendrites and the collateral branches of the apical dendrites are larger, longer, and more compact in structure, but no more numerous, than those of corresponding cells in the three-month cortex. Pedunculated bulbs are more numerous on the dendrites of the cells in C 1 ; C 2 , and C 3 than on the dendrites of corresponding cells in the three-month cortex. The bulbs are more conspicuous than in the three-month cortex, and this seems to be due to the fact that the pedicles of the bulbs are generally longer in the sixmonth cortex. The bulbs at the ends of the pedicles do not appear to be any larger than those in the threemonth cortex. The pedunculated bulbs are distributed on the dendrites in the same pattern as in the three-month cortex. FIBERS.

The axons of the cells in this area are larger and have more myelin than those of corresponding cells in the three-month cortex. The dendrites and axons of the cells in area HE 2 are in approximately the same state of development as those of corresponding cells in areas H E l a and HE l b . The mesh of fibers formed by Golgi type II cells is about the same as that in this area in the threemonth cortex. DENTATUS

width. It is illustrated in Figures 192, 193, 194, and 195. CELL-BODIES

Layer I. The molecular layer measures 0.365 mm. in the widest places. It contains 50 neurons to the unit. Layer II. The stratum granulosum measures o . n o

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THE CORTEX OF THE SIX-MONTH INFANT

mm. in width. The neurons number 485 to the unit. They are of fairly uniform size, measuring 13/6ι8/9μ. There are also a very few small neurons measuring 5/4/x. Layer III. The layer of polymorph cells measures 0.584 mm. in width in the widest places. This layer is a direct continuation of the stratum pyramidale of area HE 2 . The neurons are small and large spindle and Golgi type II cells scattered throughout the layer and a few very large cells which are located in the outer part of the layer immediately below layer II. The neurons in layer III number 60 to the unit. The small cells measure 6/4-13/ιομ, the large cells range in size from 20/ιομ to 29/13/x, and the very large cells in the outer part of the layer measure 33/1240/2 ομ. These very large cells are stained more darkly than the other neurons in the layer. The cells in layer I I I are directly continuous with the cells in the pyramidal layer in area HE 2 , and are of the same types and sizes as the latter. The cells in area HF, however, are stained more lightly than those in area HE 2 . The chromophil substance is not as densely packed in the polymorph cells as that in the cells in the pyramidal layer in HE 2 . The majority of the cells in layer I I I in area H F are of approximately the same size, but a few very large cells are widely scattered in the upper part of the layer immediately under the stratum granulosum. These very large cells contain more chromophil substance and are more darkly stained than any other cells in layer III. The cells in the stratum granulosum are much smaller, contain less chromophil substance in proportion to the size of the nucleus, and are stained more lightly than the cells in layer III. The chromophil substance is collected chiefly at the base of the cell, and is in the form of small and large clumps and small particles. The nuclear membrane is thick and has a small disc or cone of chromophil substance resting upon it immediately under the base of the apical dendrite. This dendrite is stained very lightly, and has only a few small particles of chromophil substance at the proximal end. In the six-month cortex the chromophil substance in the cells in the stratum granulosum is of about the same quantity and in about the same state of development as that in the small cells in layer II—III in

area HD. The chromophil substance in all cells in area H F in the six-month cortex is greater in quantity than that in corresponding cells in this area in the cortex of the three-month infant. In the Cajal sections the cytoplasm of all cells in this area is filled with granules which are scattered irregularly throughout the cell-bodies and processes. FIBERS. The cells in this area are arranged in the same manner as in the three-month cortex. The layer of polymorph cells (layer I I I ) is a direct continuation of the layer of pyramidal cells in area HE 2 . The polymorph cells in H F resemble the cells in C x of HE 2 in structure and in size. The apical dendrites have several collateral branches. The dendrites are of about the same size, but do not have as many pedunculated bulbs, as the dendrites of the cells in Ci. The axons of the polymorph cells are of about the same size and have about the same quantity of myelin as the axons of the cells in Ci. The dendrites of the polymorph cells are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in area H F in the threemonth cortex. The axons are larger and have more myelin than in the three-month cortex.

The dendrites of the granule cells in the stratum granulosum are larger, longer, more compact in structure, and have more pedunculated bulbs than those of corresponding cells in the three-month cortex. In the six-month cortex pedunculated bulbs are less numerous on the dendrites of the granule cells in area H F than on the dendrites of the pyramidal cells in area HE. The brains studied fall into the following sequence with reference to the depth and extent of the stain in the Weigert sections: ( 1 ) 4 1 - 9 1 , (2) 37-82, (3) 38-35, (4) 37-49· The stain in the sections of brain 41-91 is distinctly darker and more extensive than that in the sections of brain 37-82. The stain in brain 37-82 is darker but no more extensive than that in brain 38-35. The stain in brain 38—35 is both darker and more extensive than that in brain 37-49. The stain in brain 37-49 is only slightly darker, but much more extensive, than that in brain 38-128, the three-month brain with the most myelin in these areas. In each of the six-month brains MYELINATION.

RHINENCEPHALON the stain in these areas is slightly darker and more extensive than it is in area HA. The following description is based upon the sections of brain 37-82. The stain is darkest and most extensive in the crossed spheno-ammonical bundle subjacent to the subiculum. The stained fibers extend from this bundle to the alveus, in which the stained fibers are but slightly lighter in color than those in the sphenoammonical bundle. Toward the fimbria the fibers of the alveus become yet lighter in color. The stain on the vertical fibers in the core of the hippocampal gyrus is distinctly lighter in color than that on the fibers of the spheno-ammonical bundle and the alveus. Stained fibers are much more numerous and much darker in the molecular layer of the presubiculum, area HD, than in layer I in the adjacent crown of the

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hippocampal gyrus, area HC. The transition from HC to HD is quite abrupt as to the number of fibers and depth of stain. The stained fibers in the molecular layer gradually increase in number and become darker toward the subiculum, reach a maximum in both respects at the junction of areas HE l a and HE l b , than rapidly decrease in number and depth of stain in area HE 2 , and are least numerous and lightest in the molecular layer of area HF. The stained fibers in the molecular layer are much larger, more numerous, and more darkly stained than in the three-month cortex. Stained horizontal and vertical fibers throughout areas HE and H F are more numerous and darker than in the three-month brain. Stained horizontal and vertical fibers are much darker and more numerous in area HE l a than in area HE l b , and are but slightly more numerous and darker in the latter area than in area HF.

COMMENTS T H E G E N E R A L pattern of the architectonics of the cerebral cortex in the brain of the six-month infant is the same as that in the brain at the age of three months. During the interval of time between the ages of three months and six months changes occur in each of the nine features of microscopical structure which are being used in this investigation as criteria of development in the cortex. These criteria are the same as are used in the description of the three-month cortex in Volume III of this series of monographs, and are, ( i ) width of the entire cortex and of each horizontal layer, (2) number of nerve cells, (3) size of nerve cells, (4) condition of the chromophil substance, (5) neurofibrils, (6) size, compactness of structure, and length of the processes of nerve cells, (7) pedunculated bulbs, (8) size and number of exogenous fibers, (9) state of myelination. Each of these criteria of development is considered in the description of each area of the cortex which is selected for study, and these areas are the same as those which are described in the three previous volumes. The criteria are discussed below in the order in which they are listed.

( ι ) The width of the cortex and of each horizontal layer in each area is shown in Table I. The measurements are made from the cresyl violet sections. A plus sign ( + ) after a number in this table indicates that the width in the six-month cortex is greater than that in the three-month brain; a minus sign ( —) signifies that the width is less in the six-month cortex than in the three-month brain; a check sign ( V ) after a number means that the width is the same for both ages. The width is measured only in those places where the cortex is cut exactly in the transverse plane and not obliquely. In each area of the cortex there is a certain amount of variation in the width. As a rule, the total width of the cortex is greatest in the crowns of gyri, is somewhat less in the walls of sulci, and is least in the floors of sulci. As Economo has shown, this is also the rule for the individual horizontal layers, except layers I and II, which are widest in the floors of sulci and narrowest in the crowns of gyri. The measurements listed in Table I are for the cortex

in the crowns of gyri unless otherwise stated in the description of the area. Since the three-month stage of development the cortex has increased in width in every area except LA, OA, and OB. The increase in width has occurred chiefly in layers III, IV, and V. In only four areas — OA, T A , IA, and I B — is the width of layer III less in the six-month cortex than in the three-month brain. In only seven areas — F A y in the region of the lower extremity, F B , PE, OA, OB, T G , and L C — h a s layer IV failed to gain in width since the three-month stage of development. Layer V has increased in width in all but seven areas — O B , OC, T A , T B , LA, L D , and LE. Layers I and V I have increased in width in twenty-four areas, and have failed to increase in thirteen areas. Layer II has increased in width in only sixteen areas, and has failed to increase in twenty-one areas. These figures indicate that development in the cortex between the ages of three and six months is slow in the outer granular layer, is somewhat more active in the molecular and polymorphic layers, and is the most active in the two pyramidal layers and the inner granular layer. The outer pyramidal layer shows a gain in width in a greater number of areas than any other layer. An examination of the gross brain reveals that since the three-month age there has been an increase in the length and width of the gyri. This increase in linear extent of the cortex is particularly evident in the frontal lobe. Comparison of Table I for the six-month brain with the same table for the three-month cortex reveals that according to the criterion of the width of the cortex the entire area F A y has been active in development in the interval between the two ages, and in this area the regions of the lower extremity and trunk have been especially active. Growth in the posterior regions of the superior, middle, and inferior frontal gyri has been much less active than in area FAy. The middle region of the three frontal gyri and the anterior region of the superior frontal gyrus have been more active than the posterior region of these gyri. Growth has been less active in the anterior

