CHAP, ii.] THE BRAIN. 933 



below, ventral to, the corpus callosum, and here the junction 

 between the two vesicles is reduced to a thin sheet, the septum 

 lucidum; but behind, the broader basal portion of the fornix is 

 arched up so as to lie immediately under and touch the corpus 

 callosum. Hence the septum lucidum has the form of a more or 

 less triangular vertical sheet, broad in front and narrowing behind, 

 separating the two lateral ventricles. The sheet may be conceived 

 of as being double and formed by the apposition of two layers, one 

 belonging to each ventricle ; between these two layers is developed 

 a narrow 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 ventricle, 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 ventricle com- 

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



The corpus callosum or dorsal portion of the junction between 

 the vesicles spreads out, as we have said, laterally along its whole 

 length, and thus forms a broad band joining the two hemispheres 

 together ; the middle portion spreads out in a more or less straight 

 direction though curving over the ventricle upwards and downwards 

 to reach various parts of the hemisphere, while the front and hind 

 ends bend round on each side forwards and backwards to reach the 

 anterior and posterior parts. Thus through the corpus callosum 

 the thick wall 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 bifur- 

 cates 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 attachments 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 continuity 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. 



