200 COMPARATIVE PHYSIOLOGY. 



Fatigue will occur, it is well kaown, if the muscles are used 

 for an indefinitely long period, no matter how favorable the 

 blood-supply — another evidence that there is, in all probability, 

 some chemical product, the result of their own activity, depress- 

 ing them; and this is rendered all the more likely when it is 

 learned that the injection of lactic acid, to take one example, 

 produces effects like ordinary fatigue. 



It is also a matter of common experience that exercise, while 

 beneficial to the whole body, the muscles included, as shown by 

 their enlargement under it, becomes injurious when carried to 

 the point of fatigue. 



Why the use of the muscles is conducive to their welfare is 

 but a part of a larger question. Why does the use of any tissue 

 improve it ? 



When the nerve which supplies a muscle is stimulated its 

 blood-vessels dilate, and it has been assumed that the same 

 happens when a muscle contracts normally in the body ; and 

 when muscular action is increased there is a corresponding 

 augmentation in the quantity of blood driven through the 

 muscles in a given period, even if there be no actual increase 

 in the caliber of the blood-vessels, for the heart-beat is greatly 

 accelerated. 



But repose is as necessary as exercise for the greatest effect- 

 iveness of the muscles, as the experience of all, and especially 

 athletes, proves. 



That the nervous system plays a -great part in the nutrition 

 of muscles is evident from the fact, among countless others, 

 that it is not possible to use the brain to its greatest capacity 

 and the muscles to their fullest at the same time ; the individual 

 engaged in physical " training " must forego severe mental ap- 

 plication. Nervous energy is required for the muscles, and all 

 questions of blood-supply are, though important, subordinate. 

 But it would be premature to enter into a full discussion of this 

 interesting topic now. 



The sense of fatigue experienced after prolonged muscular 

 action is complex, though there can be no doubt that the nerve- 

 centers must be taken into account, since any muscular work 

 that, from being unusual, requires closer attention and a more 

 direct influence of the will, is well known to be more fatigu- 

 ing. On the other hand, the accumulation of products of 

 fatigue doubtless reports itself through the local nervous mech- 

 anism. 



