76 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



branch of a nerve-cell, and nerve-cells appear to fatigue if frequently excited 

 to vigorous action. Inasmuch as we have as yet no definite knowledge of the 

 nature of what we call the nerve-impulse, or of the character of the processes 

 by which it is transmitted along the nerve, we can afford to leave this question 

 open, and simply state that the evidence thus far obtained is opposed to the 

 view that nerve-fibres fatigue. 



Effect of Use and Disuse. — Different kinds of muscle-tissues possess very 

 different degrees of endurance. By endurance we mean the capacity to liber- 

 ate energy during long periods of time. This capacity is intimately associated 

 with irritability, for one of the first marks of failure of power is a decline of 

 irritability. In general, the more irritable a muscle the less its endurance, 

 because with an increase of irritability there is associated a more rapid and 

 extensive liberation of energy in response to irritants. For example, the rap- 

 idly responding and acting pale striated muscles of the rabbit have less resist- 

 ing power than the red striated muscles, while the sluggish unstriated muscle- 

 fibres can contract a long time without suffering from fatigue. 



The endurance of muscles of even the same kind may differ very considera- 

 bly in the same individual, but the differences are more striking in the case of 

 different individuals. One of the causes of this is the extent to which the 

 muscles are employed. Use, exercise, is the most effective method of increasing 

 not only the strength, but the endurance of the muscle. Though this fact is 

 so well known as to scarcely need repeating, the explanation of it is by no 

 means so clear. Undoubtedly one of the causes is a more perfect circulation 

 in a muscle which is often used, but this is not all. It would seem as if the 

 protoplasm of the muscle-cell was educated, so to speak, to be more expert in 

 assimilating materials containing energy, in building up the explosive compounds 

 emploved in its work, and in excreting deleterious waste matters. 



The effect of exercise upon irritability has not been thoroughly worked out. 

 It would seem as if there were a normal degree of irritability for each special 

 form of muscle-tissne, and as if either an increase or decrease of the irritability 

 above or below this level was a sign of deterioration. Exercise, if not excess- 

 ive, is favorable to the maintenance of this normal physiological condition. 

 Without doubt many of the differences which we attribute to the muscles of 

 different men are really due to differences in the central nerve-cells, the action 

 of muscles, rightly interpreted, being rather an expression of central nervous 

 activity than the result of peculiarities of the muscles themselves. To vol- 

 untarily exercise the muscles is to exercise the nerve-cells, and the effects of 

 exercise upon these nervous mechanisms is of as much importance as the 

 effect upon the muscles. In admiring visible proportions we must always 

 bear in mind that though "beef" is of use to the athlete, the muscles are 

 merely the servants, and can accomplish nothing if the master is sick. The 

 nerve-cells always give out before the muscles, and the man preparing for a 

 contest should watch his nervous system more than his muscles. He who 

 forgets this can easily over-train, and do himself a permanent injury, besides 

 failing in the race. 



