THE CAT 253 



Study the internal structure of the brain. The brain, like the 

 spinal cord, is a hollow structure. Unlike the spinal cord, how- 

 ever, in which the cavity is a simple tubular canal, the cavities 

 of the brain form a series of complicated spaces, which extend 

 throughout its five divisions and their outgrowths. The brain has 

 thus dorsal, ventral, and lateral walls, which surround these spaces. 

 These walls vary much in thickness in different places, and the 

 actual form of the brain is due largely to these variations. 



We shall study first a longitudinal median section of the brain. 

 Cut the brain with a large, sharp scalpel or a razor into two equal 

 halves by a sagittal incision. Observe the gyri and sulci on the 

 medial surface of the hemisphere. Observe the conspicuous, broad 

 band of white fibers which joins the two hemispheres ; this is the 

 corpus callosum. Its posterior end is called the splenium and is 

 not so thick as the anterior portion. The point where it turns 

 down anteriorly is called the genu ; the anterior end, which turns 

 slightly posteriorly, is called the rostrum. Extending ventrally 

 from the ventral surface of the corpus callosum is a white, arched 

 band of longitudinal fibers which is called the fornix. In the angle 

 between the anterior half of the corpus callosum and the fornix is 

 the septum pellucidum. It is a vertical partition between the two 

 anterior ventricles of the brain, one of which is in each hemisphere. 

 At the ventral end of the fornix is a small band of white fibers 

 called the anterior commissure, which extends between the hemi- 

 spheres. Extending straight ventrally from this commissure is a 

 thin plate, the lamina terminalis, which appears in the section as 

 a thin line; it is the forward boundary of the third ventricle of 

 the brain, and bears at its ventral end the optic chiasma. 



The third ventricle, which is in the second division of the brain, 

 is a very narrow, slitlike space which lies behind the lamina ter- 

 minalis ; if the section is not exactly median, it may not come 

 into view. Its lateral walls are formed on each side by the optic 

 thalamus. Ventrally the ventricle is continued into the tuber 

 cinereum and the infundibulum. 



In the dorsal portion of the third ventricle and lying just behind 

 the fornix will be seen the circular cross section of a large body, 

 the middle commissure ; it joins the two thalami. 



