THE ENERGY OF MUSCLE AND NERVE. 115 



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 con- 

 sumption 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 contrac- 

 tion ; 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 manufacture new explosive material out of 

 the comparatively raw material already present in the fibres, or the directly 

 hurtful products *of the act of contraction undergo changes by which they 

 are converted into comparatively inert bodies. A stream of fresh blood 

 may exert its restorative influence not only by quickening the above two 

 events, but also by carrying off the immediate waste products while at the 

 same time it brings new raw material. It is not known to what extent each 

 of these parts is played. That the products of contraction are exhausting in 

 their effect, is shown by the facts that the injection of a solution of the 

 muscle-extractives into the vessels of a muscle produces exhaustion, and that 

 exhausted muscles are recovered by the simple injection of inert saline 

 solutions into their bloodvessels. But the matter has not yet been fully 

 worked out. 



One important element brought by fresh blood is oxygen. This, as we 

 have seen, is not necessary for the carrying out of the actual contraction, and 

 yet is essential to the maintenance of irritability. The oxygen absorbed by 

 the muscle apparently enters in some peculiar way into the formation of that 

 complex explosive material the decomposition of which in the act of contrac- 

 tion, though it gives rise to carbonic acid and other products of oxidation, 

 is not in itself a process of direct oxidation. 



THE ENERGY OF MUSCLE AND NERVE, AND THE NATURE OF 

 MUSCULAR AND NERVOUS ACTION. 



85. We may briefly recapitulate some of the chief results arrived at 

 in the preceding pages as follows : 



A muscular contraction itself is essentially a translocation of molecules, 

 a change of form, not of bulk. We cannot say, however, anything definite 

 as to the nature of this translocation or as to the way in which it is brought 

 about. For instance we cannot satisfactorily explain the contraction between 



