88 HUMAN PHYSIOLOGY 



slender twigs, the dendrites. The nucleus in a nerve 

 cell is usually large and prominent. 



.The axons of nerve fibers are outgrowths of nerve 

 cells. This is a most important truth to grasp for it 

 explains the relation between gray and white matter. 

 While the great majority of the processes which spring 

 from nerve cells branch out to form dendrites, as just 

 described, certain other processes acquire the sheaths 

 that appertain to nerve fibers and run on without in- 

 terruption for long distances. We can return now to 



FIG. 19. A common type of nerve cell giving rise to the axon of a 

 nerve fiber. 



the motor fibers which we are seeking to trace to their 

 place of origin. The statement can now be made 

 that each of these fibers so far as its essential core or 

 axon is concerned is an outgrowth from a cell in the 

 gray matter of the cord or the brain. 



Functions of the Nerve Cells. What, then, are the 

 functions which we are to ascribe to the nerve cells? 

 The impulses which run along the fibers must have come 

 from these cells. The question remains, however, 

 whether the cells have generated the impulses or merely 

 transmitted them. We shall do well to emphasize the 



