CHAP. IL] THE CONTRACTILE TISSUES. 149 



functional activity. Whether functional activity therefore is in- 

 jurious or beneficial depends on its amount in relation to the 

 condition of the muscle. It may be here remarked that as a 

 muscle becomes more and more fatigued, stimuli of short duration, 

 such as induction-shocks, sooner lose their efficacy than do stimuli 

 of longer duration, such as the break and make of the constant 

 current. 



The sense of fatigue of which, after prolonged or unusual exer- 

 tion, we are conscious in our own bodies, is probably of complex 

 origin, and its nature, like that of the normal muscular sense of 

 which we shall have to speak hereafter, is at present not thoroughly 

 understood. It seems to be in the first place the result of changes 

 in the muscles themselves, but is possibly also caused by changes in 

 the nervous apparatus concerned in muscular action, and especially 

 in those parts of the central nervous system which are concerned 

 in the production of voluntary impulses. In any case it cannot be 

 taken as an adequate measure of the actual fatigue of the muscles ; 

 for a man who says he is absolutely exhausted may under excite- 

 ment perform a very large amount of work with his already weary 

 muscles. The will in fact rarely if ever calls forth the greatest 

 contractions of which the muscles are capable. 



Absolute (temporary) exhaustion of the muscles, so that the 

 strongest stimuli produce no contraction, may be produced even 

 within the body by artificial stimulation : recovery takes place 

 on rest. Out of the body absolute exhaustion takes place readily. 

 Here also recovery may take place. Whether in any given case it 

 does occur or not, is determined by the amount of contraction 

 causing the exhaustion, and by the previous condition of the 

 muscle. In all cases recovery is hastened by renewal (natural or 

 artificial) of the blood-stream. 



The more rapidly the contractions follow each other, the less 

 the interval between any two contractions, the more rapid the 

 exhaustion. A certain number of single induction-shocks repeated 

 rapidly, say every second or oftener, bring about exhaustive loss 

 of irritability more rapidly than the same number of shocks 

 repeated less rapidly, for instance every 5 or 10 seconds. Hence 

 tetanus is a ready means of producing exhaustion. 



In exhausted muscles the elasticity is much diminished ; the 

 tired muscle returns less readily to its natural length than does the 

 fresh one. 



The exhaustion due to contraction may be the result : Either 

 of the consumption of the store of really contractile material 

 present in the muscle. Or of the accumulation in the tissue 

 of the products of the act of contraction. Or of both of these 

 causes. 



The restorative influence of rest, in the case of a muscle 

 removed from the circulation, may be explained by supposing that 

 during the repose, either the internal changes of the tissue 