COMMENTS region of the middle and inferior frontal gyri and in the frontal pole, but has been active in the orbital gyri· In the parietal lobe development has been the most active in area PB. Next in order with reference to activity of growth come areas PG, PF, PE, PC, and PH. According to the width of the cortex as a criterion of development, in the occipital lobe growth has been active in the primary afferent area OC only, where it has been quite active. The areas in the temporal lobe fall into the following sequence with reference to activity of growth as indicated by increase in width of the cortex, beginning with the area showing the greatest increase: T G , T F , T C , T B , T E , and T A . There has been very little activity in development in areas IA and IB during the interim between the three-month and six-month stages. Growth has been only slightly active in gyrus cinguli. There has been an increase in the width of the cortex in areas HA, HD, HE l a , HE 2 , and HF. The width of the cortex in area H E l b is slightly less in the six-month cortex than in the three-month brain. The increase in all these areas is slight. Layers V and V I show more activity than any of the other layers. (2) In counting and measuring the neurons in the six-month cortex the procedures are the same as those used for the three younger brains. The methods are explained on page 3 of Volume I. The cresyl violet sections are used for counting and measuring the neurons. The cells in any layer are scattered irregularly as to number; therefore, they should be counted in horizontal rows of ten or more units. In obtaining the number of cells listed for each layer in each area in Tables II and III at least ten samplings of ten units each are counted in each of four to six sections of each area, giving four to six hundred units for each area. The averages of these counts are listed in a table for each brain. The figures in the tables of all the six-month brains in which the cells are counted are averaged, and the results are listed in Tables II and III as the number of cells for this age of the cortex. The neurons are scattered so irregularly as to number in each layer that no particular count, however obtained, can be absolute. The figure listed

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in the tables for any particular layer does not mean that every unit in that layer contains that number of cell-bodies. In a horizontal row of ten units some of the units will contain more and others less than the number listed in the table. In a very general sense the neurons in most of the cortex are arranged in vertical columns, but the columns are not continuous throughout the width of the cortex. Also, there are many neurons scattered between the columns. Comparison of Tables II and III with the same tables in Volume III reveals that, with a few exceptions, there has been a reduction in the number of neurons per unit in every layer of every area of the cortex since the three-month stage of development. No new neurons are formed in the cortex after birth, and growth of the cortex in width and length would cause a reduction in the number of neurons per unit. The percentage of reduction in the number of neurons per unit in each layer in each area is shown in Table IV. A plus sign ( + ) after a figure indicates that the number of neurons is that much greater in the six-month cortex than in the three-month brain. There are but fourteen such cases, and all but one of them are for the large cells in layer V. A check sign ( V ) after a number signifies that the number of neurons is the same for both ages. There are nine of these cases; eight of them are for the large cells in layer V, and the other is for the small neurons in this layer. According to this criterion, during the interval between the three- and six-month stages of development growth is more active in layers III, IV, and V I than in the other layers. In layer III growth is more active in laminae IHb and IIIc than in lamina I l i a . Layer IV shows more activity than any other layer except layer VI, which is about the same as IV. Growth is active throughout the frontal lobe. Within this lobe the activity is greatest in area F A y , and the activity is considerable in all layers except the two granular layers and the large ordinary and special cells in layer V. The acceleration of growth is especially evident in the giant pyramidal cells of Betz and the cells in lamina IIIc. Within area F A y the regions of the hand and head show more activity than the other two regions. In the cortex anterior to gyrus centralis anterior activity in growth, as compared with area F A y , de-

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THE CORTEX OF THE SIX-MONTH INFANT

creases in layers I and V, increases in layers I I and IV, remains quite active in layer III, decreases markedly in case of the extra-large and large ordinary and special cells in layer V. The small cells in layer V decrease slightly in growth activity. Activity in growth in layer VI is slightly less than in area FAy. There are no conspicuous differences in growth activity between the various areas anterior to gyrus centralis anterior, except that the posterior part of gyrus frontalis medius (the frontal eye-field) is especially active in all layers, and is more active than the posterior part of gyrus frontalis inferior (the speech area). In the orbital gyri, area F E (possibly a visceral center) growth is active in all layers, except the large cells in layer V. In the parietal lobe developmental activity in layer I is about the same as in area FAy but is greater than in the frontal areas anterior to gyrus centralis anterior. Growth activity in layer I I is considerably more than in any area in the frontal lobe. Activity is slightly greater in layer I I I than in area FAy, and it is less in lamina I l i a than in laminae I l l b and IIIc. Growth in layer IV is much more active than in area FAy, and is about the same as in the prefrontal areas. In layer V growth is less active than in area FAy. In case of the large and extra-large cells growth is more active than in the prefrontal areas. Growth activity in layer VI is about the same as in area FAy. Within the parietal lobe growth is more active in area P B than in any other area. Growth activity is less in area PC than in any other area in the parietal lobe, especially in layer III. Activity is about equal in areas PE, PF, PG, and PH, but less in these areas than in the primary somesthetic area PB. In the occipital lobe growth is more active in area OB than in area OA or OC. There is more activity in layers IV and VI than in the other layers. Growth is least active in layers I I I and V. In layers I and II growth is less active than in these layers in the parietal lobe. In layer I I I growth is less active in the occipital lobe than in either the parietal or the frontal lobe. Growth in layer IV is more active than in the frontal lobe, but less than in the parietal lobe. Growth in layer V is less active than in area FAy in the frontal lobe, and is about the same as in the prefrontal and parietal areas. Activity is about the same in layer VI as in the frontal and parietal lobes.

The primary afferent area for sound, TC, is the most active in development in the temporal lobe. Growth is more active in layer IV than in any other area in this lobe. Layers I I I and VI are not as active as layer IV, but more active than either layer I or II. Laminae I l l b and IIIc are more active than lamina I l i a . Growth is least active in layer V. Growth is less active in area TA than in any other area. Growth is more active in the temporal lobe than in the occipital lobe, but is less active than in the parietal lobe. Activity of development is about the same in layer I as in the frontal lobe, but is more active in layer I I in the temporal lobe than in the frontal lobe. Growth activity in layer I I I is about the same as in the frontal lobe, but is greater in layer IV in the temporal lobe than in the frontal lobe. Growth activity in layer V is less in the temporal lobe than in the frontal lobe, especially in area FAy. Growth activity in layer VI is about the same in both these lobes. Growth activity in areas IA and I B is about equal, and is about the same as in the temporal lobe. In these areas activity is greater in layer IV than in any other layer, next in order of activity come layers VI, III, II, I, and V. Growth is more active in laminae I l l b and IIIc than in I l i a . Growth is conspicuously less active in areas LA, LC, LD, and L E than in any other areas in the isocortex. (3) The nerve cells are measured in the cresyl violet sections, and the dimensions are for the cellbodies only. The measurements are made and listed in the same manner as in the younger brains described in Volumes I, II, and III. During the interim between the three- and sixmonth stages of development there has been but little, if any, increase in the size of the bodies of the neurons. The giant pyramidal cells of Betz in area FAy have larger bodies than any other cells in the cortex of the six-month infant. The giant pyramidal cells are actually smaller in all the regions of area FAy than in the three-month cortex, except the regions of the lower extremity and head. The giant pyramidal cells in area FAy gradually decrease in size from the region of the lower extremity to the region of the head. In almost all of the other areas in the frontal lobe the extra-large pyramidal cells in layer V are larger than the corresponding cells in the three-month

COMMENTS brain. In general, the size of the extra-large pyramidal cells decreases in a postero-anterior direction, and are smallest in the anterior portions of the frontal lobe. The smallest extra-large pyramidal cells in layer V are in the mid-region of gyrus frontalis medius and the anterior region of gyrus frontalis inferior. The large pyramidal cells in lamina IIIc are the next largest cells in the frontal lobe. The largest of these cells are as large as the smallest giant pyramidal cells of Betz in the regions of the trunk and hand in area FAy. In almost all areas of the frontal lobe the large pyramidal cells in lamina IIIc are larger in the six-month cortex than in the three-month brain. The large cells in lamina IIIc are of rather uniform size throughout the entire frontal lobe. The smallest of these cells are in the region of the head in area FAy, in the orbital gyri, and in the frontal pole. The pyramidal cells in laminae I l l b and I l i a show some increase in size since the three-month stage of development, but the instances of increase are fewer than in the case of the large pyramidal cells in lamina IIIc. The large ordinary and special pyramidal and spindle cells in layers V and V I have increased in size since the three-month stage in fewer areas in the frontal lobe than the large pyramidal cells in lamina IIIc. The large cells in layer V have increased in size in more areas than the large cells in layer VI. The large ordinary and special pyramidal and spindle cells in layers V and V I are smaller than the large pyramidal cells in lamina IIIc, and are about the same size as the large pyramidal cells in lamina I l l b . The cells of layer I I have increased in size in all but two areas in the frontal lobe. Cells of the minimum size in layer IV have increased in all areas of the frontal lobe, but the size of the larger cells has increased in only four areas. The small cells in layers V and VI show practically no increase in size since the three-month stage of development. The extra-large cells in layer V are the largest cells in the parietal lobe. These cells are largest in areas P B and PC and are smallest in area PH. The cells in areas P B and PC are much smaller than the giant pyramidal cells in area FAy. In all areas in the parietal lobe except area P B the extra-large pyrami-

