STRUCTURE OF THE BRAIN. 587 



closed cavity containing fluid, called the fifth ventricle. But while the lateral 

 ventricles open by the foramen of Monro into the third ventricle, and the 

 third ventricle is continuous by means of the aqueduct with the fourth ven- 

 tricle, which again passes into the central canal of the spinal cord, the whole 

 series being developed out of the same embryonic neural canal, the fifth ven- 

 tricle communicates with none of them ; it is a cavity of different origin. 



The corpus callosum or dorsal portion of the junction between the vesi- 

 cles spreads out, as we have said, laterally along its whole length, and thus 

 forms a broad band joining the two hemispheres together ; the middle por- 

 tion spreads out in a more or less straight direction, though curving over the 

 ventricle upward and downward to reach various parts of the hemisphere, 

 while the front and hind ends bend round on each side forward and back- 

 ward to reach the anterior and posterior parts. Thus through the corpus 

 callosum the thick \\all of one ventricle is made continuous with that of the 

 other. The disposition of the fornix or ventral portion of the junction is 

 very different. At its apex in front the fornix bifurcates into two bands, 

 known as the pillars of the fornix, which on each side become continuous 

 with, and take a peculiar course in the walls of the third ventricle. In like 

 manner behind, the angles of the base of the fornix are continuous with the 

 walls of the lateral ventricles, that is to say, with the thick mass of the 

 hemispheres, being also prolonged as two special strands of fibres called the 

 crura of the fornix. But along each side of the triangle, between the attach- 

 ments in front and behind, the substance of the fornix is not continued into 

 the substance of the corresponding hemisphere ; the edge of the fornix 

 appears on each side to lie loose on the dorsal surface of the optic thalamus, 

 which here forms the median portion of the floor of the lateral ventricle ; 

 between the optic thalamus below and the fornix above there seems to be a 

 narrow slit by which the cavity of the lateral ventricle communicates with 

 parts outside itself. In reality, however, there is no actual breach of con- 

 tinuity though there is a breach of nervous substance. The slit is bridged 

 over by a layer of epithelium, by means of which the edge of the fornix is 

 made continuous with the upper surface of the optic thalamus, and the 

 median wall of the lateral ventricle made complete. But this layer of 

 epithelium has the following peculiar relations to the pia mater covering the 

 brain : 



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 cal- 

 losum 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 form- 

 ing together the hind border of the junction or bridge between the two hemi- 

 spheres. The pia matter covering the dorsal surface of the brain, passing 

 forward 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 interposition, 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 pro- 

 jects 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 arrangements of the 



