212 



THE NERVOUS SYSTEM OF MAN 



4 



.. ft* <j* 



- - •* 





the human being is ever 

 to use are made before 

 birth. These cells grad- 

 ually become more active 

 and the different parts of 

 the brain work more per- 

 fectly as we go through 

 the periods of childhood, 

 youth, and maturity. 

 The brain becomes a 

 more perfect working 

 organ by making the 

 brain cells do their 

 specific work over and 

 over and over, until each 

 group of cells can be 

 relied upon to do a 

 definite thing. 



165. Reflex Action. — 



Reflex action is the 



simplest form of nervous activity in man. For example, 



when the finger is placed on a hot stove and suddenly 





i 



Figure 225. — Micro-photograph of 

 Brain. 



The nerve cells are black. 



,sl<in 



ganglion 



'sensory 

 fiber 



motor 

 fiber 



muscle 



Figure 226. — Diagram to show Reflex Action. 



The stimulus comes in contact with the skin and is carried to the 

 spinal cord. It then passes to the motor cells which carry the order 

 to the muscle. The same skin stimulus goes to several other parts of 

 the spinal cord. 



