406 HUMAN PHYSIOLOGY 



excitation are combined to bear upon it. The same will 

 apply to the motor centers of the cerebrum; these are not 

 self-stimulated, but depend on other elements to induce 

 reactions through them. It follows that muscles must 

 give their best performance when the governing centers 

 are subjected to the most multiplied stimulation and this 

 includes potentially the whole receptor system. In 

 other words, one reason for the power of the athlete is 

 his responsiveness to what he sees, hears, and feels the 

 excellence of his afferent equipment. 



We find that fatigue is greatly delayed when we are 

 doing something that we enjoy. Our enjoyment is 

 probably a measure of the richness of the afferent tides 

 in the nervous system and if we suppose that these cur- 

 rents are applied in reflex fashion to secure innervation of 

 the muscles we have a simple explanation of our own 

 endurance. Dancing would be harder work than sweep- 

 ing a room if the comparison were not entirely de- 

 stroyed by the superior means of stimulation which 

 accompany it. 



Other Effects of Exercise. We may now turn from the 

 value of exercise as a way to improve the command of the 

 motor apparatus and mention some of its influences upon 

 other systems. We may pass over the respiratory fea- 

 tures which have been given a place in Chapter XXI. 

 Something must be said of the effects upon the circula- 

 tion. We may distinguish conveniently between those 

 that relate to the heart and those that can be classed as 

 vasomotor. 



The heart is exercised whenever the skeletal muscles are 

 actively used and the demand upon it is roughly pro- 

 portional to the intensity of the effort. There is no 

 other way to give this organ vigorous use. If it is sound 

 in the beginning it responds to training like any other 

 muscle, growing somewhat more massive and much 

 more hardy with exercise. It is to be noted that a 

 heart may become larger than normal in two different 

 cases : the increase may be in the thickness of its walls or 



