THE NERVOUS SYSTEM 211 



contain fluid from the subarachnoid spaces. In old age, 

 or in cases of disease in which the convolutions become 

 shrunken, large quantities of water are present. The 

 convolutions are folds of the brain substance, this folding 

 serving to give greater extent to the circumference of the 

 parts and also a greater area of gray matter. This is 

 believed to be the location of the mental forces and of 

 intelligence. The number of convolutions and their 

 depth vary in different races. 



The under surface of the brain resembles the upper; 

 it presents the same convoluted appearance, and is made 

 up of three lobes the anterior, the middle, and the pos- 

 terior. (See Fig. 102.) 



The anterior lobe rests on the rof of the orbit, and is 

 separated from the middle lobe by the fissure of Sylvius, 

 which receives the lesser wing of the sphenoid bone. 

 It contains the island of Reil. The middle lobe occupies 

 the middle fossa of the base of the skull formed by the 

 sphenoid and temporal bones. The posterior lobe rests 

 upon the tentorium. 



We will now examine the interior of the brain. In 

 separating the hemispheres of the cerebrum in the longi- 

 tudinal fissure we find what is termed the corpus callosum. 

 In examining the brain we find it made up of two char- 

 acteristic substances the white and the gray matter. 

 The gray matter is about J inch in thickness, and 

 invests the brain like a bark, hence it is termed the 

 cortical substance. The furrows, as previously stated, 

 vary in number and in depth, according to the intelligence 

 of the individual. To the naked eye the cortical substance 

 appears as one layer, but when examined under the mi- 

 croscope six layers are seen three gray, alternating with 

 three white. The central white tissue is called the 

 medullary portion, and forms the mass of the brain sub- 

 stance.. 



