THE BRAIN. 459 



e. The cerebellum is a median elongated oval body 



marked by a number of transverse fissures. In 

 front it is in contact with the hemispheres ; 

 behind, it overlaps the medulla ; and laterally 

 it lies above the optic lobes. 



f. The medulla oblongata is thick and wide : its an- 



terior end is covered by the cerebellum, and pos- 

 teriorly it is continuous with the spinal cord. 

 <* 



2. The ventral surface of the brain. 



a. The optic chiasma, formed by the crossing of the 



optic nerve, lies between and slightly in front 

 of the optic lobes. 



b. The infundibulum is a small median process, imme- 



diately behind the optic chiasma. It is connected 

 with the pituitary body, which is usually left in 

 the skull when the brain is removed. 



C. Bisection of the Brain. 



Divide the brain into right and left halves by a median 

 longitudinal section, so as to expose its cavities. 



a. The third ventricle, or cavity of the fore-brain, is 



of considerable extent from before backwards, 

 and dorso-ventrally ; but is very narrow from 

 side to side. It opens in front by lateral aper- 

 tures, the foramina of Monro, into the lateral 

 ventricles of the hemispheres. Its roof is pro- 

 duced upwards into the stalk of the pineal 

 body, and its floor is depressed to form the 

 infundibulum. 



b. The Sylvian aqueduct, or cavity of the mid-brain, 



is small, but communicates laterally with large 

 cavities in the optic lobes. 



c. The fourth ventricle, or cavity of the hind-brain, 



is wide from side to side, but shallow dorso- 

 ventrally. 



