THB MODIFICATION OP THE BEAIN. 63 



ventricle, and arching over the foramen of Mnnro, is pro- 

 duced. 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 of the posterim' pillars of tlie foi-nix. 



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

 missure ; and a posterior coimnissure, consisting of transverse 

 nerve-fibres, is generally developed between the posterior 

 ends of the two thalami. 



In the Mammalia, a structure, which is absent in other 

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

 of that class, this corpus callosum 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 ventri- 

 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 an- 

 terior part crosses the interspace between the hemispheres 

 considerably above the level of the fornix ; so that between 

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

 hemisphei'e, vrith the intei'vening space, is intercepted. The 

 portion of the two inner walls and their interspace, thus 

 isolated from the rest, constitutes the septwm lucidum, with 

 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 Mammalia, 

 they extend forwards over the olfactory lobes, and back- 

 wards over the optic lobes and cerebellum, so as com- 

 pletely to cover these parts ; and, m addition, they are 

 enlarged downwards towards 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 sej)arated by any very sharp lines of 

 demarcation. These regions are named the frontal, parietal, 



