Journal für Hirnforschung: Band 5, Heft 6 1962 [Reprint 2021 ed.] 9783112519882, 9783112519875

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JOURNAL FÜR HIRNFORSCHUNG I N T E R N A T I O N A L E S J O U R N A L FÜR N E U R O B I O L O G I E I N T E R N A T I O N A L J O U R N A L OF N E U R O B I O L O G Y JOURNAL I N T E R N A T I O N A L E DE N E U R O B I O L O G I E B E G R Ü N D E T VON C É C I L E U N D O S K A R VOGT Unter Mitwirkung des Instituts f ü r Hirnforschung und allgemeine Biologie in Neüstadt/Schwarzwald und der Arbeitsgemeinschaft f ü r vergleichende Neuroanatomie der Fédération mondiale de Neurologie (World Fédération of Neurology) HERAUSGEBER H. Adam (Wien), J . Anthony ( P u l l ) , J. Ariem Kippen (Groningen), M. C U » (Utanbul), E. Croeby (Ann Arbor), A. Dewnlf (Corbeek-Lo), J. Enolar (Zaragoaa), I. N. Fllimonoff (Moakau), R. Hafiler (Freiburg i. Br.), E. Herzog (Concepclon), A.Hopf (Nenstadl/Sehwarawald), J . Jansen (Otlo), W. Kiraehe (Berlin),. J. Konoraki (Warschau), St. Konijtj (Peca), J. MarSala (Prag), H. A.Matike (Lawrence), D. MiikoJezy (Tlrgu-Murea),G. Pilleri (Waldau-Bern), T.Ogawa (Tokyo),BJlexed (Upaala), S.A.Sarklasow(Moikau), H.Spatx(Frankfurt a.M.), H. Stephan (Frankfurt n.M.), J. Saentágothal (Péca), W. J . C. Verhaart (Leiden), C. Vogt f (Cambridge), K. G.'Wlngatrand (Kopenhagen), W. WUnicher ' (Lelpaig) REDAKTION

J.Anthony, Paria A.Hopf, Neustadt/Schwarzwald W.Kirsche, Berlin J.Szentágothai, Pécs

BAND S • H E F T 6 • 1 9 6 2

A K A D E M I E - V E R LA G - B E R L I N

Inhalt des Heftes 6

Seite

VERHAART, W. J. C., Anatomy of the brain stem of the elephant

455

Im JOURNAL F Ü R .HIRNFORSCHUNG werden Arbeiten aus dem Gesamtgebiet der normalen Morphologie (Anatomie, Histologie, Cytologie, Elektronenmikroskopie, Histochemie) und der Entwicklungsgeschichte des Nervensystems unter Einschluß experimentelL-anatomischer Arbeiten veröffentlicht. Neuropathologische Arbeiten werden nur angenommen, wenn sie Beiträge zur normalen Struktur, den Strukturwandlungen oder deren funktionellen Bedeutungen enthalten. Zum Publikationsgebiet des Journals für Hirnforschung gehören auch Arbeiten, die sich mit der Zuordnung experimenteller Reizund Ausfallerscheinungen bzw. klinischen Symptomen zu bestimmten Strukturen des Gehirns („Lokalisationslehre") befassen. Als spezielles Publikationsgebiet ist die vergleichende Neurobiologie vorgesehen. The JOURNAL F Ü R HIRNFORSCHUNG will publish studies on normal morphology (anatomy, histology, cytology, electron microscopy, histochemistry), on the development of the nervous system, as well as experimental anatomical studies. Neuropathological studies will only be published if they contribute to the knowledge of normal structures, structural changes or their functional significance. Papers dealing with the cerebral localization of experimental excitation and deficit phenomena or clinical symptoms (localization theory) will also be published by the JOURNAL F U E R HIRNFORSCHUNG. A special part of the publication is reserved for comparative neurobiology. Le JOURNAL FÜR HIRNFORSCHUNG publiera des études sur la morphologie normale (anatomie, histologie, cytologie, microscopie électronique, histochimie), sur le développement du système nerveux ainsi que des études anatomiques expérimentales. Des études neuropathologiques seront seulement acceptées quand elles contribuent à la connaissance des structures normales, des changements structurels ou de leur signification fonctionelle. Des études sur. la localisation cérebrale de phénomènes expérimentaux ou cliniques d'excitation ou de déficit (doctrine des localisations) seront également publiées par le JOURNAL F U E R HIRNFORSCHUNG. Une partie spéciale sera réservée à la neurobiologie comparée.

Verantwortlich für den Inhalt: Prof. Dr. J . Anthony, Paris, Doz. Dr. A. Hopf, Neustadt/Schwarzwald, Prof. Dr. W. Kirsche, Berlin und Prof. Dr. J . Szentägothai, P6cs. Verlag: Akademie-Verlag GmbH, Berlin W 8, Leipziger Straße 3 —4 (Fernruf: 20 04 41, Telex-Nr. 0 1 1 773); Postscheckkonto: Berlin 35021. Bestellnummer dieses Heftes: 1018/5/6. Das „Journal für Hirnforschung" erscheint in zwanglosen Heften von verschiedenem Umfang. 6 Hefte bilden einen Band. Preis je Einzelheft 12,— DM. Ein Band 72,— DM. Satz und Druck: VEB Druckhaus „Maxim Gorki", Altenburg. Veröffentlicht unter der Lizenznummer 1325 des Presseamtes beim Vorsitzenden des Ministerrates.

JOURNAL FÜR HIRNFORSCHUNG I n t e r n a t i o n a l e s Journal für Neurobiologie BAND 5

HEFT 6

1962

Laboratory of Neuro-anatomy of the Institute of Neurological Sciences, State University, Leiden, The Netherlands. Director: Prof. Dr. W. J. C. V e r h a a r t .

Anatomy of the brain stem of the elephant

by

W. J. C. V e r h a a r t

18 Figures

Index Introduction page Material and methods Description of the preparations The medulla oblongata The pons The mesencephalon Discussion Summary Zusammenfassung References Pictures and description

455 456 457 457, fig. 1 - 3 469, fig. 4 - 8 488, fig. 9 - 1 2 504 520 520 521 522

Introduction The anatomy of the brain stem of the elephant is very unsufficiently known, evidently because it is not readily available and experiments cannot be performed. Moreover studies as yet have been mostly done with preparations, stained according to one of the classical myelin-staining-methods, which do not surpass the Wolters-Kulschitski method much in making separate fibre tracts visible. Papers about the C. N. S. of the elephant are relatively rareV o g t , Hirnforschung, Bd. 5, Heft 6

32

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D e x l e r (1907) described the entire C. N. S. of the elephant, b u t only paid attentation to its macroscopical features. B r e g m a n n (1915), described the pyramidal tract in the elephant of which according to his description, 2/3 of the fibres join the facial nerve nucleus in the low pons. He examined the pyramidal tract at the rostral and the caudal border of the pons and saw t h a t at the latter its cross surface area amounted to about one third of that in the former. As he saw crossing fibres at the level of the facial nerve nucleus and found this nucleus to be very large, he came to this conclusion about so m a n y pyramidal fibres terminating there, after crossing. P r e c e c h t e l (1925) gave a description of the brain stem with pictures of W. P. and of haematoxylin-stained slides. He points to the strong external arcuate fibres in the medulla, the large size of the facial nucleus with receives many pyramidal fibres, the conspicuous nucleus of Roller and dorsal vagus nucleus, the relative smallnes of the motor trigeminal nucleus, and the large vestibular nuclei, which show a direct continuation to the medial cerebellar nuclei, with vestibular fibres ascending to the latter together with the nuclei. He refers to the description of the inferior olive with its huge dorsal lamina by K o o y in 1916. About the cerebellum and the related systems Precechtel reported t h a t the corpus restiforme and the brachium pontis are large, the nuclei of both also being more marked than in man, as are the basal pontine nuclei. In the midbrain the red nucleus consists chiefly of large cells, even in its most rostral parts, the substantia nigra partly lies within the medial angle of the cerebral peduncle, described by Winkler and Potter as nucleus proprius pedunculi. The peduncle itself is large in a vertical direction and has forced the medial geniculate body dorsally. In the midbrain roof large cells of origin of the fifth nerve mesencephalic root are found, the fasciculus retroflexus passes closely in front of the red nucleus, to the nucleus interpeduncularis, most of its fibres are devoid of myelin, b u t at its periphery myelinated fibres are present. V e r h a a r t and K r a m e r (1958), V e r h a a r t (1959) in descriptions of the cord of the elephant reported some features of the low medulla also.

Material and Methods The C. N. S. of an almost adult african elephant, Loxodonta africana, was given me by Prof. H. S p a t z from the Max Planck Brain institute at Gießen, it had been in formalin for a number of years. Initially it was presented to Dr. L u d o van B o g a e r t at the Bunge Institute at Antwerp, who send it to Gießen because he did not intend to study it himself. Fixation was moderately good, except for the centre of the cerebral hemispheres. For the present study the brain stem was sectioned at a high mesencephalic level, it was cut in blocks about two cm. wide, fixed in Baker's formalin for 3 weeks and mordanted in a 5% potassium bichromate solution with some calcium chloride added to it.

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As material fixed in formalin for a very long period, still m a y stain resona b l y well with H a g g q v i s t ' s m e t h o d ( V e r h a a r t and V o o g d 1961), and this m e t h o d b e t t e r t h a n most others can show b o t h ganglioncells and fibres, this staining m e t h o d was chosen. F o r t u n a t e l y the fibres, axons and myelinsheaths in general stained sufficiently well and fibrepattern could be recognized all over t h e section and fibretracts delineated. Photographs of whole preparations are not very elucidating, whereas those of fibretracts m a d e with higher power magnification are m o s t l y satisfactory. A description of the macroscopic features will not be given, as this has been done already b y Dexler.