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dal cells in layer V are smaller in the six-month cortex than in the three-month brain. The large pyramidal cells in lamina IIIc are next in size in all areas in the parietal lobe, except areas P B and PC where the large pyramidal cells in layer V I are larger than the cells in IIIc. The large pyramidal cells in lamina IIIc are somewhat smaller than the corresponding cells in the three-month cortex. The cells in the parietal lobe have not increased in size since the three-month stage of development as consistently as the corresponding cells in the frontal lobe. In general, they are smaller in the parietal lobe than in the frontal lobe. The large ordinary and special pyramidal cells in layers V and VI are next in size. These cells have increased in size in all areas in the parietal lobe since the three-month stage of development. They are about the same size as corresponding cells in the prefrontal areas. The large pyramidal cells in lamina I l l b are the next in size. These cells are somewhat larger in the six-month cortex than in the three-month brain in all areas except PC and PH. The spindle cells in layers V and V I are next in size in the parietal lobe. These cells are slightly larger in area PB, but are smaller in all other areas than in the three-month cortex. The cells in lamina I l i a are next in size, and they are about the same size as in the three-month cortex. The small cells in layers III, V, and VI are next in size, and they are of about the same size as in the three-month cortex. The cells in layer IV are next in size in the parietal lobe, and they are of about the same size in all areas of the lobe. The cells of smallest dimensions in this layer are larger than in the three-month cortex, but the large cells are about the same size as at the threemonth stage. The cells in layer IV in the parietal lobe are somewhat smaller than the corresponding cells in the frontal lobe. The cells of layer I I are the smallest cells in the parietal lobe, and they are slightly smaller than the corresponding cells in the frontal lobe. In the sixmonth cortex they are about the same size in all areas of the parietal lobe, and show little, if any, increase since the three-month stage of development. The extra-large pyramidal cells in layer V are the

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THE CORTEX OF THE SIX-MONTH INFANT

largest cells in the occipital lobe. The smallest of these cells are larger in area OC than in areas OA and OB, but the largest cells are found in area OB. The extra-large pyramidal cells in layer V in areas OC and OB are smaller than the corresponding cells in areas P B and PC, respectively. The extra-large pyramidal cells in layer V in the occipital areas are slightly larger than the corresponding cells in the three-month cortex. The large pyramidal cells in lamina IIIc are the next in size in the occipital lobe. These cells are larger than the corresponding cells in the three-month cortex. In the six-month cortex the large pyramidal cells in lamina I I I c in the occipital lobe are, in general, smaller than the corresponding cells in the parietal lobe. The large pyramidal and spindle cells in layer VI are only slightly smaller than the large pyramidal cells in lamina IIIc. These cells in layer VI are, in general, slightly larger than the corresponding cells in the three-month cortex. In the six-month cortex they are smaller in areas OC and OB than in areas PB and PC, respectively. The large ordinary and special pyramidal cells in layer V are next in size. They are about the same size as corresponding cells in the three-month cortex. In the six-month cortex they are smaller than the corresponding cells in the parietal lobe. The large cells in laminae I l l b and I l i a are larger in areas OA and OC but smaller in OB than corresponding cells in the three-month cortex. In the six-month cortex the large cells in laminae I l l b and I l i a are smaller in the occipital lobe than corresponding cells in the parietal lobe. The small cells in layer V are of about the same size in all areas of the occipital lobe, and are of about the same size as corresponding cells in the threemonth cortex. In the six-month cortex these cells in the occopital lobe are of equal size to those in the parietal lobe. The cells of layer IV in the occipital areas are slightly larger than those of layer II. The cells of layer IV are slightly larger than corresponding cells in the three-month cortex. In the six-month cortex the cells of layer IV are about the same size as corresponding cells in the parietal lobe, but are smaller

than those in layer IV in the frontal lobe except in the orbital gyri and frontal pole. The cells of layer I I are the smallest cells in the occipital lobe. They are of the same size in all areas of this lobe, and are slightly smaller than corresponding cells in the three-month cortex. In the six-month cortex the cells of layer I I in the occipital lobe are smaller than those in the parietal lobe. The extra-large pyramidal cells in layer V are the largest cells in the temporal lobe. These cells are largest in area T A and smallest in area TC. Only in area TA has there been an increase in size of these cells since the three-month stage of development. In all the other areas in the temporal lobe these cells are of about the same size as corresponding cells in the three-month cortex. In the six-month cortex these cells are slightly smaller in the temporal lobe than in the occipital lobe. The large pyramidal cells in lamina IIIc are next in size to the extra-large pyramidal cells of layer V. They are smallest in area T G and are about equal in size in all the other areas. The large cells in lamina IIIc in all areas of the temporal lobe except areas T B and T G are larger than corresponding cells in the three-month cortex. In the six-month cortex the large cells in lamina IIIc in all areas of the temporal lobe are larger than corresponding cells in any area in the occipital, parietal, and frontal lobes. The large pyramidal cells in lamina I l l b are smaller than the large cells in IIIc. In lamina I l l b these cells are largest in area TA, slightly smaller in T B , and still smaller in the other areas of the temporal lobe. These cells have increased in size only in TA, T B , and T C since the three-month stage of development. The large cells in lamina I l i a in all areas except T G are larger than corresponding cells in the three-month cortex. These cells are smallest in area TC. The large cells in all the other temporal areas are of approximately uniform size. In the sixmonth cortex the large pyramidal cells in lamina I l l b in area T A are larger than any of these cells in the occipital lobe, but in all the other areas of the temporal lobe the cells in this lamina are of about the same size as corresponding cells in the occipital lobe. The large cells in lamina I l i a are about the same size in the temporal and occipital lobes, except that

COMMENTS

they are larger in area OC than in any other area in either of these lobes. The large ordinary and special pyramidal cells in layer VI are of about uniform size in all areas in the temporal lobe. They are about the same size as the large cells in lamina M b in area TB. These cells are of about the same size as corresponding cells in the three-month cortex. The large spindle cells in layer VI are largest in area TA, and are larger than corresponding cells in the three-month cortex. In all the other areas of the temporal lobe these cells are smaller than in the three-month cortex. The large pyramidal and spindle cells in layers V and VI are, in general, about the same size as corresponding cells in the occipital lobe. The large ordinary and special pyramidal cells in layer V are next in size. They are of approximately uniform size in all areas in the temporal lobe, and are of about the same size as corresponding cells in the three-month cortex. They are of the same size as the corresponding cells in the occipital lobe, but are smaller than these cells in the parietal lobe. The small pyramidal cells in layer V are next in size. These cells are of uniform size throughout the temporal lobe, and are of about the same size as those in the occipital and parietal lobes. They are smaller than the corresponding cells in the frontal lobe. The small cells in layer V in the temporal lobe are of about the same size as corresponding cells in the three-month cortex. The cells of layer IV are next in size. They are of approximately the same size in all areas of the temporal lobe, and are larger than corresponding cells in the three-month cortex. In the six-month cortex the cells of layer IV in the temporal lobe are slightly larger than the corresponding cells in the occipital lobe. The cells in layer II are of the same size in all areas of the temporal lobe, and are of the same size as those in the occipital lobe. These cells show no increase in size since the three-month stage of development. The extra-large cells in layer V in areas IA and IB are of equal size, and are the largest cells in the cortex of the insula. They are smaller than corresponding cells in area TA, are of the same size as

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those in area TG, but are larger than those in the other areas in the temporal lobe. These cells are smaller in area IA and larger in IB than corresponding cells in the three-month cortex. The cells in all other layers in both areas in the insula show no increase in size since the three-month stage of development. The extra-large pyramidal cells in layer V are the largest cells in each area in gyrus cinguli, and are of uniform size in all the areas. They are larger than in the three-month cortex in all areas except LA, where they are slightly smaller than at the threemonth stage of development. In the six-month cortex the extra-large pyramidal cells in layer V in all areas of gyrus cinguli are of about the same size as the corresponding cells in the anterior regions of the frontal pole and the posterior regions of the parietal pole. The large pyramidal and spindle cells in layers V and VI are of about the same size as the large pyramidal cells in layer III. These cells are of approximately uniform size in all areas of gyrus cinguli, and they are not any larger than corresponding cells in the three-month cortex. All other cells in the cortex of gyrus cinguli are of about the same size as corresponding cells in the three-month cortex. The stellate cells in layer II are the largest cells in area HA. The largest of these cells are larger than the smallest giant pyramidal cells in area FAy but are smaller than the largest giant pyramidal cells. The largest stellate cells in area HA are larger than any of the extra-large pyramidal cells in any area of the entire cortex. The stellate cells are larger than the corresponding cells in area HA in the three-month cortex. The large pyramidal cells in layers V and VI are the next in size, and they are larger than the corresponding cells in area HA in the three-month cortex. In the six-month cortex they are about the same size as the large ordinary pyramidal cells in layer V in gyrus cinguli. The other cells in the various layers of area HA have not increased in size since the three-month stage of development. The large pyramidal cells in layer VI are the largest cells in area HD. These cells are larger than the corresponding cells in the three-month cortex. They are much smaller than the large pyramidal cells in

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layers V and VI in area HA, and are of about the same size as the large pyramidal cells in layer I I I in HA. The other cells in area HD show little, if any, increase in size since the three-month stage of development. The large pyramidal cells in layers IV and V are the largest cells in area HE l a . They are as large as the large pyramidal cells in layers V and VI in area HA. These cells are slightly larger than in the threemonth cortex. The other cells in area HE l a are about the same size as in the three-month cortex. The cells in area HE l b are smaller than those in area HE l a . They have not increased in size since the three-month stage of development. The various cells in area HE 2 are not any larger than corresponding cells in the three-month cortex. The largest pyramidal cells are slightly larger than the largest pyramidal cells in layer VI in area HA. The cells in area H F are of about the same size as corresponding cells in the three-month cortex. The largest cells are found in the polymorph layer, and they are about the same size as the largest pyramidal cells in area HE. (4) The chromophil substance in neurons appears to undergo certain progressive changes with advancing stages in the development of the cortex. These changes are listed on page 135 in Volume III. To this list may be added increase in the quantity of chromophil substance. The appearance of the nucleus in the younger stages of development of the neurons suggests that the chromophil substance originates in the nucleus. In the earliest stages of development the nucleus constitutes the bulk of the cellbody, the nuclear membrane is thick, and small clumps of chromophil substance are clinging to the outer surface of the membrane. In later stages of development chromophil granules and clumps have increased in quantity in the cytoplasm, and clumps of chromophil substance are clustered on the external surface of the thick, darkly stained membrane of the nucleus. Concomitant with the appearance of chromophil granules in the base of the apical dendrite, a dark cone-shaped mass of chromophil substance is adherent to the external surface of the nuclear membrane immediately under the origin of the dendrite. The apex of the cone is directed toward the base of

