T x. ] THE NER VO US S YSTEM. 369 



as they advance, and are crossed superficially by two an- 

 teriorly converging round cords, the optic tracts (which unite 

 to form the optic nerves), and thus a lozenge-shaped space 

 is enclosed. In the hinder part of this space are two small 

 rounded bodies placed side by side, called the corpora 

 mammillaria. In front of these is a slight prominence 

 termed the tuber cinereum, from the middle of which pro- 

 jects a conical process, the infundibulum; at the end of the 

 infundibulum is a small oval reddish mass called the pituitary 

 body, which is received into the pituitary fossa (or sella 

 turcica) of the sphenoid bone. 



In front and beside these small median parts are those 

 voluminous masses the cerebral hemispheres, which thus 

 form the larger part of even the under surface of the brain. 



The great longitudinal fissure is seen in the middle line in 

 front, and another considerable lateral fissure (called the 

 Sylvian fissure) separates obliquely (Fig. 325) the anterior (or 

 frontal] lobe from the one behind (or temporal lobe) of the 

 same hemisphere. This Sylvian fissure receives (when the 

 brain is in place) the hinder edge of the orbital wing of the 

 sphenoid, W 7 hile the frontal lobe lies upon the orbital plate ot 

 the frontal bone, and the temporal lobe lies in that cranial 

 fossa which is bounded in front by the orbital wing of the sphe- 

 noid and behind by the petrous part of the temporal bone. 



In a groove on the under surface of each frontal lobe is a 

 body, shaped something like a life-preserver, with an oblong 

 head and a long stalk. This is the olfactory lobe, the so- 

 called " olfactory nerve." 



Upon turning back the optic tracts at their union in what 

 is called the optic commissure a delicate layer is seen to 

 connect them with the anterior end of the corpus callosum. 

 This delicate layer is called the lamina cinerea, or lamina 

 terminalis. 



When the brain is viewed in profile we see the convoluted 

 surface of one of the cerebral hemispheres with the deep 

 Sylvian fissure running backwards and slightly upwards from 

 its inferior margin, and separating the temporal lobe from 

 the frontal one. When this fissure is opened out, a triangular 

 convoluted prominence is exposed, called the Island of Reil. 

 Below and behind the cerebrum we see the cerebellum with 

 the pons Varolii in front of it, beneath which the medulla runs 

 down to merge into the spinal cord (Fig. 325). 



To obtain a more complete knowledge of the structure of 

 the brain, certain definite sections must be made. 

 B B 



