

FATIGUE. 471 



still by means of a tourniquet or a tightly applied bandage. 

 When removed from the body, a mammalian muscle soon ceases 

 to be irritable and dies, but its functional activity may be renewed 

 by passing an artificial stream of arterial blood through its 

 vessels, and an isolated muscle may thus be made to contract 

 repeatedly for a considerable time. 



On the other hand, the muscle of a cold-blooded animal will 

 remain alive for a long time many hours if kept cool and 

 moist. When its functional activity is about to fade, it may be 

 revived by means of an artificial stream of blood being caused 

 to flow through its vessels, just as in the case of the mammalian 

 muscle. 



Common experience teaches us that even when well supplied 

 with blood our own muscles become fatigued after very prolonged 

 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 sup- 

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

 of nutrition. However, if it be placed under favorable circum- 

 stances, 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 