Description of the preparations T h e m e d u l l a o b l o n g a t a . I t was cut in two blocks, one from the pyramidal crossing to t h e mid-olivary level, preparations numbered from 113 to 1, caudal to rostral, and t h e other from the midolivary level to t h a t of t h e rostral pole of the facial nucleus numbered from 1 4 4 — 1 . T h e most caudal level has already been more or less described with the pyramidal crossing and the topog r a p h y of t h e cord, H a r d e s t y (1902) V e r h a a r t and K r a m e r ( 1 9 5 8 ) ; V e r h a a r t (1960). This level is represented by preparation 113 of the first medulla oblongata block, which shows the inferior olivary complex not yet to be present, and the pyramidal decussation no more to be distinguished. The pyramids present a somewhat triangular shape with the tip dorsally as seen in a level where the decussation begins. The pyramid is characterized by its paucity of fibres over 6 [x in diameter, and therefore it can be recognized readily. Dorsal to the pyramid the thick fibres of the medial longitudinal fascicle are very conspicuous but in between thin and thick fibres are mixed and moreover a zone of moderately thick fibres also is present. This zone shows a much more homogeneous fibrepattern than the medial longitudinal fascicle and to all probability it represends the most caudal part of the medial lemniscus. — In the cat Busch could find it at this same side between the pyramid and the m. 1. f. in experimental lesions of Goll's nucleus. — However it proved not possible to find this pattern all over the borderline between the pyramid and the fascicle, especially laterally in the adjacency of the hypoglossus nerve root the fibrecomposition was very heterogeneous. The medial longitudinal fascicle in the anterior funiculus lateral and dorsal to the pyramid, characterized by a relative large number of very coarse fibres, can be followed dorsally to the central canal. Between the fascicles of both sides there are very many longitudinally cut coarse fibres, that apparently run ventrally and subsequently as external arcuate fibres either along the ventral margin of the pyramid or through it, pass to the ventrolateral periphery. At the ventral margin of the pyramid near the midline they form a fairly thick bundle. The anterolateral funiculus begins lateral to the 12th nerve bundles, it contains a number of coarse fibres, not present in the pyramid. Along the periphery an area with many more such fibres, containing those of the lateral vestibular tract cannot be identified. Within the posterolateral funiculus similar to that in the high cervical cord, the dorsal spino-cerebellar and the rubro-spinal tract can be located. The first can be recognized by its very large percentage of coarse fibres, the latter by its typical composition of bundles consisting of very coarse fibres intermingled with many fine ones. Following this pattern medially it was proved that it occupies a large area which lies all along the lateral half of the posterior horn. Maximal fibres diameters however are hardly more 32*

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t h a n in other mammals, about 22 [x, but fibres over 6 ¡x in diameter amount to only about 10%. In the dorsal spino-cerebellar t r a c t they are much more numerous ( H a a r t s e n 1961). The trigeminus descending root with its nucleus still much resembles Lissauer's marginal zone and the top of the posterior horn of the high cervical cord. Its fibres are chiefly very small but larger and even relatively large fibres are spread all over its area. Along its dorsomedial border to Burdach's funiculus only, these latter fibres are less numerous t h a n elsewhere. The nucleus consists of an outer layer of gelatinous substance and in inner portion provided with larger neurones. The latter is larger and the former smaller t h a n in a high cervical level. The posterior funiculi are not very large, relative t h e size of the medulla, they contain small nuclei, in Burdach's t r a c t laying a t the ventral border. Dorso-lateral to the m. 1. f. the small hypoglossus nucleus is found. The reticular formation, characterized by small fibrebundles containing fibres of different calibers, mixed with grey areas, is surrounded by the posterior funiculi with their nuclei, the trigeminal root, the rubrospinal tract, the anterolateral funiculus and the medial longitudinal fascicle. Moderately large ganglioncells are spread all over its extend, b u t it does not contain grey areas, other t h a n very small ones. The next level is represented in preparation 55 and 54 (fig. 1); it lies somewhat caudal to preparation 55 where the ventricle is going to open, because the latter preparation was not fit for being depicted. The pyramids are separated by a median ventral sulcus, bundles of external arcuate fibres traversing them, can be followed along the periphery along the lateral side up to the lateral tip of the external cuneate nucleus. Arcuate nuclei are not present. The medial lemniscus apparently fills the whole interolivary space as far dorsally as the dorsal accessory olive, b u t in between the nuclei of the olivary complex and around them the fibrepattern of the lemniscus is not detectable. At the dorsal border of the lemniscus the structure of the white matter gets less compact and thin fibres increase in numbers, the caliber of the thickest fibres also increasing however. The inferior olivary complex consists of the dorsal lamina of the main olive, which in t h e elephant is very wide and lacks the corrugated aspect of t h a t in primates and m a n y other ammals ( K o o y 1916), and a very short and narrow ventral lamina. Dorsal to the medial third of the dorsal lamina of the main olive lies the dorsal and medial to the lateral half the ventromedial accessory olive. Peripheral to the main olive a large number of mediumsized fibres about 4 (j. in diameter are visible, presumably olivo-cerebellar fibres, as will be evidenced in more rostral levels. Such fibres similarly are present within the hilus of the olives and longitudinally cut they travers the medial lemnisci in the interolivary space. Within the anterolateral funiculus lateral to the olive, several groups of ganglioncells are dispersed, belonging to the central p a r t of the lateral funiculus nucleus. Medial to them a few large motor cells may indicate the ambiguus nucleus. The ventral spino-cerebellar t r a c t within Gower's anterolateral fascicle is not sharply circumscribed although some accumulation of thick fibres can be distinguished along the periphery lateral to the central lateral funiculus nucleus. The dorsal spino-cerebellar and the rubrospinal tract, are readily visible, both containing groups of ganglioncells. Delineating the latter t r a c t medially from the lateral reticular formation is awkward, because its bundles gradually get more dispersed. Lateral to the descending trigeminus root a relatively thick layer of coarse longitudinally-cut fibres appears, which can be followed dorsally to the lateral tip of the external cuneate nucleus. I t is t h e beginning of the corpus restiforme and presumbly consists chiefly of dorsal spino-cerebellar t r a c t fibres, and nuclei-cerebellar fibres from the nucleus funiculi lateralis and anterioris ( B u s c h ) . At the right side within the corpus restiforme a relatively large nucleus lies lateral to the intermediate third of the descending trigeminus branch, as already mentioned by Precechtél in 1925. The descending root of the trigeminal nerve is very large. I t contains some fairly large nuclei within its fibremass. Its

Bd. 5 Heft 6

ANATOMY OF T H E B R A I N STEM OF T H E E L E P H A N T

459

nucleus contains two more or less gelatinous masses medial to its ventral third and two other very small ones more dorsally. The remaining nucleus consists of fairly large neurones scattered between fibres and not arranged in special groups. The nuclei within the root itself similarly consist of relatively large cells, unlike those in the gelatinous masses. The posterior funiculi everywhere show a number of ganglioncellgroups but large areas in the dorsomedial part of Burdach's funiculus are still devoid of them. Dorsal to the trigeminal descending root a large group of large ganglioncells represents the nucleus cuneatus externus. At the right side of the preparation which apparently lies somewhat more rostrally than the left side, the nuclei are somewhat larger than at the left side. The external cuneate nucleus at the latter side exhibits separate cellgroups situated medioventral to the main nucleus adjacent to the ventral part of Goll's nucleus. The central grey matter around the central canal contains a relatively large hypoglossal nucleus, consisting of a medial part dorsal to the medial longitudinal fascicle and a lateral part lateral the former. At the lateral aspect of the central grey matter the dorsal vagal nucleus is very distinct. Dorsal to the lateral part ot the 12th nerve nucleus a very small nucleus of small neurones represents the intercalate nucleus of Staderini. The medial longitudinal fascicle ventrally is not yet clearly separated from the medial lemniscus, although they can be distinguished because of the somewhat less compact structure and. the more marked presence of both fine and very coarse fibres in the former. A conspicuous diversity of fibre-patterns in certain areas of its cross surface area, as reported by Busch in the cat, cannot be detected. Lateral to its ventral half continuing lateral to the dorsal accessory olive the circumscribed bundles of the lateral vestibulospinal tract grow more marked. Lateral to the medial longitudinal fascicle at the ventral border of the central grey matter the fibrebundles of the lateral reticular formation form a relatively dense and circumscribed area of white matter protruding into the grey matter. I t has about the same shape as in the cat, but its seems much more richly provided with myelinated fibres, and ganglioncells are only relatively rare and widely scattered. Probably lateroventral to the dorsal vagus nucleus the bundles of the tractus solitarius can be recognized. The next preparation to be described is prep. 16 (fig. 2) in a midolivary level. The gross changes are the increase in size of the inferior olive, the posterior funiculi nuclei and the central grey matter, and the opening of the fourth ventricle. The arrangement of the elements however is essentially the same as in the previous level. The pyramid laterally lines the ventral periphery till the medial tip of the main olive. Medially it is separated from the contralateral pyramid by external arcuate fibres. Arcuate fibres both traverse it and curve around its ventromedial edge to reach the ventrolateral periphery similar to the more caudal levels. Its surface area amounts to 18 sq mm. its fibre content to about 959,000 fibres, with 35% up to 2 (JL, 44% from 2 — 4 [x, 19% from 4 — 6 (A and 1% from 6— 8 ¡x in diameter. Dorsal to it along the raphe the medial lemniscus can be followed somewhat beyond the dorsal border of the dorsal accessory olive. Between the main and the dorsal accessory and along the dorsal border of the latter, the lemniscus fibre-pattern is found, and presumably the lemniscus extends that far. The olivary complex consists of the main olive, and the ventromedial and dorsal accessory olives. At this level they show the characteristic elephantine features of the greatly swollen dorsal lamina of the main olive and the somewhat less swollen ventromedial accessory olive. The dorsal accessory olive consists only of a narrow band and the ventral lamina of the main olive is still very short. Olivo-cerebellar fibres longitudinally cut are plentiful in the hilus of the olive and in the anterolateral funiculus lateral to it. They can be followed latero-dorsally through the rubrospinal tract and along the ventral and lateroventral margin of the trigeminus descending root. Lateral to the root they are only found very medially and ventrally, the rest of the area being filled with much thicker fibres. The coarse fibres of the ventral spino-cerebellar tract mixed with olivocerebellar fibres and presumably other fibres of Gower's fascicle lie along the periphery,

