CEREBRUM. 379 



equal parts, which are separated vertically from each other by 

 the falx. This vertical separation does not extend through the 

 centre of the cerebrum, although it divides it completely before 

 and behind. A portion of the central part of the cerebrum ; 

 which is situated deeper than the under edge of the falx, is not 

 divided. 



The upper surface of the two hemispheres is convex. The 

 under surface is rather irregular ; it is divided in each hemi- 

 sphere into three lobes : the anterior, the middle, and the 

 posterior* 



The anterior lobes of the brain are situated on the front part 

 of the base of the cranium, principally on the orbitar processes 

 of the os frontis. 



The middle lobes are lodged in the fossae formed by tbe tem- 

 poral and sphenoid bones. 



The posterior lobes rest chiefly upon the tentorium, over 

 the cerebellum. 



Between the anterior and middle lobes is a deep furrow, 

 corresponding to the base of the cranium on which they rest, 

 which is called the fossa Sylvii. 



The surface of the brain resembles that of a mass of small 

 intestines, or of a convoluted cylindrical tube : it is, therefore, 

 said to be convoluted. The fissures between these con- 

 volutions do not extend very deep into the substance of the 

 brain. 



The whole surface of the brain, thus convoluted, is covered 

 by pia mater ; which is connected to every part of the surface 

 by an infinite number of small vessels and processes, that 

 appear when this membrane is peeled off from the surface of 

 the brain. 



The mass of the brain consists of two substances of different 

 colors ; one of which is, for the most part, exterior to the 

 other. The exterior substance is of a light brown color, 

 and is therefore called cineritious, or cortical, from its situa- 

 tion. 



The internal substance is white, and is denominated the 

 medullary. 



