FATIGUE. 469 



* 



exertion, and are incapable of further action. . This occurs all the 

 more rapidly when anything interferes with the flow of blood 

 through them, such as when we use our arms in an elevated posi- 

 tion ; the simple operation of driving in a screw overhead is soon 

 followed by pain and fatigue in the muscles of the forearm, though 

 the same amount of force could be exerted when the arms are in 

 a dependent posture without the least feeling of fatigue. 



The difficulties of experimenting with the muscles of mammals 

 make the frog-muscle the common material for investigation, and 

 from it we learn the following facts : 



When removed from the body and deprived of its blood supply, 

 the muscle of a cold-blooded animal slowly dies from want of nu- 

 trition. However, if it be placed under favorable circumstances, 

 and allowed perfect rest, it may live twenty-four hours. If it be 

 frequently excited to action, on the other hand, it rapidly loses its 

 irritability, becoming in fact fatigued. 



From a muscle removed from a recently killed animal, we learn, 

 moreover, that even without any blood supply the muscle-tissue is 

 capable of recovering from very well-marked fatigue, if it be 

 allowed to rest for a little time, so that the muscle has in itself 

 the material requisite for its recuperation. 



The first question then is, what causes the loss of irritability 

 which we call fatigue? And the second is, by what means is the 

 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 irrita- 

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

 be rendered active again by washing out its bloodvessels with a 

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