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ventral to the dorsal spino-cerebellar tract. The latter is still readily recognizable by its large contingent of coarse fibres, but m a n y of them already proceed dorsally. The rubrospinal tract has not changed its site and appearance, excepted the large rostrolateral portion of the lateral funiculus nucleus lying within it. The rostromedial division is much less marked, it consists of a number of ganglioncellgroups lying dorsal to the lateral third of the main olive. The corpus restiforme has much increased in size, along the' dorsolateral periphery of the medulla it reaches as far medially as the external cuneate nucleus. I t everywhere has grown much wider and lateral ,to the dorsal half of the trigeminal root it contains

Fig. 1 (Medul. 1/74). This picture shows the medulla in a low olivary level, as the scheme shows the pyramid the medial lemniscus and the medial longitudinal fascicle lie along the raphe. The inferior olive consists of a relatively small principal olive, lateral to it m a n y ganglioncell groups of the central portion of the lateral funiculus nucleus are found and lateral to them the dorsal spinocerebellar t r a c t with the caudolateral p a r t of t h a t nucleus and medial to it the rubrospinal tract. External arcuate fibres line the medulla from the pyramid to the lateral third'of the ramus descendens trigemini. The latter is large its nucleus shows m a n y areas of gelatinous grey matter. The funiculi of Burdach and Goll show their nuclei. In the ventral p a r t of the grey matter the hypogolssus and dorsal vagus nuclei are present. Magnification x S1/^-

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ANATOMY O F T H E B R A I N STEM OE T H E E L E P H A N T

4(>1

a large and a small nucleus. The former is allmost in continuity with the external cuneate nucleus, and its ganglioncells are of the same large size. The corpus restiforme all over contains a very large contingent of very coarse fibres, only along the ventrolateral margin of the trigeminal root some much smaller fibres are present. I t is sharply circumscribed against the trigeminal root because of the differences in fibrepattern, b u t against the external cuneate nucleus it cannot be demarcated as from t h a t nucleus fibres of the same large size seem to enter it. The trigeminal descending root is not much changed, only within its medioventral tip fine fibres are very numerous. Its nuclei dorsally touch those of the posterior funiculi.

Legenda. C1)V

C U

drV dsc 1)X

ea EL fp

Pars caudalis of the radix dcscendens nucleus Nucleus cuneatus Radix descendens trigemini Dorsal spino-cerebellar t r a c t Dorsal vagus nucleus Eibrae arcuatae externae Nucleus funiculi lateralis Funiculus posterior

OR i o

ml mlf P rs. N XII lvs

Nucleus gracilis Inferior olive Medial lemniscus Medial longitudinal fascicle Pyramid Rubrospinal t r a c t Nucleus nervi hypoglossi Lateral vestibulo-spinal t r a c t

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Fig. 2 (Medul. 1/'IC>) Midolivary level of t h e medulla T h e p y r a m i d keeps its site, t h e medial lemniscus spreads all over t h e olive a n d t h e i n t e r o l i v a r y space, t h e medial longitudinal fascicle is less c o m p a c t in its v e n t r a l t h i r d . A p a r t of t h e nucleus funiculi anterioris lies within it. T h e olivary complex shows t h e typical swollen dorsal l a m i n a a n d v e n t r a l accessory olive, e x t e r n a l a r c u a t e fibres are still p r e s e n t along t h e v e n t r a l a n d v e n t r o l a t e r a l perip h e r y . T h e c e n t r a l p a r t of t h e nucleus funiculi lateralis is m u c h smaller b u t t h e rostrolateral p o r t i o n within t h e r u b r o s p i n a l t r a c t is relatively large. Olivocerebellar fibres lie along t h e laterodorsal m a r g i n of t h e olive, v e n t r a l spino-cerebellar fibres mixed w i t h others, lateral t o t h e former. T r a c t u s solitarius T h e c o r p u s r e s t i f o r m e is coming t o existence, it c o n t a i n s some fairly large nuclei. T h e r a d i x descendens. trigemini shows t h e i n t e r p o l a r p a r t of its nucleus. T h e r e is a large nucleus c u n e a t u s e x t e r n u s , t h e f u n i c u l u s of Goll is no longer p r e s e n t , t h e dorsal v a g u s a n d t h e hypoglossus nuclei keep t h e same site as more caudally. T h e lateral vestibulospinal t r a c t can be distinguished lateral to t h e v e n t r a l p a r t of t h e medial l o n g i t u d i n a l fascicle. Magnification x 5.

Bd. 5, Hefts 1962

ANATOMY O F T H E BRAIX STEM O F T H E E L E P H A N T

Legenda CE cr CIT drV 1)X ea FA FL IO lvs ml mlf oc p vsc

External cuneate nucleus Corpus restiforme Cuneate nucleus Radix descendens trigemini dorsal vagal nucleus External arcuate fibres Nucleus of the anterior funiculus within the medial longitudinal fascicle Nucleus of the lateral funiculus Inferior olive Lateral vestibulospinal t r a c t Medial lemniscus Medial longitudinal fascicle Olivo-cerebellar fibres Pyramid Ventral spinocerebellar t r a c t

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The posterior funiculi have lost the large majority of their fibres, they are occupied chiefly by their nuclei. There is a very large external cuneate nucleus located dorsal and medial to the trigeminal root whereas the internal cuneate nucleus, as already quoted, is continuous with the trigeminal root nucleus. Medio-ventral to the internal cuneate nucleus the solitary tract and its nucleus are located, and medial to the latter a relatively large dorsal vagal nucleus. The hypoglossal nucleus similarly is large but it does no longer show two separate portions. An intercalate nucleus cannot be distinguished. The medial longitudinal fascicle shows a number of large nuclei in its ventral third. They are called the anterior funiculus nuclei by B u s c h in accordance with L e w a n d o w s k y (1904) because they lie in the m. 1. f. the analogen of the sulcomarginal fascicle of the anterior funiculus of the cord. The circumscribed bundles of the lateral vestibulo-spinal tract along the lateral side of the ventral half of the m. 1. f. are more marked. The reticular formation dorsal and dorsolateral to the latter tract seems less densely filled with fibre-bundles than in lower levels, grey areas are more marked, very large ganglioncells occur especially in the area of the tract. Lateral to it there are also scattered ganglioncells but they are somewhat smaller.

The second block, consisting of 144 preparations, number 144 the most caudal, contains the upper medulla and the transition to the pons. In preparation 144, the pyramid exhibits no changes. The medial lemniscus along the raphe can be distinguished with certainly only dorsal to the ventro-medial accessory olive because the latter almost touches the raphe. The fibrepattern of the lemniscus can be followed dorsally up to about the dorsal margin of the dorsal accessory olive where the structure gets less compact and both very coarse and very thin fibres are more numerous. Apart from this it is found between the accessory and the main olive, and around the latter dorsally, laterally and latero-ventrally as well as between it and the ventromedial accessory olive. The fibrepattern is not exactly identical everywhere but it is very different from the surrounding white matter especially by very coarse and very fine fibres being extremely rare. The inferior olivary complex consists of a huge dorsal lamina of the main olive, a relatively large ventromedial accessory olive somewhat increased in size in a dorsomedial direction, an extremely tiny ventral lamina of the main olive and a dorsal accessory olive represented by a small horseshoe-shaped dorsomedial and a small slender band of cells dorsolateral to the main olive. Olivo-cerebellar fibres in longitudinally cut bundles in large numbers are seen, traversing the area of the rubrospinal tract and lying around the ventro-lateral side of the trigeminal root. Along the periphery lateral to the olive the medial lemniscus fibrepattern is found, but more latero-dorsally thick fibres increase in numbers, although a circumscribed ventral spino-cerebellar tract cannot be recognized. The dorsal tract neither is very distinct, as the great majority of its fibres have shifted dorsally into the restiforme body. The rubrospinal tract shows a much less compact structure than in more caudal levels, its bundles are separated especially by relatively thick bundles of olivo-cerebellar fibres. It appears to protrude mediodorsally as far as the medioventral tip of the trigeminal root. A few small groups of ganglioncells lie between its bundles and between it and the trigeminal root, similar groups are scattered in the ventrolateral part of the reticular formation between the olive and the rubrospinal tract. They presumably belong to the lateral funiculus nucleus. The restiform body is very large, although it has still not wholly shifted to the dorsolateral part of the medulla. Its lateral two thirds about consist predominantly of coarse fibres, those of the dorsal spino-cerebellar tract presumably. Medioventrally there are relatively thin olivo-cerebellar fibres and mediodorsally nucleo-cerebellar fibres from the lateral and anterior funiculi nuclei as depicted by Busch in the cat. The latter are much thicker than the former, they may be mixed with fibres from the external cuneate nucleus that lies along the mediodorsal portion of the corpus resti-

