80 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



development of what we call the nerve-impulse. We have already seen that 

 the nerve may undergo an alteration of irritability if subjected to artificial 

 irritants. Such a change at the point of application of the irritant is hardly 

 to be regarded as a fatigue effect, however, for in many cases, at least, it is 

 due to the direct effect of the irritant on the physical or chemical structure of 

 the nerve-protoplasm rather than to molecular changes which are peculiar to 

 the development of the nerve-impulse. Thus the change of irritability which 

 results from a series of light blows, such as may be given to a nerve by 

 Tigerstedt's tetanomotor, cannot properly be said to be the result of fatigue. It 

 has been found that a medullary nerve may be excited many times a second 

 for hours, by an induced current, and still be capable of developing at the 

 stimulated point what we call the nerve-impulse. The change which is de- 

 veloped at the point of excitation and which passes thence the length of the 

 nerve, would seem to be the expression of a form of energy liberated within 

 the nerve, and since the liberation of energy implies the breaking down of 

 chemical combinations, the apparent lack of fatigue of the nerve is incompre- 

 hensible. It is the more remarkable since the nerve-fibre is to be considered a 

 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 

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



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



