126 INTRODUCTION TO GENERAL PHYSIOLOGY 



Inhibition plays a very important part in the functions of the 

 higher nerve centres, as we shall see presently. It is a matter of 

 every-day experience that we can stop a movement suddenly, if 

 necessary. This is effected not only by bringing antagonistic 

 muscles into play, but by inhibition of those producing the move- 

 ment which is required to cease. 



Fatigue 



If we perform the same movement many times in succession, 

 it is possible to arrive at a state in which we cannot make the 

 muscles contract any more. We say that they are fatigued. But 

 we must be careful to distinguish this state from one of exhaustion. 

 The store of material yielding energy to the muscles has not been 

 used up, since an electrical current applied to them directly pro- 

 duces vigorous contraction. Something has happened at a synapse 

 in the course of the nervous arc by which it ceases to be able to 

 conduct. It may be either the using up of some material in this 

 situation, or the production of some chemical substance that has 

 not been removed with sufficient rapidity. But, in any case, 

 the synapse recovers very quickly, and the fact of fatigue has the 

 useful function of preventing the possession of any particular final 

 common path by a reflex for an undue time. But why do we say 

 that it is the synapse and not the whole neurone that is fatigued ? 

 It is because of the remarkable fact that the fatigue of a reflex 

 arc, using a certain final common path as motor neurone, does 

 not affect the use of this same motor neurone by another kind 

 of reflex. The motor neurone itself seems to be very difficult 

 or impossible to fatigue. It is also impossible to fatigue nerve 

 fibres, except in the absence of oxygen, and even this has not 

 been altogether satisfactorily demonstrated. 



Fatigue of voluntary muscle itself, such as can be brought 

 about by prolonged direct stimulation of excised muscle, is un- 

 doubtedly due to the accumulation of lactic acid, which is not 

 oxidised as quickly as it is formed. If the muscle so fatigued is 

 placed in pure oxygen, it recovers to a notable degree as the lactic 

 acid disappears. But recovery is not complete, so that we must 

 admit a partial exhaustion of the store of potential energy, due to 

 inability to replace it in these abnormal conditions. The muscular 

 fatigue of normal exercise is doubtless due in part to some excess 

 of lactic acid. We know that in vigorous exercise the lactic acid 

 is not oxidised completely as rapidly as it is formed, since some of 

 it diffuses into the blood and is excreted in the urine. 