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forme. Lateral to the dorsal third of the trigeminal root a nucleus is located within the restiform body. Relatively thick fibres seemingly from the corpus restiforme traverse the trigeminal root to enter the tegmentum of the medulla, similar to arcuate fibres, possibly proceding to the medial lemniscus. This however cannot be evidenced without the help of experimental material. The trigeminal descending root contains some small areas in which very thin fibres prevail. Such fibres mostly myelinated also are seen within the nuclei possibly they are fibres nearing their termination. Its nucleus contains a number of gelatinous nuclei with small neurones besides many others with larger and some with fairly large cells. The posterior funiculi nuclei are restricted to a large external and a small internal cuneate nucleus. Medial to the latter the descending vestibular nucleus is visible, and medial to that nucleus the solitary tract and its nucleus. The dorsal vagal nucleus is very large and contains a great number of large neurones. The hypoglossus nucleus has got much less compact, it may consist chiefly of the preposite nucleus and possibly the nucleus of Roller. Still there are distinct rootbundles leaving it which can be traced along the lateral side of the m. 1. f. and the olive to their exit. The medial longitudinal fascicle contains a number of cellgroups, in its dorsal third they form a large nucleus, otherwise they are spread. The fibrepattern of the fascicle everywhere is the same, coarse and fine fibres being much more conspicuous than in the medial lemniscus. The reticular formation near the rubrospinal tract and the trigeminal root is very densely populated with fibrebundles in which thin fibres are prevalent. Large ganglioncells lie chiefly in the ventromedial part, in the area between the lateral vestibulo spinal tract, the m. 1. f. and the olives. In preparation 93 the pyramid contains few arcuate fibres, but in the midline still may longitudinally sectioned vertical fibres are present. The medial lemniscus no longer lies at the midline and between it and the raphe, loose fibre-bundles mixed with ganglioncells are located. I t actually lies between the ventral and the dorsal accessory olives and within the hilus of the main olive, with many olivo-cerebellar fibre-bundles traversing it. Within the olives there are numerous small round circumscribed bundles that resemble the lemniscus as to their fibrepattern. Around the main olive at the middle third of its lateral side, also a lemniscal pattern is found, whereas around the ventral third coarse fibres prevail very much. The inferior olivary complex consists of a very tiny medioventral accessory olive, a relatively large main and a slender dorsal accessory olive. The ventral lamina of the main olive is much narrower than formely, but it is longer and in a medio-dorsal direction extends to near the raphe. The dorsal lamina at this level lies in a ventrodorsal direction, it is less wide than in lower levels, but still much more so than in most mammals. Lateral to the olive the caudal pole of the facial nerve nucleus shows as a fairly compact group of large motor ganglioncells, with a dorsomedial extension of stray neurones over a relatively large area. The bundles of the rubrospinal tract are more widely spread than in more caudal levels, they lie partly between the facial nucleus cells partly lateral to them. Both are traversed by many olivo-cerebellar fibrebundles. Ventro-lateral to the rubrospinal bundles along the periphery many coarse fibres are to be seen, which in more rostral levels will prove to constitute the ventral spinocerebellar tract. In close proximity of the medioventral margin of the trigeminus descending root a subtrigeminal nucleus is present, in its ventral portion containing relatively large and in its dorsal small neurones. Rubrospinal tract bundles apparently do not enter it. The corpus restiforme has increased much in size, especially its medio-lateral dimension. A nucleus of moderate size lies in the centre of its ventral third, a small one along the trigeminal root, and another more dorsally. V e r y coarse fibres are very numerous in its dorsolateral quarter. Along the ventrolateral margin of the trigeminal root large numbers of small olivo-cerebellar fibres enter the corpus restiforme, they can be followed

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dorsally all over the lateral side of the root. At its dorsal end they merge with the somewhat thicker fibres of the dorsomedial part of the corpus restiforme. The trigeminal root is not conspicuously changed, its nucleus seems insignificant, but dorsomedial up to the ventral margin of the medial vestibular nucleus a fairly large nucleus lies, characterised by large numbers of dendrites filling the space between the perikaria almost entirely. Posterior funiculi nuclei are no more present. The vestibular nuclei consist of a large descending nucleus with very many distinct fibrebundles, lying medial to the dorsal quarter of the corpus restiforme, and dorsal to the trigeminal root, and medial to it the medial vestibular nucleus. I t is of smaller size than formerly, especially its

Fig. 3 (Medul. 2/78) The inferior olive is no longer present, the medial lemniscus lies laterodorsal to the pyramid, the medial longitudinal fascicle is small and lies far dorsal to the lemniscus. The nucleus nervi facialis is large, the rubrospinal tract lies chiefly within its lateral half. The lateral vestibulo-spinal tract lies in the centre of the tegmentum. The radix descendens trigemini lies far laterally accompanied by the pars oralis of its nucleus. The corpus restiforme is of large size, lateral to it parts of the cochlear nuclei are visible, and medial to its dorsal third the corpus iuxtarestiforme with the nucleus descendens and medialis vestibularis. The ventral spinocerebellar tract' lies lateroventral to the facial nerve nucleus. Magnification x 5.

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dorsoventral dimension, medially it emerges into the nucleus propositus hypoglossi. Subependymal small-celled nuclei are found dorsal to the medial vestibular nucleus and to the medial longitudinal fascicle, the latter in man called nucleus paramedianus dorsalis by Olszwesky and Baxter. The medial longitudinal fascicle ventrally can be fairly well circumscribed because it is of a more dense structure than the predorsal area. Within its dorsal half groups of ganglioncells are present, but areas of a special fibrepattern cannot be found. Within the raphe between the fascicles of both sides a fairly large number of small ganglioncells is to be seen. Ventral to them, within the praedorsal area and laterally very large ganglioncells are present. Tectospinal bundles with coarse fibres cannot be distinguished in the vicinity of the raphe.

Legenda cir CO cr drV 1)V lvs ml mlf XVII

corpus iuxtarestiforme nucleus cochlearis Corpus restiforme R a d i x descendens trigemini Xucleus descendens vestibularis Lateral vestibulospinal tract Medial lemniscus Medial longitudinal fascicle Xucleus nervi facialis

OI )V p rs vsc

Pars oralis of the radix descendens trigeminus nucleus Pyramid Rubrospinal tract Ventral spino-cerebellar tract

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The bundles of the lateral vestibulo-spinal tract lie at the same site as in the former preparation. Neither a medial nor a central tegmental tract, which could be expected because of the large size of the inferior olive, can be recognized. The first preparation rostral to the inferior olives to be described is preparation 78 (fig. 3). The pyramid shows no changes. The medial lemniscus apparently lies along the lateral two third of the pyramid, somewhat further from the midline than previously, and along the ventral periphery of the medulla lateral to the pyramid. The borderline between the pyramid and the lemniscus is distinct because of the marked difference in fibrepattern. The homogenecity of the area attributed to the medial lemniscus is not established as in the ventrolateral part along the medullary periphery coarse fibres are more numerous than elsewhere. However this part can not be sharply demarcated as it gradually changes into that of the more dorsal lemniscus. It occupies about the ventral quarter of the total area attributed to the lemniscus. The fibres all over the area are density packed and therefore it can be very distinctly circumscribed from the surrounding reticular formation. Whether the coarse-fibres ventral quarter constitutes a component from a special nucleus cannot be determined as experimental material cannot be produced. The facial nerve nucleus consists of several parts: the most medial an almost vertical rodshaped cell-group, the main part horseshoe-shaped with the hilus dorsally and the medial lamina much narrower than the lateral and the ventral curve. Within the hilus two other spindleshaped groups of neurones are located, with an other small group relatively far dorsal to the medial lamina. The nucleus area contains the majority of the rubro-spinal fibrebundles. The rest of the latter 'lie dorsal to the area, the most dorsal ones about halfway between the base of the medulla and the floor of the fourth ventricle. The subtrigeminal nucleus is still present, it is very small and consists of small neurones only. Between the 7th nerve nucleus, the ventromedial margin of the trigeminal root and the ventral periphery the ventral spino-cerebellar fibres can be recognized. The corpus restiforme is very large, division in areas of fibres of certain sizes is unfeasible because many fibres running in a horizontal direction are out longitudinally. The changed direction evidently is due to the spino-cerebellar fibres shifting from the lateral to the medial half of the body (Busch in the cat). Some small nuclei lie along the lateral margin of the trigeminal root. Lateroventral to the corpus restiforme the statoacoustical nerve enters the brain stem, some of its fibres pass along the ventral margin of the trigeminal root, to join the corpus trapezoideum not yet present in this slide. Parts of the ventral and the dorsal cochlear nuclei are seen along the ventrolateral and the dorsal margins of the corpus restiforme. The trigeminal root extends somewhat further mediodorsally, its ventral third has increased in width considerably. There are no nuclei within it and in general its fibrecontent seems not to differ from that in the former level. The nucleus consists of scattered large neurones and only two groups of small cells closely packed and the interspaces filled with dendrites, are present. One lies medial to the transition of the dorsal and the intermediate third of the root, the other medial to the mediodorsal tip. The descending vestibular nucleus is smaller than in more caudal levels, the medial nucleus has increased in size. At the site of the 12th nerve nucleus only spread small ganglioncells are present, representing the preposite nucleus. The medial longitudinal fascicle can be circumscribed ventrally because the praedorsal area is much less populated with fibrebundles. It does not contain any nucleus and areas of a special fibrepattern can not be distinguished. The lateral vestibulo-spinal tract bundles are unchanged. Within the praedorsal area special bundles of the tectospinal tract cannot be recognized large cells are spread in it, whereas in the raphe some groups of smaller cells are located.

