GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 75 



though partial, improvement in the condition on the taking of food is per- 

 haps best explained as the result of a stimulating effect on the central nerv- 

 ous system. This might be due to the change in the circulation which follows 

 the taking of food, as well as the fact that a fresh supply of uncombined and 

 hence available energy-holding substances is being received. The effect of 

 the so-called stimulants, alcohol, tea, coffee, etc., to temporarily increase the 

 ability to do work, is probably chiefly through their action on the central 

 nervous system. Their influence is a temporary one, and only markedly 

 increases the amount of work when the body has a plentiful supply of nutri- 

 ment. 1 



Fatigue of Nerves. — Muscle-, gland- and nerve-cells — in fact, almost every 

 form of protoplasm — if excited to vigorous long-continued action, deteri- 

 orate and exhibit a decline of functional activity. As we have seen, in 

 the case of muscle there are a using up of available energy-holding compounds 

 and a production of poisonous waste matters, and these two effects induce the 

 condition known as fatigue. A priori, we should expect similar changes to 

 occur in the active nerve-fibre ; almost all the experimental evidence is, how- 

 ever, opposed to this view. The form of activity which is most character- 

 istic of muscle is contraction ; that which is most characteristic of nerve is 

 conduction. In the case of the muscle it is exceedingly difficult to distin- 

 guish between the effects produced by the processes associated with the change 

 of form and those which result from the transmission of the excitatory change. 

 There is little doubt that fatigue is associated with the former ; whether 

 it is associated with the latter is not known. In the case of the nerve, where 

 the transmission process may be studied by itself, conduction does not seem 

 to fatigue (see p. 95). 



Apparently the same may be said of the processes which result in the 

 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 

 1 Schumburg: Archil) JUr Anatomic und Physiologie, 1899, supplement, S. '289. 