the dendrite and presents the appearance of material streaming toward the dendrite. The chromophil substance is more advanced in differentiation in all neurons in all areas of the cortex in the six-month infant than in corresponding neurons in corresponding areas in the cortex of the threemonth infant. In the six-month cortex there are, however, differences in the degree of differentiation between areas and between the cells in any one area. In all areas of the isocortex the cells fall into the following sequence as to the degree of differentiation of the chromophil substance, beginning with the cells in which differentiation is most advanced: ( 1 ) the extralarge pyramidal cells in layer V, (2) the large ordinary and special pyramidal and spindle cells in layers V and VI, (3) the large pyramidal cells in lamina IIIc, (4) the large pyramidal cells in lamina I l l b , (5) the large pyramidal cells in lamina I l i a , (6) the small cells in layers V and VI, (7) the cells of layer IV, (8) the small cells of layer III, and (9) the cells of layer II. On the basis of differentiation of the chromophil substance, the giant pyramidal cells of Betz in area FAy are more advanced in development than any other cells in the entire cortex of the six-month infant. There are differences in the state of development of the chromophil substance in the Betz cells in the various functional regions of area FAy. The giant pyramidal cells in the region of the hand are more advanced in development than those in the region of the trunk, shoulder, and arm. The giant cells in the latter region are more advanced than those in the region of the head, and the giant cells in the region of the head are more advanced in development than those in the region of the lower extremity. The chromophil substance in the large ordinary and special pyramidal and spindle cells in layers V and VI is less advanced in differentiation than that in the giant pyramidal cells of Betz. In all layers the chromophil substance in Golgi type I I cells is in approximately the same state of development as it is in pyramidal cells of comparable size. Proceeding anteriorly from area FAy the neurons in the frontal lobe show a gradual decrease in the degree of differentiation of the chromophil substance, and the differentiation is least advanced in the cells

COMMENTS in the frontal pole. In general, the chromophil substance in the cells in each region of the superior frontal gyrus is more advanced in differentiation than that in the corresponding region in the middle frontal gyrus, and is more advanced in the latter than in the inferior frontal gyrus. The advanced degree of differentiation of the chromophil substance in the large cells of lamina IIIc is a conspicuous feature in all areas of the frontal lobe, but especially in area F A y and in the posterior regions of the superior, middle, and inferior frontal gyri· The chromophil substance is more advanced in differentiation in the cells in area P B than in any other area in the parietal lobe. The functional regions of area P B fall into the following sequence with reference to the degree of differentiation of the chromophil substance, beginning with the region in which differentiation is the most advanced: ( i ) region of the hand, (2) region of the trunk and arm, (3) region of the head, (4) region of the lower extremity. In each of the functional regions the differentiation of the chromophil substance is less advanced than in the corresponding region in area FAy. The chromophil substance in the neurons in the four functional regions of area PC is less advanced in differentiation than in the corresponding cells in the corresponding regions in area PB. Within area PC the four functional regions fall into the same sequence as in area P B as to the degree of differentiation of the chromophil substance. The chromophil substance in the neurons in areas P E , P F , and P G is in approximately the same state of differentiation, and is less advanced than in area PC. Differentiation of the chromophil substance is less advanced in the cells in area PH than in corresponding cells in area PG. In all areas in the parietal lobe the large cells in lamina IIIc are conspicuous because of their dark stain and the advanced state of differentiation of the chromophil substance. The chromophil substance in these cells in all the areas except P B is as advanced in differentiation as it is in the large ordinary and special pyramidal and spindle cells in layers V and VI. The chromophil substance in the cells in area OA is more advanced in differentiation than in corre-

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sponding cells in area PG. In OA the chromophil substance in the largest cells in lamina IIIc is as advanced in differentiation as it is in the extra-large pyramidal cells in layer V. The chromophil substance in the cells in area OB is more advanced in differentiation than it is in corresponding cells in area OA. In area OB also the chromophil substance in the large cells in lamina IIIc is as advanced in differentiation as it is in the extra-large pyramidal cells in layer V. In area OC the chromophil substance in the extralarge pyramidal cells in layer V is more advanced in development than it is in any other cells in this area. These cells are more advanced in development with reference to this criterion than the extra-large pyramidal cells in layer V in area OB, but they are not as advanced in development as the giant pyramidal cells of Betz in the region of the head in area FAy. The large ordinary and special pyramidal cells in layer V, the large Meynert and Cajal cells in layer IV and the large pyramidal cells in lamina I I I c are in approximately the same condition as to differentiation of the chromophil substance. The cells in layer I I I are in approximately the same condition as corresponding cells in area OB. The chromophil substance is in approximately the same state of differentiation in areas TA, T B , and TC, and is less advanced than in areas OA, OB, and OC, respectively. In areas T E , T F , and T G the chromophil substance is less advanced in differentiation than it is in area TA. The chromophil substance is in approximately the same state of differentiation in the cells in both areas of the insula, and is less advanced than it is in corresponding cells in the adjoining areas in the operculum of the frontal lobe. The cells in all areas of gyrus cinguli are in approximately the same state of development as to differentiation of the chromophil substance. In area L A the chromophil substance in the extra-large pyramidal cells in layer V is in approximately the same state of differentiation as it is in corresponding cells in the cortex on the medial surface of the frontal lobe, but the chromophil substance in the cells in layer I I I is less advanced in LA. In all areas of gyrus hippocampus and of the hippocampus the large cells in the lower layers are more

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advanced in development as to differentiation of the chromophil substance than any other cells except the large cells in the glomeruli in layer I I in area HA. The chromophil substance in the cells in area HA is in approximately the same state of differentiation as in corresponding cells in area L A . The chromophil substance in the cells in areas HD, H E l a , and HE l b ) is less advanced in differentiation than it is in area HA. The cells in area H E 2 are in about the same state of development as those in area HA. (5) The state of differentiation of neurofibrils is studied in the Cajal sections. In all areas where there is any indication of the differentiation of neurofibrils in the neurons the differentiation is more advanced than in the three-month cortex. The first step in the formation of neurofibrils appears to be alignment of granules in longitudinal rows. In the six-month cortex longitudinal rows of granules or neurofibrils, if any, are present in the largest cells in layers I I I , V, and V I only. The state of differentiation of neurofibrils in the large cells in lamina IIIc, and in some areas in I H b also, is a conspicuous feature in the six-month cortex in comparison with the cortex of the three-month infant. In many areas the large cells in lamina I I I c are as advanced in development in regard to differentiation of neurofibrils as the large ordinary and special pyramidal and spindle cells in layers V and VI. The cells in each area of the isocortex fall into the same sequence with reference to the differentiation of neurofibrils as they do in regard to differentiation of the chromophil substance. Neurofibrils are more advanced in differentiation in the giant pyramidal cells of Betz in area F A y than in any other cells in the entire cortex. Neurofibrils are more advanced in differentiation in other cells in area F A y than they are in corresponding cells in any other area of the isocortex. The large ordinary and special pyramidal and spindle cells in layers V and V I and the large pyramidal cells in lamina I I I c are in about the same state of development as to differentiation of neurofibrils. The four functional regions in area F A y differ as to the development of neurofibrils in the neurons, and fall into the following sequence, beginning with the region in which the differentiation is most advanced: (x) region of the hand, (2) region of the trunk and arm, (3) region of the head, (4) re-

gion of the lower extremity. In area F A y neurofibrils are present in the giant pyramidal cells of Betz, the large ordinary and special cells in layers V and VI, and the large cells in lamina I I I c only. Proceeding anteriorly from area F A y in the frontal lobe, differentiation of neurofibrils in the cells decreases in degree. Also, there is a decrease in degree of differentiation in the three frontal gyri from above downward; differentiation is more advanced in the cells in the superior gyrus than in corresponding cells in the middle gyrus, and the cells in the middle gyrus are more advanced in development of neurofibrils than those in the inferior gyrus. In the parietal lobe, differentiation of neurofibrils is more advanced in areas P B and P C than in any of the other areas. Within areas P B and P C the cells in the functional regions of the hand and trunk are more advanced in development than those in the regions of the head and lower extremity. In each functional region differentiation of neurofibrils is less advanced than in the corresponding region in area F A y . Differentiation of neurofibrils is less advanced in area P E than in the region of the lower extremity in area PC, is less advanced in area P F than in area P E , and is least advanced in areas P G and PH. Differentiation of neurofibrils in the neurons in area OC is in approximately the same condition as in corresponding cells in area P E , and is less advanced than in the somesthetic area P B in the region of the head. The differentiation of neurofibrils is less advanced in area OB than in area OC, and is still more retarded in area OA. The state of the differentiation of neurofibrils in area OA is approximately similar to that in area PG. In the temporal lobe, differentiation of neurofibrils is more advanced in area T C than in areas T A and T B . The state of differentiation of neurofibrils in the cells in area T C is slightly less advanced than in corresponding cells in area OC. The cells in areas T A and T B are similar in regard to differentiation of neurofibrils and are in approximately the same state of development as corresponding cells in area OA. Differentiation of neurofibrils is less advanced in area T E than in T A , and is less advanced in area T F than in area T E . The state of differentiation of neurofibrils in area T G is about the same as in area T F . Differentiation of neurofibrils in the cells in areas