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Next a number of more rostral preparations show the superior olives with the trapezoid body, the caudal part of the basal pontine nuclei, the rostral half of the facial nerve nucleus with the knee and the exit of the nerve, the more rostral vestibular nuclei with the origin of the deiterso-spinal tract, and the gradual change of the pyramid into the pedunculus cerebri. P r e p a r a t i o n 64 shows the pyramid and the medial lemniscus much the same as in the previous preparation. Dorsal to the medial third of the pyramid along the raphe, reticular ganglioncells and fibrebundles are conspicuous. The borderline between the pyramid and the lemniscus is sharp, there are no pyramidal bundles visible within the lemniscus. Lateral to the lemniscus a superior olive can not yet be identified, a tiny group of small cells only being present. The facial nerve nucleus lateral to this tiny nucleus shows a large vertical medial portion with at its dorsal end three irregular groups of neurones. Lateral to it lies the horseshoe-shaped lateral portion, of which the medial lamina extends into the groups of neurones at the dorsal end of the medial portion. The ventral and the lateral parts are wide, the lateral running almost vertically. From the nucleus many fibres originate which can be traced far dorsally but a knee is not yet present. The rubrospinal tract bundles mostly lie within the facial nerve nucleus area, a number of them lie dorsal to its lateral part. The trigeminal descending root lies lateral to the facial nucleus and medioventral to the corpus restiforme. A nucleus lies between its ventromedial margin and the former nucleus, its cells are much smaller than those of that nucleus and resemble those of some descending root nuclei. Ventromedial to it and ventrolateral to the facial nerve nucleus the area of the ventral spinocerebellar tract is located. Cochlear nuclei lie around the ventral, lateral and dorsal sides of the corpus restiforme. The vestibular nuclei consist of a small medial nucleus with small neurones and a distinct descending nucleus characterized by the many fibrebundles mixed with its neurones. From them fibres proceed medially and probably cross near the floor of the ventricle; vestibulo-spinal fibres travelling in the medial longitudinal fascicle. Within the predorsal area large ganglioncells are found, but special bundles containing thick fibres to be recognized as tectal bundles are not visible. The lateral vestibulo-spinal tract lies somewhat more dorsally than before. P r e p a r a t i o n 50 shows a very distinct superior olive, consisting of one rodshaped nucleus with many relatively small cells, lying parallel to and very near the medial portion of the facial nerve nucleus. The trapezoid body between the olives of both sides is only small. The facial nerve nucleus fibres can be followed into the large genu from which bundles penetrate the trigeminal descending root and emerge at its lateral side. The knee is very wide and extends from the medial vestibular nucleus to very near the medial longitudinal fascicle. Deiter's nucleus large neurons lie spread between the bundles of the vestibular descending tract. Medial to them the medial nucleus is still present it is not very distinct as its neurones are small and scarce. Abducens nerve root fibres pass through the lateral part of the medial lemniscus to their exit. Many transverse, longitudinally cut fibrebundles pass through the pyramid, presumably belonging to the corpus trapezoideum. Between the lemniscus and the facial nerve nucleus the superior olive is conspicuous, it consists of a double layer of cells ventrally connected, lying parallel to the medial side of the facial nerve nucleus. Medial to it a small nucleus of which the neurones are somewhat larger may represent the trapezoid body nucleus, it however is very small relative the superior olive. The facial nerve nucleus consists of a medial part, ventrally very narrow but dorsally ending in a large group of cells, with another small group medial to its dorsal end, and the lateral portion horseshoe-shaped of which the medial lamina extends much further dorsally than the lateral. Within the hilus lie two more groups of neurones.

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Fig. 4 (Medul. 2/31) Caudal end of the pons The pyramid increases in size, it is traversed by trapezoid body fibres, the medial lemniscus lies at its dorsal and lateral sides. Lateral to the lemniscus lies the superior olive and lateral to the latter the huge facial nerve nucleus. The rubospinal t r a c t lies within its lateral half- and more dorsally. The facial nerve root fibres proceed dorsally to the genu and from there the outgoing root bundles pass ventrolaterally along the trigeminal nucleus. The abducens nerve nucleus lies ventral to the genu of the facial nerve. The ventral spino-cerebellar t r a c t ventrolateral to the facial nucleus is obliterated by the trapezoid body fibres traversing it. The radix descendens trigemini is only partly visible its oral nucleus extends relatively far dorsomedially. Dorsal to it the lateral and medial vestibular nuclei are located, with the corpus iuxtarestiforme. The reticular formation shows no special features. Magnification x 5.

ANATOMY OK T H E B R A I N STEM OK T H E

ELEPHANT

I .egenda be cjr ct tlrV g VII LY MY ml N VI N YII XP

Brachium pontis Corpus iuxtarestiforme Fibres of the corpus trapezoideum Radix descendens trigemini Genu n. facialis Lateral vestibular nucleus of Deiters Medial vestibular nucleus Medial lemniscus Nucleus nervi abducentis Nucleus nervi facialis Nuclei pontis basalis

V o t f t , H i n i i o r s c h u n g , B t l . 5, H e f t G

ODY P rf rs SO VII

Pars oralis of the trigeminal nucleus Pyramid Reticular formation Rubrospinal tract Superior olive Xervus facialis outgoing root

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Rootfibres in great numbers can be traced to the knee, from where large bundles pass ventrally to the most dorsal trigeminal root nuclei and subsequently traverse that root to their exit lateral to it. Other bundles do not traverse the root but join the facial nerve at its exit, ventral to the trigeminal root and medial to the brachium pontis. Deiters nucleus is very marked because of its large neurones, which extend almost as far medially as to reach the facial nerve knee. The medial vestibular nucleus however is indistinct. Ventromedially along the facial nerve knee a few cells are present which in more rostral levels prove to belong to the abducens nerve nucleus. Because of the large size of the knee they lie more ventrally than in most mammals. As will appear in the following preparations it is only small. P r e p a r a t i o n 31 (fig. 4) shows the basal pontine nuclei increasing in size, the pyramid also gets somewhat larger, it is traversed by many small bundles of coarse fibres that cross and presumably are derived from the superior olive, the trapezoid body nucleus and the cochlear nuclei. The medial lemniscus is very conspicuous, it lies over the dorsal side, the most medial quarter excepted and the lateral side of the pyramid, separated from the latter by a basal pontine nucleus. It is of a compact texture all over its surface area, but in its ventral part lateral to the pyramid thick fibres are most numerous. Anything like a borderline or an area of different texture however is not present between these two parts. Within the ventral part a few pyramidalbundles are located. Medial and mediodorsal to the pyramid the reticular formation contains a moderately great number of large neurones. Lateral to the ventral part of the medial-lemniscus the trapezoid body nucleus lies and lateral to it the superior olive. The former is small but its neurones are larger than those of the olive; ventral and ventrolateral to the olive similar cells are found even in larger numbers. The superior olivary nucleus is rodshaped, it lies parallel and very near to the medial portion of the facial nerve nucleus, it cells are small, the dendrital plexus is dense. Abducens nerve fibres run between the trapezoid body nucleus and the lemniscus. The facial nerve nucleus still is very large, it presents a slender medial portion that dorsally fuses with the dorsal portion. The latter extends very far dorsally and has a lateral extension dorsal to the medial lamina of the lateral horseshoe-shaped portion. This latter portion does not extend very far dorsally, its ventral part is only small, but a group of cells in its hilus dorsally fuses with the dorsal portion. Many rootfibres proceed dorsomedially to the genu, and from the genu a strong bundle travels through the nucleus of the trigeminus descending root to leave the brainstem along the medio-ventral margin of the root. The brachium pontis is of huge dimensions it had to be cut about in the middle for technical reasons, in its ventral part fibres are tiny. The corpus restiforme contains a large number of very coarse fibres. The trigeminus descending root is large and dorsally it reaches Deiters' nucleus, it is accompanied by a fairly large nucleus. The vestibular nuclei complex consists almost entirely of Deiters' nucleus, which at this level contains neurones, as large as those of the facial nerve nucleus. In its lateral half only they lie between fibrebundles. From them the coarse fibres of the lateral vestibulo-spinal tract arise andbegin to proceed ventromedially. The abducens nerve nucleus lies along the medio-ventral side of the facial nerve knee it is only small and contains a moderate number of neurones. The medial longitudinal fascicle is relatively small, the praedorsal area ventral to it is very poorly provided with bundles containing coarse fibres and pretectal bundles cannot be recognized, but large neurones are relatively numerous. P r e p a r a t i o n 21 shows the basal pontine nuclei getting larger, but the pyramid still lies entirely dorsal to them. Dorsal and dorsolateral to it, the medial lemniscus is densely packed with fibres all over its surface area. Aberrant pyramidal bundles lie in its medioventral part. Nuclei are found at several places between the pyramid and the lemniscus.

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The superior olive surrounded by lateral lemniscus fibres lies between the medial lemniscus and the 7th nerve nucleus. The lateral lemniscus can be distinguished from the medial at this level by its more irregular fibre-pattern. The facial nerve nucleus has decreased in size, especially its medio-lateral dimension. I t consists of a relatively small mediodorsal portion, the lateral horseshoe-shaped portion and a tiny group of large ganglioncells much more dorsally. Numerous fibrebundles proceed dorsally to the knee. The rubrospinal tract bundles are spread over the ventral and the lateral part of the horseshoe-shaped portion and relatively far dorsally outside it along the medial margin of the facial nerve outgoing root. The ventral spino-cerebellar tract can be distinguished ventrolateral to the facial nerve nucleus, it contains a relatively large contingent of coarse fibres. Lateral to the facial nerve nucleus the trigeminal descending root dorsally deviates somewhat to lateral, lateral to it the huge brachium pontis with an immense number of fibres is present. The nucleus of the trigeminal root is very large. Dorsal to it vestibular nuclei cells lie scattered between fibrebundles but large cells are very rare. The facial nerve knee is very wide, an abducens nerve nucleus can no more be distinguished, only three scattered cells being found ventromedial to the knee, a small-celled subependymal nucleus lies along its medial side. The medial longitudinal fascicle is not very distinct, neither are the predorsal area fascicles. Thick fibres however are fairly numerous in the dorsomedial part of the tegmentum, medial to the 7th nerve knee, the area near the raphe excepted. In general the tegmentum is crowded with myelinated fibres, nerve-cells are rare and real grey areas are only found along the pyramid, and the ventral half of the tegmental raphe. A small subependymal nucleus lies near the raphe. An area resembling the human central tegmental tract with regard to its homogeneous fibrepattern cannot be found within the tegmentum. Within the basal pontine nuclei pyramidal fibre-bundles cannot be distinguished, but as the pedunculus cerebri during its course along the nuclei caudally decreases considerably in size as will appear later, it may be assumed that large numbers of fibres enter them to their termination. P r e p a r a t i o n 6. The pedunculus cerebri at this level is somewhat larger especially its ventro-dorsal dimension. I t is sharply demarcated because of its large contingent of fine fibres, it is the only tract lacking fibres over 8 in diameter. Below this size however fibres of all calibers are present those between 3—6 (i being most numerous. The fibrepattern all over its surface area apparently identical, is the same as found in the pyramid in preparation 144 of the medulla second-block. The dorsal part is still traversed by trapezoid body fibres, within the medial part some groups of ganglioncells are spread. The supsrior olive is still found, it is surrounded by a much more conspicuous lateral lemniscus than in lower levels. The 7th nerve nucleus lateral to the lateral lem-' niscus is much smaller than previously and consists of a medial part only. However very much more dorsally very near the genu another group of motor neurones lies. As its neurones are identical to those of the nucleus described and fibres from it course to the genu, it may be assumed to belong to this nucleus. Rubrospinal tract fibre-bundles lie spread over the main facial nerve nucleus, the basal pontine nuclei lateral to it and the area between it and the dorsal group, the most dorsal bundles lying very close to the facial nerve knee. Ventrolateral to the 7th nerve nucleus the ventral spino-cerebellar tract is fairly circumscribed; at its medial ventral and lateral sides it is bordered by basal pontine nuclei and dorsally by the rubrospinal tract area; The trigeminal descending root deviates laterally, evidently because it nears the levels where the nerve enters the pons. I t proves to contain a larger contingent of medium-sized fibres than in more caudal levels. Medial to the root only a nucleus consisting of scattered large neurones is visible but more dorsally over a wide area, nuclei similar to those described in more caudal areas are seen. Dorsal to the nucleus spread neurones are present, some large, the majority of medium size, they are neither numerous nor form a compact nucleus, and presumably represent the superior vestibular nucleus of v. Bechterew. Medial to them the 7th nerve genu is very much reducted in size, a small number of outgoing fibres lie ventrolateral to it. 33*

