472 MANUAL OP PHYSIOLOGY. 



muscle enabled to return to a state of functional activity? We 

 know that the mere life of a tissue must be accompanied by certain 

 chemical changes which require (1) a supply of fresh material, 

 and (2) the removal of certain substances which are the outcome 

 of the tissue change. In the case of muscle this chemical inter- 

 change is constantly but slowly going on between the contractile 

 substance and the blood. When the muscle contracts much more 

 active, and probably different, changes go on in the contractile 

 substance, more new material being required, and more effete 

 matter being produced. It is probable that the accumulation of 

 these effete matters is the more important cause of the loss of irri- 

 tability in a muscle, for a frog's muscle when quite fatigued may 

 be rendered active again by washing out its blood vessels with a 

 stream of salt solution of the same density as the serum (.6 per 

 cent. NaCl), and thus removing the injurious "fatigue stuffs," as 

 they have been called. We know, also, that a very minute quan- 

 tity of lactic acid injected into the vessels of a muscle destroys 

 its irritability, and brings it to a state resembling intense fatigue. 

 Of the new material required for the sustentation of muscle 

 irritability we know that oxygen is among the most important, 

 though its supply is not absolutely necessary for the recuperation 

 of a partially exhausted, isolated frog's muscle. 



The slow recovery of a bloodless muscle from fatigue may be 

 explained by supposing time to be necessary for the reconstruction 

 of new contractile material, and probably, also, for a secondary 

 change to take place in the effete materials by which they become 

 less injurious. 



When working actively, then it is obvious that the muscles 

 require an adequate supply of good arterial blood in order to 

 ward off exhaustion ; and, as already explained in speaking of 

 the vasomotor influences, a muscle does in reality receive a much 

 greater supply of blood when actively contracting than when in 

 the passive state. 



The irritability of a muscle and the rate at which it becomes 

 exhausted may be said to depend upon : 



1. The adequacy of its blood supply: the better the supply of 

 new material and the more quickly the injurious effete materials 