COMMENTS ΙΑ and I B is in approximately the same state as in corresponding cells in area FCBm. In all areas of gyrus cinguli the differentiation is in approximately the same condition, and is less advanced than in areas IA and I B . Area HA is less advanced than area T G with reference to the differentiation of neurofibrils. The cells in area H D are in about the same condition as corresponding cells in area HA. Longitudinal rows of granules are present in the largest cells only in area H E l a . No neurofibrils or longitudinal rows of granules are present in any cells in any other of the hippocampal areas. (6) The dendrites and axons of all cells in all parts of the cortex have increased in size, length, and compactness of structure during the interim between the three-month and six-month stages of development. There has been no increase in the number of apical or basal dendrites or in the number of branches of the basal dendrites or of collateral branches of apical dendrites. The terminal branches of apical dendrites which end in layer I, however, have increased in number since the three-month stage, especially in case of the extra-large pyramidal cells in layer V and the large pyramidal cells in layer I I I . The dendrites of Golgi type I I cells have increased in size, length, and compactness of structure, but not in number. Each nerve cell has but one axon. Since the threemonth stage of development the axons of all nerve cells in all parts of the cortex have increased in size, length, and compactness of structure. There has been no apparent increase in the number of branches on any of the axons, but the branches have increased in size and length, and there has been an increase in the number of cells whose axons have branches, especially in layer I I I . The axons of a greater number of cells bear indications of myelin than in the threemonth cortex, and the myelin has increased in quantity on the axons of cells whose axons are myelinated at the three-month stage of development. The axons of the Golgi type I I cells form a mesh of fibers in every area in the cortex, but the mesh is not of the same density in all areas. In some areas this mesh is more dense in the six-month cortex than in the corresponding areas in the three-month brain. In each area the mesh is more dense in layer IV than in the other layers, and the density decreases more or

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less abruptly external and internal to this layer. The mesh is more dense in laminae Vb and V i a (the inner band of Baillarger) than in lamina Va. This mesh is formed entirely by the axons of Golgi type I I cells and does not receive any contribution in the form of branches from either horizontal or vertical exogenous fibers. With reference to the size, length, and compactness of structure of the dendrites and to the size and degree of myelination of the axons, the cells in the various layers in each area of the isocortex fall into the following sequence, beginning with those with the most highly developed processes: ( i ) the extra-large pyramidal cells in layer V, (2) the large ordinary and special pyramidal and spindle cells in layers V and VI, (3) the large pyramidal cells in lamina IIIc, (4) the large pyramidal cells in lamina IHb, (5) the large pyramidal cells in lamina I l i a , (6) the small pyramidal cells in layers V and VI, (7) the cells of layer IV, and (8) the cells of layer II. The Golgi type I I cells are in about the same state of development as the pyramidal cells of comparable size. There are also differences between areas in the cortex as to the degree of development of the axons and dendrites of the cells. The cells in area F A y are more advanced in development of the processes than the cells in any other area in the cortex of the six-month infant. The dendrites of the giant pyramidal cells in the regions of the lower extremity, hand, and trunk and arm are in approximately the same state of development. The axons of the giant cells in the region of the hand are larger and have more myelin than those of corresponding cells in the region of the lower extremity and trunk. The dendrites and axons of the giant pyramidal cells in the region of the head are less advanced in development than those of corresponding cells in the other regions of area FAy. A mesh of fibers formed by Golgi type I I cells is present in area FAy, but it is thinner than in any other area in the frontal lobe. Proceeding anteriorly in the frontal lobe from area F A y there is a gradual decrease in the size of the apical dendrites of all neurons, in the size, number, and compactness of structure of terminal branches in layer I, and in the size, length, and compactness of structure of the basal dendrites and of the collateral

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branches of the apical dendrites. There is also a gradual decrease in the size, compactness of structure, and degree of myelination of the axons. The mesh of fibers formed by the axons of the Golgi type I I cells increases in density in a posteroanterior direction, but it is of about the same density in the anterior regions of the frontal lobe as it is in the middle regions. There has been little, if any, increase in the density of the mesh since the threemonth stage of development. The cells in area P B are more advanced in development of their processes than corresponding cells in any other area in the parietal lobe, but they are less advanced than corresponding cells in the corresponding regions in area FAy. The processes of the pyramidal cells appear to be about equally developed in all four regions of area PB. Throughout the entire extent of area P B the large pyramidal cells in lamina IIIc are conspicuous because of the coarseness and length of their dendrites. The mesh of fibers formed by the axons of Golgi type I I cells is considerably more dense in area PB than it is in any area in either the frontal or the parietal lobe. In fact, all the primary afferent areas (koniocortex) are characterized by an especially dense mesh of fibers formed by the axons of Golgi type I I cells. This mesh extends the entire length of area P B from the region of the foot in the paracentral lobule to the region of the head in the inferior end of gyrus centralis posterior. The mesh varies in density in the various functional regions, however, and these regions fall into the following sequence, beginning with the one in which the mesh is densest: ( i ) region of the head, (2) region of the trunk and arm, (3) region of the hand, (4) region of the lower extremity. In each region the mesh is more dense than that in the corresponding region in the three-month cortex. Throughout area P B the mesh is much more dense in layer IV than in any other layer in the cortex. In area PC the dendrites and axons of the pyramidal and spindle cells in layers I I I , V, and V I are slightly less advanced in development than those of corresponding cells in area PB. The mesh of fibers formed by the axons of Golgi type I I cells is much less dense than that in area PB, but much more dense than the mesh in area FAy. The dendrites and axons of the pyramidal and

spindle cells in areas PE, P F , and PG are in approximately the same state of development, and are less advanced than those of corresponding cells in area PC. The mesh of fibers formed by the Golgi type I I cells is more dense than that in area PC. The mesh of fibers formed by the axons of Golgi type I I cells in area P H is about the same as in area PG. The processes of the pyramidal and spindle cells in area PH are less advanced in development than those of corresponding cells in area PG. The advanced degree of development of the large pyramidal cells in lamina IIIc is a conspicuous feature in all areas of the cortex in the parietal lobe. The axons and dendrites of the pyramidal and spindle cells in area OA are in approximately the same state of development as those of corresponding cells in the region of the head in area PC, and are more advanced in development than those in area PG. The mesh of fibers formed by Golgi type I I cells is more dense than that in area PC in the region of the head. The axons and dendrites of the pyramidal and spindle cells in area OB are more advanced in development than those of corresponding cells in area OA. The advanced degree of development of the dendrites and axons of the large pyramidal cells in lamina IIIc in area OB is a conspicuous feature in this area. The mesh of fibers formed by the axons of Golgi type I I cells is slightly more dense than that in area OA. The processes of the large pyramidal and spindle cells in layers V and VI in area OC are more advanced in development than those of corresponding cells in area OB, but the axons and dendrites of the cells in layer I I I are in about the same state of development in these two areas. The processes of the pyramidal and spindle cells in all layers in area OC are less advanced in development than corresponding cells in the region of the head in area PB. The mesh of fibers formed by the axons of Golgi type I I cells is but slightly more dense than that in area OC in the three-month cortex. In the six-month cortex the mesh is less dense than that in the region of the head in area PB. In the temporal lobe the dendrites and axons of the large pyramidal and spindle cells in layers V and V I in area TA are slightly more advanced in development than those of corresponding cells in area OA. The

COMMENTS processes of the pyramidal cells of layers II, III, and IV are about equally developed in these two areas. The mesh of fibers formed by the axons of Golgi type I I cells is of approximately the same density in areas T A and OA. The axons and dendrites of the neurons in area T B are in approximately the same state of development as corresponding cells in area OB, and are more advanced in development than those in area TA. The advanced condition of development of the pyramidal cells in lamina IIIc in area T B is especially noticeable. The mesh of fibers formed by the axons of Golgi type I I cells is more dense in area T B than in area TA. The dendrites of the cells in the area of koniocortex in the temporal lobe, area TC, are about the same size and length as corresponding cells in the koniocortex in the occipital lobe, area OC, but the axons of the cells in T C have less myelin than those in OC. Both the axons and dendrites of the cells in area T C are less advanced in development than those of corresponding cells in the region of the head in area PB. The mesh of fibers formed by the axons of Golgi type I I cells is much more dense in area T C than in area T B . The mesh of fibers in area T C is of approximately the same density as that in area OC, but is not as dense as that in the region of the head in area PB. The axons and dendrites of the pyramidal and spindle cells in areas T E and T F are in about the same state of development. They are less advanced in development than the processes of corresponding cells in area TA, but are more advanced than the axons and dendrites of corresponding cells in area PH. The mesh of fibers formed by the axons of Golgi type I I cells is of approximately the same density in areas T E and T F , and is not as dense as the mesh in area TA. The processes of the neurons in area T G are in about the same state of development as those of corresponding cells in area T E . The dendrites and axons of the pyramidal and spindle neurons in areas IA and I B are in about the same state of development as those of corresponding cells in areas F D and FCBm in the opercular parts of the frontal lobe, but are more advanced than those of corresponding cells in the most anterior parts of the frontal lobe. The mesh of fibers formed by the axons of Golgi type I I cells is more dense in area I B than

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that in area IA. The mesh in I B is much less dense than that in area TC. The processes of the neurons in areas LA, LC, LD, and L E are in approximately the same state of development, and are slightly more advanced than corresponding cells in F C and F D on the medial surface of gyrus frontalis superior and in area P E on the medial surface of the precuneus immediately above gyrus cinguli. The axons and dendrites of the large cells in layers V and VI in the uncus, area HA, are slightly more advanced than those in the temporal pole, area TG. The processes of the cells in layer I I I in these two areas are in about the same state of development. The processes of the pyramidal and spindle cells in area HD are slightly less advanced in development than those in area HA. The mesh of fibers formed by the axons of Golgi type I I cells in area HD is of about the same density as that in area TC, and indicates that area HD is a primary afferent area. The mesh is more dense than that in area HD in the three-month cortex. The processes of the various kinds of cells in all parts of area H E are about equally developed, and are more advanced than those of corresponding cells in areas HA and HD. The processes of the polymorph cells in area H F are in approximately the same state of development as those of the large pyramidal cells in area HE 2 . (7) The pedunculated bulbs are more numerous on all cells in all areas of the entire cortex of the six-month infant than on corresponding cells in corresponding areas in the three-month cortex. The increase in quantity of the bulbs is particularly apparent on the terminal branches of apical dendrites which end in layer I. On each pyramidal cell, whatever the size, the pedunculated bulbs are most numerous on the apical dendrite, much less numerous on the basal dendrites and collateral branches of the apical dendrite, and least in quantity on the terminal branches. The bulbs are present in about equal quantity on the basal dendrites and collateral branches of the apical dendrites, and are much less numerous on the terminal branches. The bulbs are present on the dendrites of Golgi type I I cells in about the same quantity as they are on the basal dendrites of pyramidal cells of comparable size. No pedunculated

