The Central Nervous System. 275 



(k) The parts of the metathalamus are distinguishable 

 externally as two rounded projections of the lateral surfaces 

 of the thalamencephalon. One of them, the lateral geni- 

 culate body (corpus geniculatum laterale), lies to the 

 lateral side of the pulvinar, and is only separated from it by 

 a faint depression of the surface. The medial geniculate 

 body occupies a more posterior and medial position. 



(1) The optic tract (tractus opticus) connects the geniculate 

 bodies, especially the lateral one, with the ventral surface of 

 the brain, ending in the optic chiasma. 



(m) On the ventral surface, the optic chiasma (chiasma opti- 

 cum), forms a conspicuous elevation, the posterior portion 

 of which is traceable into the optic tracts, the anterior 

 portion into the bases of the second cranial, or optic 

 nerves. 



(n) The hypophysis, or pituitary body, lies immediately 

 behind the optic chiasma. 



On. account of its enclosure by the walls of the hypophyseal fossa, 

 and also its slight attachment to the brain, the hypophysis is commonly 

 detached in removing the brain from the skull, in which case a slit-like 

 aperture, representing the ventral portion of the third ventricle is 

 exposed. 



(o) The tuber cinereum is a small elevation of grey matter 

 appearing on the ventral surface after the removal of the 

 hypophysis. It is the base of attachment of the infundi- 

 bulum, the latter being the slender extension of the brain 

 downward toward the hypophysis. 



(p) The mamillary body (corpus mamillare) forms a con- 

 spicuous rounded elevation, lying at the posterior end of the 

 tuber cincereum. The structure is single, but there is an 

 indication of lateral lobes. 

 in the mesencephalon: 



(a) The dorsal surface is marked by four elevations, the corpora 

 quadrigemina. The anterior pair, distinguished as the 

 colliculi superiores, are much larger than the posterior 

 pair, the colliculi inferiores. 



(b) The ventral surface is occupied by a pair of divergent cords, 

 the cerebral peduncles (pedunculi cerebri), separated by a 

 faint median depression, the interpeduncular fossa (fossa 

 interpeduncularis) . 



