GROWTH OF THE NERVOUS SYSTEM 



211 



Figure 223. — Nerve Cells. 



special nerve pathways which end in nerve cells thai 



interpret stimuli. 



The nerves which connect the central nervous system, 



that is, brain and spinal 



cord, witli all parts of 



the body, consist of 



many long nerve libers. 



Each nerve looks like a 



small white thread and 



is covered with a thick, 



fatty sheath (medullary 



sheath). In the living animal, this fatty sheath is white 



and the nerve fibers so covered are found to occupy a 



certain part of the spinal cord and 

 brain. Thus, we get the name white 

 substance. Other of the nerve fibers 

 and cell bodies are not covered with a 

 sheath and so have a gray appearance. 

 Thus we have the term gray substance in 

 connection witli the nervous system. 



164. Growth of the Nervous System. — 

 The nervous system of man, like all 

 other parts of the body, lias a definite 

 beginning and grows in an ordered 

 manner. Not only is this true in man, 

 but also in the frog and fish. The 

 tissue of the embryo, which is to grow 

 into brain and spinal cord, gradually 

 changes until the adult parts are formed. 

 During this early period of growth, the 

 nerve cells send out processes which 

 become nerve fibers, so that at birth the 



Figure 224. -Nerve nervous system is ready to go to work. 

 Cells. Indeed, nearly all the nerve cells which 



