CHAP. IL] THE CONTRACTILE TISSUES. 97 



tractions, 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 pro- 

 ducing 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 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 

 solutions into their blood-vessels ; moreover lactic acid and indeed 

 other acids injected into a muscle cause rapid exhaustion; and 

 we may suppose that carbonic acid, with the other substances 

 which after a contraction tend to give rise to an acid reaction, 

 when generated too rapidly to be neutralized by the alkaline 

 lymph in which the fibres are bathed, in part at least determine 

 the exhaustion. 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. 

 It is probably of use as what may be called "intramolecular 

 oxygen " in preparing the explosive material whose decomposition 

 gives rise to the carbonic acid, and other products of contraction. 



F. 



