188 Anatomy of the Rabbit. 



sion 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 opticum), 

 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 hypophysial 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 mammillary body (corpus mammillare) forms a con- 

 spicuous rounded elevation, lying at the posterior end of the 

 tuber cinereum. 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) . 



(c) The third cranial, or oculomotor nerve (n. oculomotorius), 

 arises from the ventral surface of the cerebral peduncle. 



IN THE RHOMBENCEPHALON: 



(a) The cerebellum forms an arch over the posterior portion of 

 the brain, and is supported by stout pillars from its sides. 

 In addition to the ridges of its surface the whole structure is 

 divided into several lobes, including the median portion or 

 vermis, the lateral hemispheres, and the stalked bodies, or 

 flocculi, arising from the hemispheres. 



(b) The anterior medullary velum (velum medullare anterius) is 

 the thin membrane underlying the anterior portion of the 

 cerebellum and connecting the latter with the inferior 

 colliculi. It forms a small anterior portion of the roof of the 

 fourth ventricle. 



