100 



MESENCEPHALON. 



side, and the cases which are sometimes met with of conjugate paralysis involving 

 the internal rectus of one side, and the external rectus of the other side, which 

 are accompanied by atrophy of the nucleus of the sixth, are thus accounted for. 



The prolongation of the tipper nucleus of the fifth nerve consists of a small 

 number of large globose cells (figs. 75, 76, d.V), which lie at the extreme lateral 

 margin of the grey matter of the aqueduct close to the bundles of the descending 

 root of the fifth nerve, towards which their axis-cylinder processes are directed. This 

 nucleus and root gradually become smaller, and disappear before the superior end of 

 the mesencephalon is reached. 



The crura cerebri (fig. 32, P) emerge from the upper border of the pons and 

 diverge from one another, leaving between them the posterior perforated space and 

 the corpora mamillaria and disappearing in the cerebral hemispheres under the 

 optic tract. The triangular interval seen at the base of the brain to be enclosed 

 between the diverging crura has been termed trigonum inter peduncular e by 

 Schwalbe. Near the point of the angle of divergence the roots of the third nerve 

 issue in several bundles from a groove along their inner side (fig. 32, ///.) ; and 

 this groove serves to indicate the separation between the more prominent ventral 

 part of the peduncle ( pes s. basis s. crusta pedunculi, fig. 75, cr.) and the dorsal 

 and larger part (tegmentum, /.) which is in great measure concealed from view by 



Fig. 75. OUTLINE OF TWO SECTIONS ACROSS 

 THE MESENCEPHALON. Natural size. 

 (E. A. S.) 



A, through the inferior pair of the corpora 

 quadrigemina : B, through the superior pair. 



cr, crusta ; s.n., substantia nigra ; t, teg- 

 mentum ; s, Sylvian aqueduct with the cen- 

 tral grey matter ; e.g., grey matter of quad- 

 rigeminal bodies ; l.g., lateral groove ; p.L, 

 posterior longitudinal bundle ; d. V, descend- 

 ing root of fifth nerve; s.c.p., superior cerebellar peduncle ; /. fillet. The dotted circle in B indicates 

 the tegmental nucleus. 



the pes when viewed from below and in front. A section into the crus cerebri 

 shows the two parts of which it is composed to be separated from one another by a 

 tract of dark coloured grey substance known as the substantia nigra (fig. 75, sii), 

 which comes to the surface on the inner side at the groove above mentioned from 

 which the third nerve issues (sulcus oculomotor ii), and on the outer side also along 

 a grooved line the sulcus lateralis (fig. 75, l.g.}. 



Of the two main parts of each peduncle the crusta (cr) is formed almost entirely 

 of lamellated bundles of longitudinal fibres, some of which are continuous with the 

 pyramid-fibres of the medulla oblongata and pons, whilst others are superadded ; and 

 the tegmentum is a continuation of the formatio reticularis of those parts, with the 

 addition of much grey matter and white fibres, amongst the latter being those of 

 the superior peduncle of the cerebellum. The two ventral portions (crustse or pedes) 

 are entirely distinct from one another (as shown in the accompanying sections, 

 fig. 75), and each is marked off externally from the tegmentum of the same side by 

 the grooves just mentioned ; but the two tegmenta are united in the median plane 

 by a prolongation of the raphe, and extend dorsally at the sides of the aqueduct to 

 become continuous with the bases of the corpora quadrigemina. 



Crusta. The crusta is semilunar in section, the substantia nigra projecting 

 into it with an irregular convex border. It is made up of longitudinal white fibres 

 which become arranged in the higher parts of the mid-brain into flattened bundles, 

 with their edges dorsal and ventral, separated from one another by processes of pia 

 mater. The main part is a direct prolongation of the longitudinal bundles of the 

 pops and passes superiorly towards the internal capsule of the cerebral hemisphere. 



