Chap, xviii.] CEREBRUM AND CEREBELLUM. 161 



211. The cms cerefori of each side consists of an 

 anterior, middle, and posterior portion. The anterior 

 or ventral portion is the crusta, or pes , the 

 posterior or dorsal portion is the tegmentum. Be- 

 tween the two is the substaiitia nigra. The crusta is 

 composed of longitudinal tracts of medullated nerve- 

 fibres passing from the margin of the pons Varolii 

 to the internal capsule of the thalamencephalon, and 

 farther into the white matter of the hemisphere. 



212. The tcgmcntum has been mentioned above 

 as being situated in front of the grey matter forming 

 the anterior wall of the aqueductus Sylvii. The 

 tegmentum is a prolongation of the reticular formation 

 of the pons Yarolii and medulla (see above), i.e., small 

 masses of grey substance separated by tracts of nerve- 

 fibres, most of which run in a longitudinal or transverse 

 direction. The longitudinal bundles include a con- 

 tinuation of the white matter of the cerebellum, 

 mentioned in a previous page as the superior 

 peduncle of the cerebellum, or the pedunculus cere- 

 belli ad cerebrum. These undergo total decussation in 

 the upper part of the mesencephalon, and ultimately 

 enter the thalamus opticus. 



213. The substaiitia nigra is grey matter 

 situated between the two above ; it has received its 

 name from the numerous dark pigment granules 

 lodged in the substance of its ganglion cells. These 

 are small and multipolar. 



214. V. The ttialamencephalon and corpus 

 striatum. The former comprises the parts of the 

 brain situated round the third ventricle, the most im- 

 portant being the thalamus opticus, the pineal gland, 

 the corpora albicautia, the infundibulum and tuber 

 cinereum, and the hypophysis cerebri. The corpus 

 striatum is the ganglion of the cerebral hemisphere, 

 with which it originates from the same part i.e., the 

 frontal part of the first cerebral vesicle of the embryo. 



L 



