XII.] THE CORPUS CALLOSUM. 383 



and below with the corpora striata (Figs. 120 and 123 spt). 

 It is by a series of differentiations within this septum, 

 the greater part of which gives rise to the septum luci- 

 dum, 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 this septum there become first formed, below and 

 behind, the transverse fibres of the anterior commissure 

 (Fig. 120 and Fig. 123 cma), while above and behind 

 these the vertical fibres of the fornix are developed 

 (Fig. 120 and Fig. 123 frx 2). The vertical fibres meet 

 above the foramen of Monro, and thence diverge back- 

 wards, as the posterior pillars, to lose themselves in the 

 cornu ammonis (Fig. 123 amm). Ventrally they are 

 continued, as the descending or anterior pillars of the 

 fornix, into the corpus albicans, and thence into the 

 optic thalami 1 . 



The corpus callosum is not formed till after the 

 anterior commissure and fornix. It arises in the upper 

 part of the septum formed by the fusion of the lateral 

 walls of the hemispheres (Figs. 120 and 123 cal\ and 

 at first only its curved anterior portion the genu 01 

 rostrum is developed. This portion is alone found 

 in Monotremes and Marsupials. The posterior portion, 

 which is present in all the Monodelphia, is gradually 

 formed as the hemispheres are prolonged further back- 

 wards. 



1 Recent observations tend to show that the anterior pillars of the 

 fornix end in the corpus albicans ; and that the fibres running from 

 the latter into the optic thalami are independent of the anterior 

 pillars. 



