THE BRAIN AND ITS MEMBRANES. 



771 



is not a real fissure or sulcus, but a rent in part of the floor of the body and in 

 the inner wall of the descending cornu of the lateral ventricle. The cleft formed 

 by removal of the plexus of the body of the ventricle leads mesially into a space 

 whose upper boundary is the under surface of fornix and corpus callosum, the 

 lower boundary being the upper surfaces of the optic thalami on each side, while 

 in the middle part is seen the cavity of the third ventricle, which has necessarily 

 been unroofed by the removal of the velum interpositum. Posteriorly, this space 

 continues into the larger one separating splenium of corpus callosum above and 

 pineal gland and corpora quadrigemina below ; while in its turn this interval is 



Oenu 



Post, pillar of fomix 

 Pe hipp. 

 Corp.ftmb. 



. major 

 . collect. 



Hipp, minor 



FIG. 462. The lateral ventricles from above. The corpus callosum is removed and the velum interpositum 

 has been pulled out from beneath the fornix. (Henle.) 



prolonged posteriorly into the still larger interspace between under surfaces of 

 occipital lobes and upper surface of cerebellum (Fig. 463). 



The Surface Aspect of the Hemispheres. 



Each hemisphere, as already stated, has four main lobes, frontal, parietal, tem- 

 i-nl or temporo-sphenoidal, and occipital. The white substance or medullary 

 centre of each of these lobes lies next to the corresponding portion of the ventri- 

 cle, and is of course directly continuous with that of an adjacent lobe, so that, as 

 far as the white matter is concerned, there is no actual demarkation between the 

 lobes. The surfaces of these lobes, however, can be fairly accurately separated 

 from each other; but, since they constitute, all taken together, the surface of the 

 entire hemisphere, it is more convenient to consider this first, and it is to be 

 remembered that the various " lobes " to be mentioned are really the surfaces of 

 these lobes. 



The surface of each hemisphere presents the following general points for con- 

 sideration : Its un<I> r surface or base is of an irregular form, resting in front on 

 the anterior and middle fossae of the skull, behind upon the tentorium cerebelli. 

 There is a small portion of the under surface which is adherent. This is equal 

 in width to the internal capsule, and is the line of junction between its fibres and 

 those of the crusta (see Fig. 460). Its upper surface is of an ovoid form, 

 broader behind than in front, convex in its general outline, and separated from 