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bulbs are present on either the cell-body or the axon of any cells. In every area of the cortex the neurons fall into the following sequence with reference to the quantity of pedunculated bulbs, beginning with the cells on whose dendrites the bulbs are the most numerous: ( i ) the extra-large cells in layer V, (2) the large ordinary and special pyramidal cells in layers V and VI, (3) the large pyramidal cells in lamina IIIc, (4) the large pyramidal cells in lamina I l l b , (5) the large pyramidal cells in lamina I l i a , (6) the small pyramidal cells in layers V and VI, (7) the cells of layer IV, and (8) the cells of layer II. There are differences among the various areas as to the quantity of pedunculated bulbs on the dendrites of the cells. The bulbs are more numerous in area FAy than in any other area in the entire cortex. Within area F A y the bulbs are present in approximately equal quantity in all four functional regions, except they are somewhat less numerous on the dendrites of the giant pyramidal cells of Betz in the region of the head than in the other three regions. Proceeding anteriorly from this area in the frontal lobe, the quantity of bulbs gradually decreases, and they are least numerous in the anterior part of the lobe. Pedunculated bulbs are more numerous on the dendrites of neurons in area P B than in any other area in the parietal lobe, but are less numerous than in area F A y in the frontal lobe. The bulbs are present in approximately the same quantity in all four functional regions of area PB. Pedunculated bulbs are less numerous on the dendrites of the cells in area PC than on corresponding cells in area PB. No differences between the various functional regions of area PC as to the quantity of bulbs are apparent. Pedunculated bulbs are present on the dendrites of the cells in areas PE, P F , and PG in approximately equal quantity, and are less numerous than on the dendrites of corresponding cells in area PC. The bulbs are less numerous on the dendrites of the cells in area PH than on corresponding cells in area PG. In the occipital lobe, pedunculated bulbs are more numerous on the dendrites of neurons in area OC than on the dendrites of corresponding neurons in area OB. The bulbs are less numerous on the dendrites of the neurons in area OC than on the dendrites

of corresponding cells in the region of the head in area PB, and are about the same as in the region of the head in area PC. Pedunculated bulbs are less numerous on the dendrites of the cells in area OA than on those of corresponding cells in area OB. Pedunculated bulbs are more numerous on the dendrites of the large extra-large cells in layers V and VI in area T A than on the dendrites of corresponding cells in area OA, but are present on the dendrites of other cells in these two areas in about equal quantity. The cells in area T B have more pedunculated bulbs than those of area TA, and about the same quantity as the cells in area OB. Pedunculated bulbs are more numerous on the dendrites of neurons in area T C than on the dendrites of corresponding cells in area T B , but are less numerous on the cells in area T C than on corresponding cells in area OC. Pedunculated bulbs are present on the dendrites of the neurons in areas T E and T F in approximately equal quantity, but are less numerous than on corresponding cells in area TA. Pedunculated bulbs are less numerous on the dendrites of cells in area T G than on the dendrites of corresponding cells in areas T E and T F . Pedunculated bulbs are present in approximately equal quantity on the dendrites of the cells in areas IA and I B , and are of about the same number as on the dendrites of the cells in the anterior part of the frontal lobe. There seems to be but little, if any, increase in the quantity of pedunculated bulbs on the dendrites of cells in area L A since the three-month stage of development. The bulbs are present in about equal quantity on the dendrites of the cells in areas LC, LD, and L E , and are more numerous than in these areas in the three-month cortex. The dendrites of the large cells in layers V and V I in area HA have more pedunculated bulbs than those of corresponding cells in area T G which adjoins the uncus. Pedunculated bulbs are not any more numerous on the dendrites of the cells in layer I I I in area HA than in area TG. Pedunculated bulbs are slightly less numerous on the dendrites of the cells in area HD than on the dendrites of corresponding cells in area HA. The pedunculated bulbs are present in approximately equal quantity on the dendrites of the neurons

COMMENTS in the various parts of area H E . The bulbs are more numerous on the neurons in area H E than on corresponding neurons in areas H A or H D . The dendrites of the large cells in area H E have more pedunculated bulbs than those of the polymorph cells in area H F . (8) The number and size of the exogenous fibers increase with advancing age. The number of fibers per unit of width ($ομ) in each layer of the various areas of the isocortex is shown in Table IV. With a few exceptions there has been an increase in the number of all exogenous fibers in each layer in each area since the three-month stage of development, as becomes evident upon comparing the figures in Table IV with those in the same table in Volume I I I . The tangential fibers in layer I show less variation in number throughout the entire isocortex than any other fibers. Usually the tangential fibers are larger, more numerous, and more darkly stained in the floors and walls of sulci than in the crowns of gyri. T h e source of the tangential fibers in layer I is not known. Cajal believed that they are the axons of the large horizontal cells in layer I. T h e bodies of these cells disappear, however, and very few are present in the cortex of the six-month infant. Some Cajal cells are present in the areas which are the least advanced in development. It seems unlikely that the processes of neurons would remain after the cell-bodies disappear. Short branches only are present on tangential fibers, and these branches are few in number. In every area of the isocortex the horizontal exogenous fibers are least in number in layer I I , silghtly more numerous in lamina I l i a , are still more numerous in lamina I l l b , and reach a maximum number in lamina I I I c and layer IV, where they constitute the outer band of Baillarger. T h e external margin of this band is indistinct, but the internal margin is usually well defined by an abrupt decrease in the number of horizontal exogenous fibers in lamina Va. T h e horizontal fibers increase in number in laminae Vb and Via, where they constitute the inner band of Baillarger, and are usually of about the same number as those in the outer band. T h e horizontal fibers in lamina V l b may be of the same quantity as those in the inner band of Baillarger, or may be slightly more or less numerous than the latter. A short branch is occasionally seen on a horizontal fiber, but no long branches have been observed.

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In all areas of the isocortex except areas F D p , T E , IA, IB, and H A the subcortical association fibers are more numerous than the horizontal exogenous fibers in any one layer. T h e subcortical association fibers are especially numerous in those areas where development is most active according to other criteria, viz., areas FAy, PB, OC, and T C . N o branches have been observed on any subcortical association fibers. The vertical exogenous fibers are, without exception, more numerous in every area than in the corresponding area in the three-month cortex. These fibers are most numerous in areas FAy, PB, OC, and T C . The vertical fibers ascend into the cortex in greater quantity than in the three-month brain, but few of them reach layer IV. No collateral or terminal branches have been observed on any vertical exogenous fibers. Each vertical fiber ends as a single shaft, and does not contribute any fibers to the mesh formed by the axons of the Golgi type I I cells. The tangential, horizontal exogenous, and subcortical association fibers are more numerous in area FAy than in any other area in the frontal lobe. Within area FAy there is very little, if any, difference between the four functional regions as to the quantity of tangential and horizontal fibers. Subcortical association fibers are the most numerous in the region of the lower extremity, are less in quantity in the region of the trunk and arm, are still less in the region of the hand, and are least in number in the region of the head. The vertical exogenous fibers are most numerous in the regions of the lower extremity and trunk, and are less in number in the other two functional regions. Proceeding anteriorly from area FAy there is a gradual decrease in the number of the tangential, horizontal exogenous, subcortical association, and vertical exogenous fibers. There is little, if any, difference between the various areas in the parietal lobe as to the number of tangential fibers in layer I. T h e horizontal exogenous and subcortical association fibers are more numerous in area P B than in any other area in the parietal lobe. Within area P B these fibers are the most numerous in the region of the lower extremity. They are of about equal quantity in the regions of the trunk and head, where they are less numerous than in the region of the lower extremity, and they are least in number in the region of the hand. There is little, if any, differ-

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THE CORTEX OF THE SIX-MONTH INFANT

ence between areas PB and FAy as to the number of horizontal exogenous, subcortical association, and vertical exogenous fibers. The horizontal exogenous and subcortical association fibers gradually decrease in number in the other areas in the parietal lobe in the following order: PC, PE, P F and PG, and PH. The vertical exogenous fibers are most numerous in the region of the leg in area PB, but there is not much variation in the quantity of these fibers in the other areas. Tangential, horizontal and vertical exogenous, and subcortical association fibers are present in about equal quantity in areas OA, OB, and OC. Horizontal fibers in layer IV and vertical exogenous fibers are slightly more numerous in area OC than in either area OB or OA, and are of the same quantity as corresponding fibers in the region of the lower extremity in area PB. Subcortical association fibers are slightly less numerous in area OC than in area PB, except in the region of the head. Tangential fibers in layer I are present in about equal quantity in all areas of the temporal lobe except in area TG, where they are less numerous than in the other areas. Horizontal exogenous fibers and subcortical association fibers are more numerous in area TC than in any other area in the temporal lobe. These fibers are present in about equal quantity in areas TA and TB, and are less numerous than in area TC. They are still less numerous in area TE, and are least in quantity in areas T F and TG. Horizontal exogenous fibers are more numerous in layers II, III, V, and VI in area TC than in area OC, but in layer IV they are of the same quantity in these two primary afferent areas. Subcortical association fibers and vertical exogenous fibers are slightly less numerous in area TC than in OC. Horizontal and vertical exogenous fibers are present in about equal quantity in areas TA, OA, TB and OB, but subcortical association fibers are more numerous in areas OA and OB than in areas TA and TB. The horizontal exogenous and subcortical association fibers are less in number in area TE than in area TA. These fibers are present in equal quantity in areas T F and TG, and are less numerous than in area TE. Vertical exogenous fibers are present in about equal quantity in areas TE, TF, and TG. All the fibers are present in areas T F and

