412 ANATOMY AND PHYSIOLOGY OF THE BRAIN. 



reaching the medulla diverge from each other, becoming ' 

 applied against the inner side of the restiform bodies, with 

 which they seem to coalesce. Arnold has, however, traced 

 them up into the brain proper. The interval between the 

 posterior pyramids is the floor of the fourth ventricle of 

 the brain, at the anterior part of which is the opening of a 

 minute canal, which is continued along the whole length 

 of the spinal cord between its grey and white commissures. 



Thepons Varolii is a quadrilateral elevation on the lower 

 aspect of the brain, and forming, the anterior limit of the 

 medulla and the posterior of the brain proper. It is broader 

 in front than behind, and has well-defined anterior and 

 posterior borders. It is composed of grey matter, and of 

 longitudinal and transverse fibres ; a large number of the 

 latter pass upward to form the middle peduncles of the 

 little brain. 



In the cerebrum or brain proper, the white matter is 

 placed on the surface and the white within, so that it differs 

 in this respect from the parts already noticed. Viewed 

 from above the cerebrum is seen to be composed of two 

 symmetrical halves (hemispheres), divided by a deep median 

 longitudinal fissure. The surface is not smooth, but thrown 

 into a great number of convolutions with intervening fissures, 

 penetrating more or less deeply into the white substance, 

 and thereby largely increasing the superficial extent of the 

 grey matter. The grey matter of these convolutions preside 

 over mental acts ; and accordingly in animals, with a large 

 intellectual development, a proportionate increase in the 

 number and depth of those convolutions is met with. A 

 transverse groove on the lateral aspect of each, between 

 the anterior and middle third, is known as the fissure of 

 Sylvius. In this fissure are some semi-detached convo- 

 lutions, known as the island of Reil. These are the first 