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Within the dorsomedial quarter of the tegmentum most bundles contain a number of very coarse fibres, but they neither form a compact tract nor lie close to the raphe and a distinct medial longitudinal fascicle cannot be identified. Within the reticular formation groups of 'neurones are not found, except along the ventral 2/3 of the tegmental raphe and dorsal to the medial third of the cerebral peduncle. The tegmentum similar to lower levels is crowded with myelinated fibres, straylarge ganglioncells occur in its centre whereas the fibrebundles are separated by longitudinally cut fibres of various sizes, the majority transversely running coarse ones.

Fig. 5 (Pons 1/10) Pons, trigeminal entrance level. The cerebral peduncle lies at the dorsal border of the basal pontine nuclei, dorsal and lateral to it the medial lemniscus. Lateral to the latter lies the lateral lemniscus and more lateral the subrospinal, and ventral spino-cerebellar tracts and the motor trigeminus root. The motor nucleus lies more dorsally with part of the rubrospinal tract in its medioventral part. The nucleus princeps sensibilis is very large, and dorsally reaches to the lateral wall of the ventricle. Medial to the dorsal part the mesencephalic root lies near the ventricle and lateral to it the ventral part of the brachium conjunctivum. Possibly medial to the medial lemniscus the medial tegmental tract can be distinguished. Except nuclei along the raphe and around the cerebral peduncle no distinct elements can be identified in the medial half of the tegmentum. Magnification x f>.

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The next proximal preparation is number 'iO (fig. 5) of the lower pontine block I. Preparation 4(5 is the most caudal of this block, but preparation 40 is complete and better suited for a description. Lt shows the pons and the fourth ventricle with the brachium conjunctivum along its lateral wall. The most conspicuous formation is the trigeminal nuclear complex with the entering sensory root. The pedunculus cerebri is somewhat larger than in the level previously described, it lies at the medioventral corner of the tegmentum, and shows no signs of splitting in separate bundles t h a t enter the basal pontine nuclei. Similar to the low pontine levels it is a compact body of white matter

Legenda be bp cp 11 lm mt MV mV l'Y

Brachium conjunctivum Brachium pontis I'edunculus cerebri Lemniscus lateralis Lemniscus medialis Medial tegmental tract Motor trigeminal nucleus Mesencephalic root of the trigeminus nerve Nucleus sensibles princeps trigemini

rs V vsc

Rubrospinal tract Nervus trigeminus Ventral spino-cerebellar tract

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only in its ventrolateral and medial parts showing narrow areas of grey matter. The medial lemniscus seemingly is going to divide in a ventrolateral and a dorsomedial portion, with a less compact area in between part. The former lies between the cerebral peduncle and the lateral lemniscus, the latter dorsal to the lateral 2/3 of the peduncle. Nuclei are scattered between the peduncle and the lemniscus and around the medial part of the latter. The lateral lemniscus is more sharply circumscribed than formerly, it contains two small nuclei in its centre. Lateral to it lie some basal pontine nuclei with the ventral spino-cerebellar tract and dorsal to them the rubrospinal tract bundles, which spread very far dorsomedially along the medial side of the motor trigeminal nucleus as will appear presently. More laterally the motor trigeminal root runs from very dorsal ventrally till about the ventral spino-cerebellar tract. A small motor neuronal group lies in its most dorsal part and a much larger medial to it, the latter lying along the mediodorsal margin of the rubrospinal tract bundles. Lateral to the motor root an immense main sensory nucleus stretches ventrodorsally from the ventral spino-cerebellar tract area to the lateral wall of the fourth ventricle. To all probability its largest cross surface area lies between the blocks, rostrally it fairly rapidly decreases in size. Along the lateral wall of the ventricle it lies between the brachium conjunctivum and the trigeminal mesencephalic root. Its consists of a large and wide ventral and a much smaller dorsal portion. Most of the ganglioncells of both portions lie in more or less well circumscribed clusters and only relatively few lie scattered over its area. The latter are fairly large but those in the clusters are almost as large. Medially the cells are somewhat smaller than laterally and in the dorsal portions some of the cellgroups contain still smaller cells. Between the cells in all clusters dendrites are conspicuous, only in some they form a dense network. The dorsal part within its area and around it contains a large number of thin fibres about 2 — 3 in diameter all of them of about the same size. The trigeminal descending root has decreased very much in width. The mesencephalic root consists only of a small number of coarse fibres partly lying near the ventricular floor partly near the motor trigeminal root. As will appear, in more rostral levels it contains a much larger number of fibres, therefore at the present level most of them have already joined the motor root. Coarse fibres are relatively numerous in the lateral half of the medio-dorsal quarter of the tegmentum, they lie in bundles mixed with finer fibres. More medially they get much less numerous and in the vicinity of the raphe a circumscribed medial longitudinal fascicle cannot be recognized. Dorsal to the raphe underneath the ependym a number of coarse fibres cross, but neither their origin nor their destination can be determined. Predorsal fibrebundles containing some very coarse fibres can be found within the medial part of the tegmentum although not very near the raphe, they do not form a distinct area as they lie relatively far apart. As already stated medial to the medial lemniscus near the raphe there are fairly large white areas, distinguishable from the lemniscus by their larger contingent of fine fibres. Especially the most ventral area consists nearly only of fibres about 2 [i in diameter and therefore resembles the medial tegmental fascicle of other animals somewhat. Nuclei within the tegmentum besides those of the trigeminal and lateral lemniscus systems are more numerous than in lower levels. Along the raphe they are relatively large and extend from the subependymal coarse crossing fibres to the pes. There are fairly large nuclei around the medial and ventral margin of the medial lemniscus, and in the centre of the tegmentum small grey areas between the reticular formation fibre-bundles are much more numerous than in previously described levels. In the raphe many coarse fibres running a vertical course are cut longitudinally, they may be those apparently crossing near the ependym, possibly originating in reticular paramedian nuclei and proceeding to the cerebellum by way of the brachium pontis. P r e p a r a t i o n 4, at the rostral end of block pons 1, represents a supratrigeminal level. The psdunculus cerebri is not appriciably changed some increase in size excepted,

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the medial lemniscus lateral part is fairly conspicuous at the lateral side of the peduncle. Contrarily the mediodorsal part is not very distinct, and its continuity with the lateral part is vague. In the tegmentum, the only compact mass of fibres presenting the fibrepattern of the lemniscus of lower levels lies dorsal to the'middle third of the peduncle. I t however cannot be circumscribed nor can it be estimated how much of it belongs to the medial lemniscus. The .lateral lemniscus is fairly distinct, it has not changed its side along the lateral side of the lateral portion of the medial lemniscus. In its centre and dorsal portion some nuclei are present, dorsal to them the lemniscus apparently has its border as its fibrestructure is no longer found. The ventral spinocerebellar tract is much smaller, coarse fibres are seen to proceed dorsally along the ventro-lateral and lateral sides of the brachium conjunctivum. The rubrospinal tract bundles are restricted to an area at the dorsolateral side of the centre of the tegmentum much smaller than in lower levels. Lateral to it a small motor trigeminal nucleus is still present, its neurones are rare and relatively wide apart. Laterodorsal to this nucleus the dorso-rostral tip of the main sensory nucleus is located, it contains some small groups of neurones, surrounded by a large number of fibres about 2 — 3 [x in diameter. I t lies between the medial side of the ventral third of the brachium conjunctivum and the trigeminus mesencephalic root. The latter is much larger than in lower levels, it lies very near the lateroventral wall of the fourth ventricle and consists chiefly of very coarse fibres. Within the area of the main sensory nucleus and its fibresystem some bundles' of fine fibres, partly cut obliquely, can be distinguished, their fibrepattern resembling that of the pedunculus cerebri. Possibly as will become somewhat more evident in more rostral levels, they belong to the cortico-tegmental tract described by B a g l e y in 1922. In the mediodorsal quarter of the tegmentum the bundles containing relatively many very coarse fibres lie somewhat more ventrally than in the more caudal preparations, probable tectospinal bundles lie more medially, their coarse-fibre content is less. In the dorsomedial part a number of fibrebundles are assembled, which might be considered as the medial longitudinal fascicle. Along the raphe a relatively wide zone is occupied by nuclei, which stretch from the floor of the ventricle to the pedunculus cerebri. In the vicinity of the latter they are wider than near the ventricle. Other nuclei lie along the dorsal margin of the peduncle, within and around the medial lemniscus and within the white matter in the ventral part of the dorsomedial quart of the tegmentum. A small number of very large ganglioncells are spread over the area dorsal to the lateral portion of the medial and to the lateral lemniscus. The next block pons 2 runs from 1 to 41 caudo-rostrally. The first preparation representing the whole tegmentum is preparation 4, in which the brachium conjunctivum has shifted somewhat ventrally, without sending fibres in a medial direction to their crossing (fig. 6). The cerebral peduncle has increased somewhat in width (ventrodorsal diameter), the lateral portion of the medial lemniscus and the lateral lemniscus have shifted laterodorsally over some distance the former however still lying besides the cerebral peduncle. The dorsomedial portion of the medial lemniscus cannot be difined with some degree of certainly. Dorsal to the medial half of the cerebral peduncle a number of large nuclei divide the white matter in separate portions and dorsal to the lateral quarter of the peduncle many small nuclear areas do the same. Compact areas of a more or less typical ...fibrepattern evidently are hard to recognize therefore. The lateral part of the medial lemniscus and the lateral lemniscus lie side by side, they differ somewhat in fibrepattern as the lateral contains more fine fibres and the size of its most coarse fibres exceeds that in the medial lemniscus somewhat. The lateral lemniscus contains a relatively large nucleus in its dorsomedial part. Dorsolateral to the lateral lemniscus ventral spino-cerebellar tract fibres can be found in a small number, other similarly coarse fibres proceed dorsally along the lateral side of the brachium "conjunctivum.

