NERVOUS SYSTEM OF THE VERTEBRATA. 443 



is formed outside it. It constitutes a fold projecting far into the 

 cavity of the lateral ventricle, and together with the vascular 

 connective tissue in it gives rise to the choroid plexus of the 

 lateral ventricle (figs. 260 and 261 pi}. 



It is clear from the above description that a marginal fissure 

 leading into the cavity of the lateral ventricle does not exist in 

 the sense often implied in works on human anatomy, in that the 

 epithelium covering the choroid plexus, which forms the true 

 wall of the brain, is a continuous membrane. The epithelium of 

 the choroid plexus of the lateral ventricle is quite independent 

 of that of the choroid plexus of the third ventricle, though at the 

 foramen of Munro the roof of the third ventricle is of course con- 

 tinuous with the inner wall of the lateral ventricle (fig. 260 s). 

 The vascular elements of the two plexuses form however a con- 

 tinuous structure. 



The most characteristic parts of the Mammalian cerebrum 

 are the commissures connecting the two hemispheres. These 

 commissures are (i) the anterior commissure, (2) the fornix, and 

 (3) the corpus callosum, the two latter being peculiar to Mam- 

 malia. 



By the fusion of the inner walls of the hemispheres in front 

 of the lamina terminalis a solid septum is formed, known as the 

 septum lucidum, continuous behind with the lamina terminalis, 

 and below with the corpora striata (figs. 255 and 259 spf). It is 

 by a series of differentiations within this septum that the above 

 commissures originate. In Man there is a closed cavity left in 

 the septum known as the fifth ventricle, which has however no 

 communication with the true ventricles of the brain. 



In the septum lucidum there become first formed, below, the 

 transverse fibres of the anterior commissure (fig. 255 and fig. 

 259 cma), and in the upper part the vertical fibres of the fornix 

 (fig. 255 and fig. 259/0: 2). The vertical fibres meet above 

 the foramen of Munro, and thence diverge backwards, as the 

 posterior pillars, to lose themselves in the cornu ammonis (fig. 

 259 amm}. Ventrally they are continued, as the descending or 

 anterior pillars of the fornix, into the corpus albicans, and thence 

 into the optic thalami. 



The corpus callosum is not formed till after the anterior 

 commissure and fornix. It arises in the upper part of the region 



