96 VARIATIONS OF IRRITABILITY. [BOOK i. 



flows through the muscle, and this increased flow continues for 

 some little while after the contraction of the muscle has ceased. 

 But, apart from the blood-supply it is probable that the ex- 

 haustion caused by a contraction is immediately followed by a 

 reaction favourable to the nutrition of the muscle ; and this is a 

 reason, possibly the chief reason, why a muscle is increased by use, 

 that is to say, the loss of substance and energy caused by the 

 contraction is subsequently more than made up for by increased 

 metabolism during the following period of rest. 



Whether there be a third factor, whether muscles for in- 

 stance are governed by so-called trophic nerves which affect their 

 nutrition directly in some other way than by influencing either 

 their blood-supply or their activity, must at present be left 

 undecided. 



A muscle, even within the body, after prolonged action is 

 fatigued, i.e. a stronger stimulus is required to produce the same 

 contraction; in other words, its irritability may be lessened by 

 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 exertion, 

 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 under- 

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

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

 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 con- 



