70 CONSCIOUS NERVOUS OPERATIONS 



89. In order to maintain all the nice adjustment and 

 balance of muscular force constantly demanded, the brain 

 and nerves must be continually at work. Just enough 

 stimulus must be supplied to each set of muscles and 

 supplied at exactly the right moment or something 

 will at once go wrong. This harmony of muscular ac- 

 tion and regulation of the complex relations between 

 the hundreds of muscles in the body is called coor- 

 dination. It is easy to show that it depends upon the 

 nervous system. 



If one falls asleep or receives a blow which " stuns " 

 the brain, the muscles are relaxed, and, unless supported, 

 the body falls to the ground. Sudden nervous or emo- 

 tional excitement, as surprise, grief, or fear, may cause 

 the muscles of the heart to stop their action and the body 

 to fall in a " faint." In some cases the effect of sight 

 upon the brain is to destroy the power to control the 

 muscles, as when the sight of the moving waves of lake 

 or ocean renders one giddy. The perception of certain 

 odors may have the same effect, and in many other ways 

 the control of the muscles is affected by that which affects 

 the central nervous system. 



The study of the brain has shown that the cerebellum 

 is the great center for the coordination of muscular move- 

 ment and especially of those muscular actions which have 

 to do with maintaining the equilibrium of the body. 



90. Exhaustion of Muscles. Even when we are quite 

 awake, and the brain is active, our muscles sometimes refuse 

 to act. Muscular fiber cannot contract continuously for a 

 long time. It must have periods of rest. That is the 

 reason we require frequent changes of position, one set 

 of muscles being thus allowed to rest while another set 

 is called into action. If a weight be held out at arm's 



