280 



PRACTICAL ANATOMY. 



to be composed of from three to seven layers of gray or 



vesicular matter, be- 

 tween which pass com- 

 municating and com- 

 missural axis-cylinders. 

 Through the middle of 

 the convolution pass 

 the fibres of the corona 

 radiata of the subcorti- 

 cal white matter. The 

 entire thickness of the 

 cortical matter varies in 

 different brains and in 

 the different parts of 

 the same brain. Its 

 average is about three- 

 sixteenths of an inch. 



The convolutions of 

 each hemisphere are 

 divided into primary 

 and secondary. The 

 primary are twenty- 

 eight in number ; the 

 secondary vary both in 

 number and position, 

 being principally an- 

 nectant. 



The outer surface of 

 each hemisphere is di- 

 vided into four regions 

 or lobes, frontal, pari- 

 etal, occipital, and the 



temporo - Spheiioidal, - 



so named after the bones which overlie them, but with 





FIG. 120. SECTION THROUGH THE CORTEX 

 OF A FRONTAL CONVOLUTION. 