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Fig. 6 (I'011s 2/',) High pontine level, the cerebral peduncle has increased in size, the medial lemniscus medial part is indistinct, the lateral part lies laterodorsal to the peduncle, the lateral lem niscus laterodorsal to the medial. The ventral spino-cerebellar t r a c t shifts dorsally along the lateral side of the brachium conjunctivnni. The rubrospinal tract shows a latcromedial arrangement in stead of a ventrodorsal, it lies fairly far dorsally, medial to the brachium conjunctivum. Dorsal to it lie the fibres from the dorsal part of the nucleus princcps and the mesencephalic trigeminal root. The brachium conjunctivum is of large size. Magnification >< ;">.

479

Legenda be bp cp 11

Brachium conjunctivum Brachium pontis Cerebral peduncle ^Lateral lemniscus

lm mV rs vsc

Medial lemniscus Mesencephalic trigeminal root Rubrospinal tract Ventral spinocerebellar tract

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The brachium conjunctivum shows as a huge tract at the laterodorsal aspect of the tegmentum, its dorsal half lying along the lateral side of the ventricle. Its fibre-content cannot be very exactly studied because the dorsal part is not well stained. Coarse fibres within the ventral half seem to be more numerous than within the dorsal half. At its lateral and dorsal sides accumulation of mostly very thick fibres are present, presumably ventral spino-cerebellar fibres and Russel's tract fibres. Along its medial side also a cluster of very thick fibres is visible, at about the site of the ventral spinocerebellar tract. The rubrospinal tract bundles medial to the ventral tip of the brachium conjunctivum lie spread over an oval area, with its longer axis in an almost horizontal direction. They lie relatively close to each other, though not so close as to form a compact tract, as they are separated only by some small bundles of foreign fibres, mostly longitudinally cut. Dorsal to the rubrospinal tract, fibres from the dorsorostral main sensory trigeminal nucleus, similar to Wallenberg's dorsal trigeminal tract fibres are distinguishable. As will appear in more rostral levels, they cannot be followed very far rostrally and therefore they cannot be identified with Wallenberg's tract. In the same area, especially its ventral two third, scattered bundles of fine fibres are present, probably belonging to Bagley's cortico-tegmental tract. Medial to this area the trigeminal mesencephalic root is very conspicuous, its fibres are more numerous and lie more close to each other than in more caudal levels. The bundles with a high contingent of very coarse fibres of the mediodorsal tegmentum of the previous preparation in the present level have shifted laterally and lie somewhat more apart. Medial longitudinal fascicle bundles are somewhat more distinct, predorsal tectospinal tract bundles lie spread lateral to the raphal nuclei in the dorsal half of the tegmentum. The large nuclei along the raphe have already been mentioned, around the medial longitudinal fascicle bundles similar nuclei are lying. Large reticular formation ganglioncells are especially found in the centre of the tegmentum, medial to the rubrospinal tract, but they lie far apart and are only scanty. In the vicinity of the raphe several areas of small fibres can be distinguished but they also contain some less small ones and identifying them with Ogawa's medial tegmental tract is speculative. P r e p a r a t i o n 41 is the most rostral of this block to be examined. I t shows the brachium conjunctivum ventral portion going to split up in bundles and to proceed medially to its crossing. The cerebral peduncle has much increased in size, especially its medio-lateral dimension being much larger. I t as yet covers most of the pes pontis, stretching from the raphe almost to the brachium pontis. I t can be fairly sharply delineated over most of its border because of its characteristic fibrepattern, that apparently is the same all over its surface area. Along the medial half of its dorsal side there are large nuclei, which continue mediodorsally in those along the raphe of the tegmentum. Along the lateral half the nuclei are much smaller with many fibre-bundles in between. These bundles mostly show the same fibrepattern as the cerebral peduncle, although dorsally thick fibres gradually increase a little in number. A mediodorsal portion of the medial lemniscus can not be defined with certainly. The lateral portion of the medial lemniscus similar to the previous level, lies along the dorsalateral tip of the cerebral peduncle, but as the peduncle has distended much in a lateral direction it lies very near the lateral periphery of the brain stem, only a relatively narrow dorsal extension of the brachium pontis lying more laterally. I t is fairly well circumscribed, within its ventral tip some small nuclei are located, otherwise its structure is compact and its fibrepattern is identical to that in lower levels. The lateral lemniscus has shifted dorsally and lies between the lateroventral margin of the tegmentum, and the ventral quarter of the brachium conjunctivum, it contains a nucleus in its centre. The rubrospinal tract lies along the medial side of the brachium conjunctivum, relatively far dorsally, its most lateral bundles intermingle with the fibres of the latter.

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Medial to it about in the centre of the tegmentum the bundles with a large contingent of very coarse fibres are located. Dorsal to them the area of Wallenberg's dorsal secundary trigeminal tract does not contain many typical fibres any more, and apparently the main sensory nucleus fibres have either left it. Bundles of mainly thin fibres similar to the cerebral peduncles are fairly numerous in this area. Contrary to the previous preparation they are cut perpendicularly and their fibrepattern appear to resemble that of the cerebral peduncle very closely. The mesencephalic root of the trigeminal nerve lies along the lateral third of the central grey matter, and medial to it in a nucleus with relatively large pigmented cells represents the locus coeruleus. The brachium conjunctivum is present over whole its extend, as already mentioned its ventral third is going to split up in separate bundles and to shift medially, but its dorsal 2/3 is still very compact, its fibre-content can be seen to be enormous. I t appears as if very coarse fibres are much more numerous in the ventral than in the dorsal half. The ventral spino-cerebellar tract can no more be found. The medial longitudinal fascicle is somewhat more marked than previously, it contains a number of bundles provided with some coarse fibres, but similar bundles also lie more laterally along the ventral border of the central grey matter. Ventral to the fascicle, bundles with mostly fine fibres can be seen, but they do not form a more or less compact, circumscribed tract. Tectospinal fibrebundles within the praedorsal area are very indistinct, possibly they lie more laterally than in most mammals. Underneath the ependym very coarse fibres cross, laterally they can be followed to the bundles along the ventral border of the central grey matter. The nuclei along the raphe in this level do not reach the dorsal third, more ventrally they are large and can be continued around the medial and the ventral margin of the possible medial portion of the medial lemniscus, dorsal to the medial half of the cerebral peduncle. Large neurones are especially found scattered over the ventrolateral quarter of the tegmentum, and somewhat less in quantity over the dorsolateral quarter. There is a small-celled nucleus within the medial third of the central grey matter dorsal to the medial longidutinal fascicle. In the dorsomedial quarter of the tegmentum small grey areas lie between the fibrebundles. The next block is numbered pons 3, it is represented by a series of 40 preparations of which the lower numbers are the more caudal. The first preparation examined is number 4 (fig. 7). I t lies at the rostral end of the pons, the basal pontine nuclei are much reduced in size, within the tegmentum the brachium conjunctivum has shifted very much medially, especially its ventral half. The tegmentum medial to it therefore is small, the area lateral to it is much larger than formerly. The pedunculus cerebri has increased in size especially its ventrodorsal dimension which amounts to about 2/3 of its mediolateral dimension. Its shape is nearing that of the mesencephalic pedunculus, as will appear presently. Dorsal to its medial half the tegmentum is composed of bundles of the brachium conjunctivum, of the medial portion of the medial lemniscus and the normal reticular formation part. The brachium conjunctivum fibrebundles can be distinguished by their large content of coarse fibres obliquely sectioned, and the medial lemniscus is represented by bundles in fibrecontent resembling the lemniscus in lower levels, lying between the former bundles. Dorsal to the lateral half of the pedunculus cerebri dorsomedially bordered by the brachium conjunctivum lies a fibre-field which contains lemniscus medialis and pedunculus cerebri bundles intermingled. Laterally it fuses with the lateral portion of the medial lemniscus, which shows the same fibre-pattern as in lower levels. The latter portion laterally lies along the lateral wall of the tegmentum, ventrally it apparently fuses with the lateral sixth of the peduncle, from which it cannot be distinguished in a low power magnification. B y their different fibrepattern however both fibre-systems can be sharply delineated. Dorsal to it the lateral lemniscus lies between the brachium conjunctivum and the lateral wall of the tegmentum, medially intermingling with bundles of the former and with apparent pedunculus cerebri bundles that are found there much dorsal. Its structure is less compact than formerly, but only small groups of small neurones and tiny grey areas lie

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Fig. 7 (Pons 3/4) Most rostral pontine level with'begin of decussation of the brachium conjunctivum. The cerebral peduncle has much increased in size, and lies more laterally, ventrally surrounded by basal pontine nuclei and ponto-cerebellar fibrebundles. The medial lemniscus lies along the lateral margin of the brain stem apparently continuous with the peduncle and the lateral lies dorsal to it. The medial part of the medial lemniscus is indistinct and lies within the crossing brachium conjunctivum bundles. The medial longitudinal fascicle again is very distinct, its ventral portion consists of a relatively large area of fibres of about equal size. Both nuclei of von Gudden are present at the lateral side of the fascicle with the nucleus of the trochlear nerve, and medially the nucleus centralis superior along the raphe. The brachium conjunctivum is very large, the fibres of its ventral part proceeding to their decussation, fill the ventromedial quarter of the tegmentum. The mesencaphalic root of the trigeminal nerve is more distinct than in more caudal levels. The ruborspirial tract is partly enveloped by brachium conjunctivum fibres running medioventrally. Magnification x 5.

