934 GENERAL STRUCTURE. [BOOK in. 



We have said that the roof of the third ventricle, like that of 

 the fourth ventricle, consists only of a layer of epithelium devoid of 

 nervous elements. We have further seen that the fornix, and the 

 hind part of the corpus callosum with which it is continuous 

 overlie the third ventricle, the free base of the fornix with the 

 rounded hind end of the corpus callosum above forming together 

 the hind border of the junction or bridge between the two 

 hemispheres. The pia mater covering the dorsal surface of the 

 brain, passing forwards under this curved border, spreads over the 

 top of the third ventricle, becoming adherent to the layer of 

 epithelium just referred to, and thus forms a vascular sheet called 

 the velum interpositum, which serves as the actual roof of the 

 third ventricle, immediately below, ventral to, the fornix ; it 

 cannot be seen without previously removing the fornix. At the 

 lateral edge of the fornix, on each side, this same vascular sheet of 

 pia mater projects from beneath the fornix into the lateral ventricle 

 carrying with it the layer of epithelium which, as we said, made 

 the edge of the fornix actually continuous with the rest of the 

 walls of the lateral ventricle ; the part of the pia mater thus seen 

 projecting beyond the edge of the fornix when the lateral ventricle 

 is laid open is called the choroid plexus. To this peculiar intrusion 

 of the pia mater, by which the nutrition of the brain is assisted, 

 we shall return when we come to speak of the vascular arrange- 

 ments of the brain. Meanwhile we may point out, that while this 

 vascular ingrowth seems to make the cavity of the third ventricle 

 continuous with that of the lateral ventricle on each side, and all 

 three with the exterior of the brain, it really does not do so. The 

 cavity of the third ventricle is made complete by the layer of 

 epithelium forming its roof, and the cavity of the lateral ventricle 

 is made complete by the layer of epithelium passing from the 

 lateral edge of the fornix over the choroid plexus to the other 

 parts of the wall of the ventricle. To pass along this line from 

 the actual cavity of the lateral into that of the third ventricle one 

 must first pierce the epithelium covering the choroid plexus, thus 

 gaining access to the pia mater of the plexus and of the velum, 

 and then again pierce the epithelium coating the under surface 

 of the velum and forming the roof of the third ventricle. It is 

 only by the foramen of Monro that a real communication exists 

 between the cavity of the lateral and that of the third ventricle. 



Thus by the large growth and backward extension of the 

 cerebral hemispheres, the third ventricle comes to form as it were 

 the front end of the cerebrospinal axis, the crura cerebri expanding 

 on each side of the third ventricle into the cerebral hemispheres 

 which cover up the ventricle on the dorsal surface but leave its 

 walls exposed on the ventral surface. Attached to the dorsal 

 surface of the third ventricle at its hind end, ventral to and 

 somewhat projecting beyond the base of the fornix, lies the pineal 

 gland with its attachments, the^ remnants of a once-important 



