590 



THE NERVOUS SYSTEM 



detailed arrangements of the cortical gyri and sulci the reader is referred to 

 figures 390, 392 and to text-books of anatomy. 



In a transverse section of the cerebral cortex there is shown an external 

 gray layer chiefly composed of nerve cells and an internal white portion of 

 nerve fibers. The folding of the cortex into convolutions increases the total 

 mass of gray matter enormously. 



The gray or cellular external part of the cerebral cortex has an average 

 thickness of about 3 mm.; being thin in the occipital and frontal region, 

 2mm., and thick in the precentral, 4 mm., and postcentral convolutions. 



Several types of nerve cells have been described as present in the cortex, 

 the exact type and relative proportion varying somewhat in different regions. 



Solo. 



FIG. 392. Right Hemisphere, from Within. (After Eberstaller.) 



The typical characteristic cell, however, is the pyramidal cell. The py- 

 ramidal cell, as its name implies, has a pear-shaped cell body with numerous 

 protoplasmic processes. The apex of the cell is directed toward the surface 

 of the cortex, and supports numerous branches which extend out into the 

 adjacent territory, bringing it into contact with a relatively large number of 

 nerve cells. These processes are dendritic in character. The base of the 

 pyramidal cell always has a single axis-cylinder process which is directed 

 down into the white matter, and which in some cases ultimately finds its 

 course through the corona radiata into the pyramids below. The axis- 

 cylinder processes give off collaterals both in the immediate neighborhood 

 of the cell and somewhat deeper along its course. 



In the superficial layer of the cortex there is a peculiar type of small cell, 

 first described by Cajal. Most of these cells are fusiform in shape, with the 

 long axis parallel to the surface of the convolution. They give off usually two 

 axones which run along parallel to the surface and send down numerous 

 fine collaterals at right angles. Another form of Cajal cell, triangular or 



