568 



THE NERVOUS SYSTEM 



striata, optic thalami, etc. The structure and function of these basal nuclei 

 have already been given briefly, so we may turn our attention now to the cere- 

 bral cortex. 



Structure of the Cerebral Cortex. The cerebral cortex forms a large 

 part of the mass of the cerebrum, in fact of the whole brain. Its superficial 

 appearance presents a series of ridges and folds, the gyri and sulci. For gen- 

 eral convenience anatomists have divided the cerebral cortex into five lobes : 

 the frontal, that portion in front of the fissure of Rolando extending down to the 

 Sylvian fissure; the parietal, extending from the Sylvian fissure to the parieto- 

 occipital fissure, and bounded below by the Sylvian fissure; the temporal lobe, 

 just ventral to the parietal; the central lobe, or island of Reil; and the oc- 

 cipital lobe, which includes the posterior portion of the cortex behind the 



caUoso-Tnarg. 



FIG. 397. Left Hemisphere, from Without. (After Eberstaller.) 



parieto-occipital fissure. And, finally, the olfactory and limbic lobes together 

 make up the olfactory division of the brain. For the detailed arrangements of 

 the cortex the reader is referred 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, 2 mm., 

 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. 

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

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

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



