LATERAL VENTRICLES. 381 



into the medullary matter in each hemisphere, about half an 

 inch ; the whole of this unconnected surface, the middle of 

 which is directly at the bottom of the great fissure, is >ermed 

 corpus callosum. 



When the hemispheres are cut away to the level of this sur- 

 face, and the corpus callosum is examined, two raised lines 

 appear in the middle, which extend from one end of it to the 

 other ; and between them is a small groove of the same length. 

 This groove is called the raphe, or suture of the corpus callo- 

 sum. From the raised lines or bands on each side of the raphe, 

 small lines less elevated pass across the corpus callosum, and 

 are lost in the medullary matter. The hemispheres being thus 

 cut off at the level of the corpus callosum, on the cut surface is 

 to be seen the interior mass of medullary matter, with the cor- 

 tical part exterior, its edge exhibiting the convoluted surface of 

 the brain, and the pia mater, following the convolutions. 



The medullary surface, thus exhibited, with the corpus cal- 

 losum in the centre, is denominated the centrum ovale. 



The Ventricles. 



In the brain there are four cavities called ventricles : three 

 of these are formed in the substance of the cerebrum : the 

 fourth is situated between the cerebellum, the pons Varolii, 

 and medulla oblongata. The two largest are called the lateral 

 ventricles, from their situations ; the others are named, from 

 the order in which they occur, the third and the fourth ven- 

 tricles. 



Another ventricle has more lately been discovered called the 

 fifth. It is very small and is situated between the two layers 

 of the septum lucidum. It communicates downwards between 

 the anterior crura of the fornix with the cavity of the third 

 ventricle. 



The lateral ventricles are cavities of an extremely irregular 

 figure : they are situated in each hemisphere a little below the 

 level of the corpus callosum : and, with the exception of the 

 partition which separates them, are directly under it. They 

 commence anteriorly, nearly on a line with the termination of 