TG in approximately the same quantity as corresponding fibers in area PH. Fewer vertical exogenous fibers reach layer IV in area TC than in area OC. In the insula, fibers of each category are present in equal quantity in areas IA and IB, and are of approximately the same number as in area PH. In gyrus cinguli the tangential fibers in layer I are more numerous in areas LD and LE than in areas LA and LC. Horizontal exogenous and subcortical association fibers are more numerous in area LA than in any other area in this gyrus. These fibers are present in approximately equal quantity in the other three areas. Vertical exogenous fibers are present in equal number in all areas in this gyrus. Horizontal exogenous and subcortical association fibers are slightly more numerous in area LA than in area FC in gyrus frontalis superior, but vertical exogenous fibers are less numerous in area LA than in FC. Horizontal and vertical exogenous and subcortical association fibers are less numerous in area LC than in area PE. Tangential, horizontal exogenous, and subcortical association fibers are present in approximately the same quantity in area HA as in the adjoining area TG in the temporal pole. Vertical exogenous fibers, however, are less numerous in HA than in TG. The tangential, horizontal and vertical exogenous, and subcortical association fibers have increased in size in all areas of the cortex since the three-month stage of development. In each area of the cortex each of these four categories of fibers appears to fall into three groups as to size, viz., small, intermediate, and large. The largest fibers only are measured and the sizes are given in Table V. The measurements of the largest fibers in the three-month cortex are included in this table because such a table was omitted from Volume III. Through oversight, a reference to such a table was not deleted from the text on page 142 in Volume III. The fibers are measured in the GolgiCox preparations, but they are of the same size in the Cajal sections. The fibers of each category are largest in areas FAy, PB, OC, and TC. In area FAy the fibers of each category are smaller in the region of the head than in the other three functional regions. The tangential and horizontal exogenous fibers are of equal size in the regions of the lower extremity, trunk, and

COMMENTS hand. The subcortical association fibers are of equal size in the regions of the lower extremity and trunk, but are larger in the region of the hand. T h e vertical exogenous fibers are largest in the region of the hand, smaller in the region of the trunk, and still smaller in the region of the lower extremity. Proceeding anteriorly in the frontal lobe the fibers of each category gradually decrease in size and are smallest in the most anterior areas of the lobe. In area P B the fibers of each category in the region of the lower extremity are of the same size as corresponding fibers in this region in area F A y . In the region of the trunk the tangential and vertical exogenous fibers are smaller in area Ρ Β than in area F A y , but the horizontal and subcortical association fibers are of the same size in these two areas. All the fibers are smaller in the region of the hand in area P B than in area F A y . In the region of the head the fibers are of the same size in areas P B and F A y , except the tangential fibers, which are smaller in P B . Proceeding posteriorly from area P B the fibers of each category decrease in size, and are of about equal size in areas P E , P F , P G , and P H . T h e fibers in these four areas are larger than corresponding fibers in the most anterior areas in the frontal lobe. T h e fibers of all categories are larger in area O C than in areas OA and O B . T h e fibers in O C are of the same size as the fibers in the region of the lower extremity in area P B , and are larger than those in the region of the head. In the temporal lobe the fibers of each category are largest in the primary afferent area T C . In the other areas in this lobe the fibers decrease in size in the following order: T B , T E and T G , T A , T F . T h e fibers of each category in area T C are about the same size as corresponding fibers in area O C . T h e fibers in area T B are the same size as those in area O B , but the fibers in area T A are smaller than those in area OA. T h e fibers of all categories are of equal size in areas I A and I B , and are of the same size as the fibers in the region of the head in area P B and in area F C B m . In gyrus cinguli the fibers are largest in areas L D and L E . T h e fibers in area L A are smaller than corresponding fibers in area F C on the medial surface of the superior frontal gyrus.

173

T h e fibers in the uncus, area H A , are smaller than the fibers in the adjoining area P G in the temporal pole. (g) T h e condition of myelination is studied in sections prepared b y the Weigert and Pal-Weigert methods. T h e number of stained fibers per unit of width (ζομ) in each area of the isocortex is given in Table I V . Comparison of this table with the same table in Volume I I I reveals that, with few exceptions, there has been an increase in the number of myelinated fibers in the cortex since the three-month stage of development. Comparison of the photographs of the Pal-Weigert sections with those in Volume I I I reveals that myelin has increased in quantity in those areas where it is present in the three-month cortex, and that it has appeared in areas where it is lacking at the three-month stage of development. Throughout the entire brain myelination is much more advanced in development on the vertical exogenous fibers in the cores of gyri subjacent to the cortex than in the cortex itself. T h e stained vertical exogenous fibers in the cores of gyri are not only more numerous but darker in color than any myelinated fibers in the cortex. T h e myelinated vertical exogenous fibers decrease in size, number, and depth of stain as they ascend in the cortex. T h e other myelinated fibers fall into the following sequence as to number, size, and depth of stain: ( 1 ) the subcortical association fibers, (2) horizontal fibers in lamina V I b , (3) horizontal fibers in the outer band of Baillarger, (4) horizontal fibers in lamina Va, (5) horizontal fibers in the outer band of Baillarger. There are practically no stained fibers external to lamina IIIc, except in layer I where stained tangential fibers are present in small quantity in some areas, particularly F A y , P B , O C , and T C . In general the pattern of distribution of myelin in the cortex of the six-month infant is much the same as that in the three-month cortex, and conforms to Flechsig's descriptions of the development of myelination. Myelination is more advanced in area F A y than in any other area, and areas P B , O C , and T C follow in the order named. Within area F A y the state of development of myelin is about the same in the regions of the lower extremity and the trunk and is more advanced than

174

THE CORTEX OF THE SIX-MONTH INFANT

in the region of the hand. Myelination is least advanced in the region of the head. Proceeding anteriorly in the frontal lobe, there is a gradual decrease in the degree of myelination, and myelin is least in quantity in the most anterior areas. In the parietal lobe, myelination is more advanced in development in area P B than in any other area. Within this area myelin is present in about equal quantity in the regions of the lower extremity, trunk, and hand, and is least in the region of the head. Myelination is less advanced in area PC, and the four functional regions are the same as in area P B as to the degree of development of myelin. The state of myelination is about the same in all the other areas in the parietal lobe and is only slightly less advanced in development than in area PC. In fact, myelination is more advanced in area P F than in the region of the head in area PC. Myelination is more advanced in area OC than in area OB, and is less advanced in area OA than in OB. Myelination is more advanced in area OC than in the region of the head in area PB, but is less advanced than in the other regions of area PB. In area OB myelination is more advanced than in the region of the head in area PC, but is less advanced than in the other functional regions of area PC. In the temporal lobe myelination is slightly more advanced in area T C than in area T B , and is but slightly more advanced in T B than in TA. Areas TC, T B , and TA are less advanced in myelination than areas OC, OB, and OA, respectively. Myelination is more advanced in area T C than in the region of the head in area PB, but is less advanced than in the other functional regions in P B . Area T B has more myelin than the region of the head in area PC, but less myelin than the other regions in PC. Myelination is about the same in area T E as in TG, and is less advanced than in area TA. The development of myelin is less advanced in area T F than in any other area in the temporal lobe. Myelination is less advanced in the insula than in the adjoining regions in the operculum of the frontal lobe and in area T G . T h e degree of development of myelin is about the same in all areas in gyrus cinguli. In area LA myelination is less advanced than in area F C or F D on the medial surface of gyrus frontalis superior, and it is

slightly more advanced in area LC than in area P E in the precuneus. Myelination is in about the same state in area H A as in area TG. I t is more advanced in development than in area H A in the three-month cortex. Myelination is more advanced in all hippocampal regions than in the three-month cortex. T h e degree of myelination is not the same in all of the six-month brains used in this study, as is shown in Table VII. The number of the brain in which myelination is the most advanced for each area of the cortex is placed in the first column at the left, and the numbers of the other brains are placed in the columns toward the right in the order of the degree of myelination. Owing to an unfortunate accident several blocks of tissue for the Weigert method were lost, so the table is not complete for all the areas. I t can be seen, however, that the brains fall into the following sequence with reference to the degree of myelination: ( i ) 41-91, (2) 3 7 - 8 2 , (3) 38-44, (4) 38-35, and (S) 37-49· This order is consistent for all areas of the cortex. Brain 37-49 is consistently in the column farthest to the right, showing that it is the least advanced in the development of myelin. This is the brain of an infant which was in a state of chronic malnutrition from the time of birth. The lack of definite knowledge of fiber connections between the various parts of the cerebral cortex (intercortical or association fibers), and between the cortex and lower neuron centers (extracortical or projection fibers) places any attempt to correlate structure and function in the realm of speculation. A satisfactory anatomical method is the chief obstacle to obtaining definite knowledge of both intercortical and extracortical fiber connections. Stimulation methods of various kinds have produced many physiological data, but no exact information as to structure. Experimental degeneration methods on animals and pathology in humans have many limitations, and have yielded a comparatively small amount of reliable information as to fiber connections in the human cerebral cortex. For instance, the exact origin and destination of the fibers in the pyramidal tract are still unknown, and this bundle of fibers was one of the first to be identified. The attempt in this investigation to correlate the development of structure with the development of

