546 THE CEREBRUM. 



the fornix, to the point of each desceBcling cornu. They consist of a 

 highly vascuhar villous membrane. The villi with which they are 

 covered are again divided npon their surfaces and at their borders into 

 small processes, along which fine vessels are seen to run. Numerous 

 small vessels pass between the plexuses and the surface of the corpora 

 striata, as well as other neighbouring parts, and the epithelium of the 

 ventricles is continued over their surface. Thus it is only at the 

 foramen of Monro that the epithelial lining of the lateral ventricles is 

 continuous with that of the third ventricle. 



The epithelium changes its character where it covers the plexus. It is there 

 composed of large spheroidal corpuscles, in each of which is seen, besides a 

 distinct nucleus, several yellowish granules, and one or more dark round oU- 

 di'ops. According to Henle each of these cells is provided with short, slender, 

 acuminate, transparent, and colourless pioccsses. 



On raising the velum interpositum, two slight vascular fringes are 

 seen running along its under surface, and diverging from each other 

 behind. They form the choroid plexuses of the third ventricle. 



The choroid artery enters the velum interpositum at the point of the 

 descending cornu ; and other arteries enter from behind, l3eneath the 

 corpus callosum. The greater number of the veins terminate in two 

 principal vessels named the veins of Galen, which run backwards on 

 the velum interpositum, and passing out beneath the corpus callosum 

 pour their blood into the straight sinus, having generally .first united 

 into a single trunk. 



The velum having been removed, the optic thalami are brought fully 

 into view, together with, the cavity of the third ventricle situated 

 between them, while, behind the third ventricle, between it and the 

 upper surface of the cerebellum, are seen the pineal body, the corpora 

 quadrigemina, the valve of Vieusseus, and the superior peduncles of the 

 cerebellum. 



The third ventricle is a narrow longitudinal cleft placed between 

 the optic thalami, which bound it on its two sides. It is covered above 

 by the velum interpositum and the fornix. Beneath, its floor is formed 

 by the following parts, which have been already described as seen on 

 the base of the cerebrum ; viz., commencing from behind, the posterior 

 perforated space, the corpora albicantia, the tuber cinereum and infandi- 

 bulum, and the lamina cinerea, the last of which also serves to close it 

 in front, as high as the anterior commissure. Behind, is the anterior 

 opening of the aqueduct of Sylvius. The cavity is crossed by three 

 commissures, named from their position, anterior, middle, and posterior. 



The middle or soft commissure is composed almost entirely of grey 

 matter, and connects the two thalami. It is variable in size, and some- 

 times wanting ; it is liable to be torn across in examining the brain. 



The (interior commissure is a round bundle of white fibres, placed 

 immediately in front of the anterior pillars of tlie fornix, and crossing 

 between the corpora striata. It marks the anterior boundary of the 

 ventricle ; its fibres extend laterally through the corpora striata a long 

 way into the substance of the cerebral hemispheres, connecting the 

 xwo temporo-sphenoidal lobes. 



The posterior commissure, also white but of smaller size, is placed 

 across the back part of the ventricle, between the posterior parts of the 

 thalami, immediately before and below the pineal body, with which and 

 with the corpora quadrigemina it is intimately connected. It consists 



