MANUAL OF CATTLK-FEEBIIfG. 219 



deatli, and tlie rigor mortis is caused by tlie solidifieation 

 of tlie jelly-like viyosin^ which is albo one of the buhstances 

 formed in mn&cular action. 



4. A mobt important condition of contiimed muscular 

 activity is found in the capacity which the body has of 

 storing up witlun itself, during rest, a reserve of force, to 

 be used later as demands are made upon it. 



After working for a certain time w^e experience a feeling 

 of fatigue, or, if the e^vertion be continued long enough, of 

 exhaustion, and require a period of rest before the muscles 

 are capable of again performing work. The same thing is 

 true of the involuntary muscles. Even those which, like 

 the heart and the respiratory muscles, seem to work con- 

 tinually, really have relatively long intervals of rest between 

 each exertion. Thus, the heart is calculated to be at rest 

 for about one-third of the time. Work is oxily possible 

 when alteraated with periods of rest ; and while the ner- 

 vous system has undoubtedly much to do with the need for 

 1 est, there is no doubt that it is also required by the nms- 

 cles, to enable them to repair the waste occasioned by 

 work. 



This well-known fact is sufficient to show that the force 

 of muscular exertion is not produced by a direct combus- 

 tion of mnscle substance by means of tlie oxygen of tlie 

 blood, as coal is burned under a boiler, since, if this were 

 the case, there would be no reason why it should not go 

 on indefinitely. It is the sudden utilization of latent en- 

 ergy wdiich has been laid up during rest. 



That the seat of this latent energy is in the muscles 

 themselves is shown by the fact that they are capable of 

 contraction for a time after their blood-supply has been 

 cut off, or even after their removal from the body. A 

 f i'og^s heart, when removed from the body and freed from 



