THE MODIFICATION OF TUE liRAIN. 59 



'^eriorly, these longitudinal fibres are continued backward and 

 downward along the inner wall of the cerebral hemisphere, 

 following the juncti(^n of the corpora striata and optic thalami, 

 and pass into a thickening of the wall of the hemisphere, 

 which projects into the lateral ventricle, and is called the 

 lilppocampus major. Thus a longitudinal commissural band 

 of nervous fibres, extending from the floor of the third ven- 

 tricle to that of the lateral ventricle, and arching over the fora- 

 men of Munro, is produced. The fibres of opposite sides unite 

 over the roof of the third ventricle, and constitute what is 

 called the body of the fornix. Behind this union the bands 

 receive the name oi \)i\Q posterior pillars of the fornix. 



The optic thalami may be connected by a gray soft com- 

 missure / and a posterior comtnissure, consisting of transverse 

 nerve-fibres, is generally developed between the posterior ends 

 of the two thalami. 



In the Mamynalia^ a structure, which is absent in othef 

 Vertehrata, makes its appearance ; and, in the higher members 

 of that class, this corpus calloswn is the greatest and most im- 

 portant mass of commissural fibres. It is a series of trans- 

 verse fibres, which extends from the roof of one lateral ventr- 

 cle to that of the other, across the interval which separates 

 the inner wall of one hemisphere from that of the other. 



When the corpus callosum is largely developed, its ante- 

 rior part crosses the interspace between the hemispheres con- 

 siderably above the level of the fornix ; so that between the 

 fornix and it, a certain portion of the inner wall of each 

 hemisphere, with the intervening space, is intercepted. The 

 portion of the two inner walls and their interspace, thus 

 isolated from the rest, constitutes the septum lucidicra^ mth 

 its contained fifth ventricle. 



The Modifications of the Brain. — The chief modifications 

 in the general form of the brain arise from the development 

 of the hemispheres relatively to the other parts. In the lower 

 vertebrates the hemispheres remain small, or of so moderate a 

 size as not to hide, by overlapping, the other divisions of the 

 brain. But, in the higher 3Iarnrnalia, they extend forward 

 over the olfactory lobes, and backward over the optic lobes 

 an i cerebellum, so as completely to cover these parts ; and, in 

 addition, they are enlarged downward toward the base of the 

 brain. The cerebral hemisphere is thus, as it were, bent round 

 its corpus striatum^ and it becomes distinguished into regions, 

 or lohes^ which are not separated by any very sharp lines of 

 demarcation. These regions are named the frontal^ parietal^ 