COMMENTS behavior is likewise speculative. Certain inferences as to correlation, however, may be attempted. All nine of the criteria of development used in this investigation indicate that the primary motor area FAy is more advanced in development than any other area in the six-month cortex. This suggests that motor phenomena predominate in the behavior of the infant at the age of six months, and the many investigations in child behavior confirm this. This is true also at the age of three months, but all nine criteria show that the cortex in area FAy is more advanced in development in the infant at the age of six months than at the age of three months. The increased state of development of the primary motor area in all four functional regions may be correlated with the increase of inhibitions of random movements of reflex character in all parts of the body in the six-month infant as compared with the three-month infant. Certain differences in the development of structure are apparent between the four functional regions of area FAy. Whereas in the three-month infant the region of the upper trunk, shoulder, and arm is more advanced in development than any of the other regions, at the age of six months the region of the hand is as well developed as that of the trunk and arm. This may be correlated with the increase in voluntary movements of the trunk, arm, and hand at the age of six months as compared with the three-month infant. The six-month infant cannot sit with the vertebral column fully extended. While lying in the prone position he can hyperextend the head and upper one-half of the vertebral column. He turns the head from side to side easily and readily, but not very quickly. He can roll over to the supine position from the prone position, and does so by twisting the upper one-half of the trunk laterally and pushing the lower extremity against the floor. These movements are performed slowly and with difficulty, success usually requiring several attempts. At six months the infant moves the wrist joint and joints of the digits less readily and easily than the elbow and shoulder joints. But the movements of the hand are performed with greater ease and control than at the age of three months. The six-month infant moves the wrist, abducts and adducts the fingers, and opposes the thumb to the fingers, but performs all these movement awkwardly while playing with toys. He grasps a rod either by having thumb and

175

fingers on the same side or with the thumb on one side and the fingers on the other. He shows no preference for either the right or left hand. Development of the cortex is less advanced in the region of the lower extremity in area FAy than in the region of the hand and arm, and the movements of the lower extremity are less controlled than those of the upper extremity. The movements of the lower extremity are less random and more voluntary at six months than at three months, and the cortex in this area is more advanced in the six-month infant. The six-month infant moves all the joints in the lower extremity, but flexes and extends the hip and knee joints more readily than the joints of the foot. The two lower extremities are usually moved at the same time, but one extremity can be moved independently of the other. The lower extremities are used in pushing against the floor or side of the play-pen. While in the prone position the six-month infant can raise the anterior part of his body by extending both elbow joints and propel his body backward by pushing with his upper and lower extremities, but does this with considerable difficulty. While lying in the supine position he beats his entire lower extremities in unison in a random manner. He exercises much less control over them than over the upper extremities. The cortex in the region of the head is less advanced in development than any other region in area FAy, but is more advanced than the region of the head in the three-month infant. Movements of the muscles in the head are more controlled in the sixmonth infant than in the three-month infant. At the age of six months there is less voluntary movement of the muscles in the head than in other regions of the body. The eyeballs move freely and converge for near and distant vision, but the eyes are held on one object for only a comparatively short time. The facial muscles are in repose much of the time, but are moved in smiling, vocalizing, eating, etc. The infant mumbles, babbles, laughs, and cries readily and easily. Each of the nine criteria shows that there is a gradual decrease in the degree of development of the cortex in the frontal lobe in a postero-anterior direction, and development is least advanced in the most anterior parts of the lobe. Each area of the cortex in the frontal lobe in the six-month infant is more ad-

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THE CORTEX OF THE SIX-MONTH INFANT

vanced in development than at the age of three months. Area P B is next to area F A y as to the degree of development of the cortex, and is more advanced than any of the other areas in the parietal lobes. The cortex in area P B is more advanced in development than in the three-month infant. There are differences between the functional regions in area PB. The regions of the hand and of the upper trunk and arm are more advanced than the regions of the head and lower extremity. This is the same order as in the threemonth cortex. The exogenous fibers and myelin are about the same in the region of the lower extremity as in the regions of the upper trunk, arm, and hand. By all the criteria area PC is less advanced in development than PB, areas PE, P F , and P G are in about the same condition and are less advanced than PC, and area PH is less advanced than PG. Area P B is believed to be the primary afferent area for general somesthetic and proprioceptive impulses, including discriminative touch. The six-month infant has undoubtedly increased his experience in all of these sensations since the three-month stage of development. According to most of the criteria the primary afferent area for vision, OC, is less advanced in development than area P B in the region of the head. Vision is still in an early state of development in the sixmonth infant. Some investigators believe that bifocal vision is just beginning at this age. The cortical area for macular vision is believed to be located in the occipital pole. Myelination is less advanced in the cortex in the posterior limb of the calcarine sulcus than in that in the anterior limb, which would indicate that macular vision is less advanced in the six-month infant than peripheral vision. B y most of the criteria the cortex in area OB is less advanced in development than that in area OC, and is more advanced than that in area OA. The cortex in the primary acoustic area, TC, is more advanced in development than any other area in the temporal lobe, but is less advanced than area OC. Areas T B and T A are in about the same state of development, and are less advanced than TC. Areas T B and T A are less advanced in development than areas OB and OA, respectively. The cortex in area T E is less advanced in development than that in area

T B . Areas T F and T G are in about the same state of development, and by most of the criteria are less advanced than area T E . By some criteria areas T F and T G are more advanced than area PH; by other criteria these three areas are in about the same state of development. All areas in the temporal lobe are more advanced in development than in the threemonth cortex. Hearing does not seem to play as important a role as vision in the behavior of the infant at the age of six months. He has, however, begun to make associations based on hearing. The cortex in the insula is in the same state of development in IA and I B and has advanced but little since the three-month stage. It is in about the same state of development as the adjoining cortex in the operculum of the frontal and parietal lobes. Nothing is known regarding the function of the insula. There has been but little advance in development in the cortex of gyrus cinguli since the three-month stage. The cortex is in about the same condition in all areas in this gyrus, and is, in general, less advanced in development than the cortex in areas F C and F D on the medial surface of the frontal lobe and in area P E in the precuneus, but the processes of the cells and myelination are more advanced in areas LC, LD, and L E than in the cortex in the precuneus. The cortex in the uncus, area HA, is in about the same state of development as that in the adjoining temporal pole, area T G ; the cortex in area HA is more advanced in development in the infant at the age of six months than at three months. This area and all other areas in the hippocampus are believed to be the cortical areas for olfaction. Experimental investigations indicate that the cortex in the temporal pole, TG, is also an olfactory area. Area HD has all the characteristics of koniocortex, and is probably a primary afferent area, but the function of the area is not known. The mesh of fibers formed by the axons of Golgi type I I cells is of about the same density as that in area TC. Myelination is more advanced in area HD than in area HA. The cortex in areas HE l a , HE l b , HE 2 , and H F is in about the same state of development, and in all of these areas it is more advanced in development than in area HA. The cortex in all of the hippocampal areas is more advanced in the six-month infant than in the three-month infant.

COMMENTS As is true in the cortex of the three-month infant, also in the six-month infant the extra-large and large cells in layers V and VI are more advanced in development in all areas than any other cells. The horizontal and vertical exogenous and subcortical association fibers are more advanced in development in layers V and VI than in any other layers. This would indicate that function in the six-month cortex is more active in these two layers than in any of the other layers. The apical dendrites of the extra-large pyramidal cells in layer V in all parts of the isocortex end in layer I, and their terminal branches are more numerous, larger, more compact in structure, and have more pedunculated bulbs than in the threemonth cortex. These terminal branches end among the tangential fibers in layer I, and these fibers are, in the majority of the areas, somewhat more numerous and larger than in the three-month cortex. Comparatively few of the tangential fibers are myelinated, and there is probably some, but little, function in layer I. The cells in layer IV have advanced in development, and the horizontal exogenous fibers have increased in number, size, and myelination since the three-month stage. The increase in development is particularly evident in the primary afferent areas PB, OC, and TC. In these areas, particularly, the mesh of fibers formed by the axons of Golgi type II cells is more dense than in the three-month cortex. Further-

177

more, a greater number of vertical fibers ascend to layer IV and more of them are myelinated than in the three-month cortex. All of these factors would seem to indicate that there is more function in layer IV in the six-month infant than at the age of three months. One of the most conspicuous differences in the cortex of the six-month and three-month infants is the advance in development of the large pyramidal cells in lamina IIIc, and to a much lesser degree some of the large cells in lamina Illb. The six-month infant has more ability in association than the three-month infant. Are these two phenomena related? Some investigators have suggested that the cells in layer III are association neurons. The termination of their axons is unknown. The cells and fibers in layer II and lamina I l i a are still in a low state of development in the six-month cortex. In the areas of low development these cells are in about the same condition as corresponding cells in the three-month cortex. They probably have little, if any, function in the transmission of impulses. The uniformity of the microscopical structure of the cortex in all areas strongly suggests that the cells in any one area have no specificity of function, and that any differences in function are based upon fiber connections only. Even the allocortex in the hippocampus resembles the isocortex sufficiently to suggest that it has been modified solely by mechanics.

TABLES

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Fig. 14.

Cajal method

Area FA7. Posterior wall of gyrus centralis anterior in the region of the hand

PLATE

IX

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Fig. 126.

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Fig. 128.

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Fig. 129.

Cajal method

Area TA. Crown of gyrus temporalis superior

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Fig. 130.

Golgi-Cox method

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Fig. 133.

Cajal method

Area T B . Lateral part of gyrus temporalis transversus anterior

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Fig. 134.

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Fig. 135.

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Fig. 137.

Cajal method

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Fig. 138.

Golgi-Cox method

Fig. 139.

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Fig. 181.

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Figs. 184-187.

Fig. 185.

Cajal method

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PLATE

fe-Tflfc

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Figs. 188-191.

Fig. 189.

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Fig. 190.

Golgi-Cox method

Fig. 191.

Drawings from Golgi-Cox preparations

PLATE

XCVII

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Figs. 192-195.

Fig. 193.

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XCIX

Fig. 196.

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Fig. 197.

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PLATE

Fig. 198.

Gyrus centralis anterior (at the bottom) and gyrus centralis posterior in the region of the hand

Fig. 199.

Gyrus centralis anterior (at the bottom) and gyrus centralis posterior in the region of the head

C

PLATE

CI

Fig. 200.

Fig. 201.

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Longitudinal section through gyrus temporalis transversus anterior showing area T C (at the top and left)

PLATE

-Μ nj ΟΜ ^^ 'C öcd u ιη ω 4-> Υ Ö Λ CD -Μ Ö Ο 02 > FA S O

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CVII