DIGESTION AND FOOD. 77 



distance to a fire, he is not ready to take hold of the brakes 

 of the engine and pump with successful vigor. Nor, after 

 fatiguing the arms with the engine, is he inclined to make 

 equal exertion with the feet in running a race homeward. 

 102. Fatigue of the body affects the mental powers in 

 the same way. When the laborer has finished a very hard 

 day's work, or when the fireman returns from his violent 

 exertions at afire, he is disinclined to active thought, and, 

 perhaps, even to read, and may fall asleep over his book. 

 Nor at any time do we readily think upon any serious sub- 

 ject, or attend to any business that requires grave thought, 

 immediately after we have made great and fatiguing exertion. 



163. While any limb or organ is in action, there is a 

 greater waste of particles. At the same time, there is a 

 greater flow of blood to supply this waste, and of nervous 

 power to quicken the action. But, if the action be violent, 

 the waste is greater than the new supply, and consequently 

 the part is exhausted, and the body feels fatigued. The 

 exhaustion of particles and the fatigue remain after the 

 action is over. If we are then quiet, the blood and the 

 nervous energy still continue to flow in unusual quantity, to 

 restore the previous waste, and to revive the diminished life. 

 By this means, we rest, and recover lost powers. While the 

 brain and the blood-vessels are thus restoring any fatigued 

 part, they cannot sustain a vigorous action in another ; and 

 if we then attempt to exercise the muscles, or stomach, or 

 brain, and work, digest, or think, we shall do it but languid- 

 ly, probably unsuccessfully, because the blood and ner- 

 vous energy which are needed to sustain these actions are 

 wanted and used elsewhere. 



164. During the first process of digestion, the stomach 

 requires a greater flow of blood and of nervous energy to 

 sustain this action. But if these be still required in re- 

 storing the waste, and the power of other parts, exhausted 

 and fatigued by previous exertion, they cannot be given to 

 the stomach. If, therefore, when we are much fatigued 

 with exertion of the muscles or of the brain, we fill our 



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