130 CAUSES OF EXHAUSTION. [Book ii. 



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

 strongest stimuli produce no contraction, may he 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 

 manufacture new explosive material out of the comparatively raw 

 material already present in the fibres, or the directly hurtful pro- 

 ducts 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 effects, is shewn 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 solu- 

 tions into their blood vessels. 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 contraction, though it 

 gives rise to carbonic acid and other products of oxidation, is not 

 in itself a process of direct oxidation. 