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Legenda be bp cp CS ])G n ml mlf mV N IV PG

Brachium conjunctivum Brachium pontis Cerebral peduncle Nucleus centralis superior of von Bechterew Nucleus dorsalis tegmenti of von Gudden Lateral lemniscus Medial lemniscus Medial longitudinal fascicle Mesencaphalic trigeminal root Nucleus nervi trochleari Nucleus dorsalis tegmenti profundus of von Gudden

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between its bundles. Dorsal to it a relatively large nucleus is visible but only few of its fibres seem to enter it. The rubrospinal tract bundles lie along the medial side of the brachium conjunctivum with about one third intermingling with the latter. Similar to the previous level the tract lies relatively far dorsally much more so than in the cat and the monkey. The area medial to the dorsal third of the brachium conjunctivum does not show any appreciable rest of the dorsal secundary trigeminal tract but it contains a relatively large number of finefibred bundles. Such bundles similarly are found more medially till more than halfway the distance between the brachium conjunctivum and the raphe. The trigeminus mesencephalic root is relatively large, it is well circumscribed, and consists chiefly of very thick fibres, although especially in its medioventral part numbers of very fine fibres also occur. Medial to it in the central grey matter and ventral to it pigmented and non-pigmented large neurones lie spread, pigmented cells ventrally being much less frequent than dorsally. The central grey matter is larger than in more caudal levels, especially its dorsoventral dimension has increased. Within its medio-ventral quarter a nucleus with moderately large cells can be seen. Ventral to it large motor cells lie partly in the central grey matter partly between the lateral bundles of the medial longitudinal fascicle, presumably the 4th nerve nucleus. In the adjacent tegmentum ventral and ventrolateral to it another relatively large nucleus lies between the reticular formation fibrebundles, with that in the central grey matter representing the nuclus dorsalis tegmenti and dorsalis tegmenti profundus of von Gudden. Ventral to the latter fibrebundles are very numerous, some of them contain a few coarse fibres, especially at the ventrolateral side of the nucleus. The medial longitudinal fascicle is much more marked than in lower levels, coarse fibres are found predominantly in its dorsomedial part. Ventral to it a large body of fibrebundles lies along the lateral side of the raphe nuclei, it does not contain very large fibres. The fibrepattern is not as homogeneous as that of the medial tegmental tract in the cat, and the goat, but it is more so than that of surrounding fibrebundles. The raphal nucleus is very large, and extends all over the length of the tegmental raphe, in its ventral third neurones are more numerous and larger than in the dorsal parts. The latter resemble the basal pontine nuclei neurones very much and may represent the nucleus reticularis tegmenti pontis of von Bechterew. Crossing fibrebundles of the brachium conjunctivum traverse this nucleus. Stray very large reticular formation neurones are located predominantly in the ventrolateral quarter of the tegmentum, between the deep von Gudden nucleus and the brachium conjunctivum and somewhat more ventrally. The large nuclei dorsal to the medial quarter of the penduculus cerebri have already been mentioned, smaller ones lie along its intermediate third, but otherwise reticular grey matter is rare.

The next preparation deserving an elaborate description is preparation 39 (fig. 8) of the same pons 3 block. It offers a different aspect, as the basal pontine nuclei are very much smaller and most of the brachium conjunctivum is engaged in its crossing. Lateral to the brachium conjunctivum a large area of mostly grey matter has been formed. Before describing this preparation in detail some features of intermediary levels have to be reported, because they are no longer present in the present preparation. In preparation 14 both von Gudden nuclei and the 4th nerve nucleus are more marked than in 4, but in 17 they already are much reduced in size. The nucleus centralis superior of von Bechterew lying within the raphe between the predorsal areas of both sides already indicated in prep. 4 gets much more distinct in prep. 14 and 26 but in 39 it is no longer found. The lateral lemniscus in prep. 26 shows a relatively large nucleus in its dorsal half, which

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lies halfway the lateral side of the tegmentum but not so far dorsally as the inferior colliculus in the cat. It should however be emphasized that in the cat also the lemniscus terminates at its ventral border and can no more be distinguished in more dorsal areas. In prep. 39 this nucleus is not nearly so marked and the lateral lemniscus has much decreased in size. The cerebral peduncle in prep. 39 is no longer surrounded by basal pontine nuclei and brachium pontis fibre-bundles, and only medially they are still noticiable. I t is very large and has the shape of a half-circle with its dorsal margin as the 'middle-line. At first sight it looks like one body of fibres but the lateral portion of the medial lemniscus forms its dorsolateral tip. Within its medial and its dorsal parts, relatively close to the border there are some moderately large nuclei. Medial to it lies a basal pontine nucleus and medial to that nucleus the interpeduncular nucleus. Dorsal to the medial half of the peduncle some stray neurones are seen which may belong to the compact part of the substantia nigra because they are fairly large, but they do not contain pigment. The lateral portion of the medial lemniscus by its fibre-pattern can be distinguished at the dorsolateral tip of the peduncle, the border is sufficiently sharp although peduncular bundles lie within the medial lemniscus area. Medial to the area of this lemniscus portion and dorsal to the peduncle over a fairly large area between obliquely cut brachium conjunctivum bundles, others are noted that resemble the medial lemniscus very much in fibre-pattern. This area can be followed medially to about the lateral border of the medial quarter of the tegmentum. In its lateral half pedunculus cerebri bundles also occur, but in its medial half they seem to be absent. The lateral lemniscus apparently has lost the majority of its fibres, presumably in the nuclei of the lateral aspect of the tegmentum. Dorsal to it fourth nerve bundles course laterally. The rubrospinal tract seemingly is no longer found, but close scrutiny reveals its bundles between those of the brachium conjunctivum about in the centre of the tegmentum. They lie in an area dorsal to the lateral three quarters of that of the presumable medial portion of the medial lemniscus, they are so conspicuous because of their coarse fibres that there can be no doubt about their identity. In the lateral third of their area some pedunculus cerebri bundles also can be recognized. Within the lateral half of the dorsal quarter of the brachium conjunctivum such bundles also are present differing only by a very small contingent of somewhat thicker ones. They occur medially as far as the border of the central grey matter. They may belong to Bagley's cortico-tegmental tract as already alluded to. Along the lateral side of the central grey matter the trigeminus mesencephalic root still is very marked, along its dorsal tip fourth nerve bundles course laterally. The central grey matter is somewhat smaller than in lower levels, it does not contain special nuclei but ganglioncells mostly large and mediumsized, occur all over its surface area. They are most frequent in its centre. The medial longitudinal fascicle is a large compact body of white matter although it is still divided in bundles, which however are separated by a few longitudinally cut fibres and tiny grey areas only. Coarse fibres are restricted to its dorsal and medial borders, otherwise the fibrepattern is fairly homogeneous, but the fibres are not very thin. Lateral to it thin fibres get more numerous and grey areas grow larger, although a von Gudden nucleus can no more recognized. Small bundles with a very high coarse fibre contingent lie ventrolateral to the medial longitudinal fascicle. The tegmentum between the raphe and the lateral margin of the brachium conjunctivum is mostly occupied by the latter and its decussation, and only its mediodorsal quarter around the medial longitudinal fascicle is free from them. Within this decussation the rubrospinal tract bundles and the medial portion of the medial lemniscus can be recognized. Decussating fibrebundles are especially numerous in the medioventral quarter of the tegmentum, where thick bundles proceed to the

Fig. 8 (Pons 3/.'i9) At this level only some fibrebundles of the pes pontis along the medial border of the cerebral peduncle are left. The brain stem is characterized by the huge pedunculus at its ventral periphery and the crossing brachium conjunctivum filling the medial two third of the tegmentum. The cerebral peduncle is very large, areas of a special fibre-pattern cannot be found in it. Its apparent dorsolateral tip however is formed by the lateral part of the medial lemniscus. The medial part of the latter lies within the ventrolateral part of the brachium conjunctivum. The lateral lemniscus has decreased in size and shifts dorsally to its nuclei of termination. Dorsal to it root fibres of the trochlear nerve run to their exit. The brachium conjunctivum shows a large area of decussation in the middle line whereas in the intermediate third of the tegmentum its bundles shift medioventrally. The mesencephalic root of the trigeminus nerve is distinct along the lateral margin of the central grey matter. The medial longitudinal fascicle is very large, dorsally and dorsomedially it contains coarse fibres, whereas the core consists of mediumsized fibres of about equal caliber. The rubrospinal tract is hidden in the lateral part of the brachium conjunctivum. Magnification x ">.

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