150 CAUSES OF EXHAUSTION. [BOOK i. 



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. 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. 



