THE MUSCULAR SYSTEM 77 



of mankind it is happily possible to get all needful exer- 

 cise while doing useful work. 



102. The Nervous System as involved in Muscular Exercise. 

 We have already learned that contraction of voluntary 

 muscular tissue depends upon the stimulus brought to 

 each minute muscle cell by a nerve fiber. Without such 

 stimulus a man's limbs are motionless and the whole frame 

 a lifeless mass. If the nervous system is enfeebled by 

 disease or by exhaustion, the action of the muscles becomes 

 weakened or deranged. The disease called Saint Vitus's 

 dance, Avhich causes muscular movements beyond the con- 

 trol of the will, is not a disease of the muscles, but of the 

 nerves. So in other disorders which derange the action 

 of the muscles, the real trouble is seldom with the muscu- 

 lar tissue itself. The direct effect of muscular activity, 

 as suggested in section 91, is to poison the nervous cen- 

 ters. The greater the demand upon the muscle in the 

 way of rapid and frequent contraction, the greater the con- 

 sumption of living material and the greater the amount of 

 poisonous, dead, waste matter which passes into the circu- 

 lation. These waste products, if not promptly removed 

 from the system through the excretory organs, are found 

 to have a powerful injurious effect upon the central nerv- 

 ous system, an effect that is soon manifest in the weak- 

 ened action of the muscles themselves. 



Anything, therefore, which affects injuriously the nerv- 

 ous system interferes with the free and easy play of the 

 muscles. And, conversely, anything which promotes a 

 high level of health in the nervous system is an aid to mus- 

 cular vigor also. Exercise undertaken for the carrying 

 out of some worthy purpose a purpose in which the 

 mind is deeply interested and the whole man engaged 

 is the most healthful exercise ; while that which is disliked 



