CORPUS CALLOSUM. 419 



taken. At this point, beneath the vein of Galen, the arach- 

 noid canal, or canal of Bichat, is seen. 



Corpus Callosum , (Figs. 116, 117.) By separating widely 

 the hemispheres, this body is seen presenting a brilliant 

 white appearance, and quadrilateral shape. It occupies 

 the centre of the brain ; its greatest length, which is about 

 three or four inches, being in the longitudinal direction. 

 It forms the roof of the lateral ventricles, and covers the 

 for nix. Its upper surface is white, convex, and marked 

 with two or three longitudinal lines called the raphe, from 

 which transverse lines are seen on either side. Its anterior 

 portion is round and bends downward into the anterior 

 lobes and base of the brain. Its posterior end is thick, 

 round, and continuous with the fornix and hippocampi; 

 externally, portions of it curve downwards and join the 

 thalami and corpora striata. The corpus callosum con- 

 nects the two hemispheres of the cerebrum, and is called 

 the great cerebral commissure. 



The Septum Lucidum (Fig. 116) is the membranous par- 

 tition between the lateral ventricles, and extends from the 

 median surface of the corpus callosum to the fornix. It 

 is described as consisting of four laminee. The first comes 

 from the lateral ventricle ; the second consists of gray mat- 

 ter ; the third is a white, fibrous layer ; and the fourth is 

 a very delicate layer containing a cavity called the f/tli 

 ventricle. It is generally regarded as being composed of 

 two laininse, between which is the fifth ventricle; and 

 each of these is capable of being divided into two the 

 outer layer being cineritious, the inner medullary. This 

 ventricle, in the natural state, is supposed to be closed; but 

 it is thought by others to form a portion of the third ven- 

 tricle, as in the early periods of uterine life. Tiedenman 

 found it to communicate with the latter cavity. The func- 

 tion of the septum lucidum is regarded as commissural. 



On making a horizontal section of one of the hemispheres 

 of the cerebrum, in the centre there is observed a mass of 

 white, fibrous, medullary matter, surrounded by a wavy 

 line of gray. This central appearance is called the centrum 



