418 HISTORY OF ' THE HUMAN BODY 



Still further down, ventral to the hippocampus, and partly 

 enclosed by the surrounding parts, the same inner walls of the 

 two hemispheres come in contact and form a thin double par- 

 tition known as the septum pellucidum. The two walls en- 

 close a small space to which the name " fifth ventricle " was 

 formerly given. It is unnecessary to state that this is not a 

 true ventricle and has no connection with the lumen of the 

 neural tube. 



The telencephalon of all higher mammals is further distin- 

 guished by the formation of an extensive bridge or commissure 

 across the middle line between the two lobes. This lies dorsal, 

 and is easily seen by drawing the hemispheres a little apart 

 and looking down from above. It is called the corpus callo- 

 sum, and consists of fibers of white matter that form a me- 

 dium of intercommunication between corresponding parts of 

 the two hemispheres and insure harmony of action. Aside 

 from this extensive commissural system, which has evidently 

 arisen in mammals in connection with the added needs coming 

 from larger hemispheres, there are three smaller transverse 

 bundles, common also to the brain of lower forms, the ante- 

 rior, middle and posterior commissures. Of these the first 

 alone comes within the province of the telencephalon, the others 

 are respectively di- and mesencephalic. 



The diencephalon, never an extensive element in the verte- 

 brate brain, becomes nearly or wholly covered dorsally and 

 laterally in the higher forms by the excessive development of 

 other parts, but though small and of subordinate interest in 

 itself, it is especially characterized by the formation of second- 

 ary organs, either as in- or out-pushings. Some of these latter 

 become of fundamental importance while others appear to be 

 more or less vestigial, presumably inherited from preverte- 

 brate ancestors and of problematic significance. 



Several of these formations occur along the dorsal aspect, 

 where over a considerable area of debatable territory between 

 tel- and di-encephala the roof remains thin. The most an- 

 terior consists of an extensive invagination into the third 

 ventricle, which lies just beneath this region. This invagina- 



