470 MANUAL OF PHYSIOLOGY. 



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

 they have been called. We know also that a very minute quantity 

 of lactic acid injected into the vessels of a muscle destroys its irri- 

 tability, and brings it to a state resembling intense fatigue. Of 

 the new material required for thesustentation of muscle irritability 

 we know that oxygen is amongst the most important, though its 

 supply is not absolutely necessary for the recuperation of a par- 

 tially 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 re- 

 quire an adequate supply of good arterial blood in order to ward 

 off exhaustion ; and, as already explained in speaking of the vaso- 

 motor 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 

 are removed, the more work a muscle can do without becoming 

 exhausted. 



2. Temperature has a marked effect on the irritability as well 

 as form of contraction of muscles. Very low temperatures ap- 

 proaching zero C. diminish the irritability of a muscle, but do 

 not seem to tend towards more rapid exhaustion. High tempera- 

 tures approaching 30 C. increase the irritability, and at the 

 same time rapidly bring about fatigue. At about 35 C. an iso- 

 lated frog's muscle begins to pass into heat tetanus, and soon loses 

 its irritability forever. 



3. Functional activity is accompanied by an increased blood- 

 supply, and a more perfect nutrition of the muscles, and hence 

 use is advantageous for their growth and power; while, on the 

 other hand, continued and prolonged inactivity causes a lowering 



