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THE CENTRAL NERVOUS SYSTEM. 275 
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. 
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. 
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. 
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. 
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. 
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,. 
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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. 
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). 
