CHAP, ix MID- AND INTER-BKAIN 487 



Viewed in section the base of the peduncles is crescentic in form ; 

 the bundles of which it is composed are separated by prolonga- 

 tions of the pia mater. The pyramidal bundles of the cord, medulla, 

 and pons, are the largest element, and occupy the median part,, of 

 the crus of the peduncles. They arise from the Eolandic or central 

 region of the cerebral cortex, pass through the internal capsule, 

 and run to the nuclei of origin of the motor nerves in the pons, 

 bulb, and cord. The external or lateral segment of the pes is 

 formed of bundles which are the prolongation of the lateral 

 bundles of the pons ; these take origin in the occipito-temporal 

 regions of the cerebral cortex, and terminate in the cells of the 

 nuclei of the pons which give rise to the fibres that form the 

 ponto-cerebellar path. The internal median segment of the pes 

 is composed of fibres which develop late as compared with those of 

 the pyramidal bundle ; they pass through the anterior portion of 



FIG. 242. Outline of two sections across the mesencephalon. Natural size. (Schiifer.) A, through 

 inferior pair of corpora quadrigemina ; B, through superior pair, cr, crusta ; s.n., substantia 

 nigra ; t, tegmentuin ; s, Sylvian aqueduct with central grey matter ; c.q.. grey matter of 

 quadrigeminal bodies ; l.g., lateral groove ; p.L, posterior longitudinal bundle ; d. V, descending 

 root of 5th nerve ; s.c.p., superior cerebellar peduncle ; /, fillet. The dotted circle in B 

 indicates the tegmental nucleus. 



the internal capsule, and come from the prefrontal region of the 

 hemisphere. 



The substantia nigra consists of pigmented cells and nerve- 

 fibres of which the destination is unknown. They form the 

 ventral stratum of the tegmentuin, which contains much grey 

 matter, consisting of scattered nerve-cells intersected by longi- 

 tudinal, transverse, oblique and arcuate fibres, which give the 

 same appearance to the lower part of the mesencephalon as the 

 formatio reticularis of the bulb or pons (Fig. 228). Besides the 

 scattered bundles of longitudinal fibres, we have to consider 

 the dorsal longitudinal bundle, the superior cerebellar peduncles, 

 and the fillet of Keil (Fig. 244). The first arises from the nuclei 

 of the motor cerebral nerves, and especially from the 3rd, 4th, and 

 6th pairs; the second, as we have seen, decussate near the red 

 nucleus of Stilling, and pass to the ventral portion of the optic 

 thalamus ; the fillet originates principally in the nuclei of the 

 dorsal columns of the opposite side, and comes into relation with 

 the corpora quadrigemina and optic thalamus. 



The aqueduct is surrounded by a layer of grey matter, which 



21 2 



