NERVOUS SYSTEM. 167 



above the crura cerebri ; the anterior pair, or the nates are gray ; 

 the posterior, or testes, are white. 



The optic thalajni are two masses of gray matter placed above 

 the crura cerebri ; they are separated by a deep groove, and form 

 the sides of the third ventricle. At their posterior border are 

 found two projections, the external d.ndi internal geniculate bodies. 



T\\& pineal gland or conarium is a cone-shaped body, of a red- 

 dish brown color, situated in front of the testes, with two peduncles 

 passing forward on the inner sides of the optic thalami. It is 

 composed of amorphous tissue, holding salts of lime, etc. 



The pitiHta?j gland is a small, circular body attached by the 

 infundibulum to the tuber cinereitni, a layer of gray matter in the 

 floor of the third ventricle. It has no cavity or definite structure. 

 (For interior of isthmus see Ventricles.) 



Cerebellum. 



The cerebellum is that part of the brain situated behind and 

 beneath the cerebrum, above the isthmus and separated from the 

 cerebrum by the tentorium. 



It is divided into three lobes by a median constriction, which 

 is divided into the superior and inferior vermiform p?'ocesses. 



The lateral lobes are convex, and show many convolutions or 

 depressions. 



Structure. 



Gray externally, white internally, with a tree-like arrangement 

 called the arbor vitce. 



Cerebrum. 



The cerebrum is composed of two portions, the hemispheres 

 elongated antero-posteriorly, separated by a deep fissure running 

 in the same direction and united by a transverse commissure — 

 the corpus callosum. 



The longitudinal fissure. 



This passes antero-posteriorly, separating the two hemispheres, 

 and bounded below by the corpus callosum. Behind, at the an- 

 terior and superior portion of the medulla, it changes its direction, 

 transmits the velum interpositum, and is called the fissure of 

 Bichat or transverse fissure. The longitudinal fissure lodges 

 vessels and the/^Zr cerebri. 



The fissure of Sylvius is a transverse depression in front of the 

 mastoid lobule, lodging the middle cerebral artery. 



