MUSCLES 279 



circulation may be open to question, but there is no doubt 

 that muscles function best when warm. Experiments with 

 cold blooded animals, like the frog, show that their muscles 

 will contract and relax five times as rapidly when warm as 

 when cold. When cooled to 40 F., they will not contract at 

 all, a condition known as cold rigor. On the other hand, if 

 raised to a temperature much above 104 F., they become 

 stiff and will not contract, a condition called heat rigor. 

 The effect of heat upon the muscles of warm blooded animals 

 is essentially the same. The numbness of the human muscles 

 when chilled excessively is an illustration of the effect of cold 

 which has come within the experience of almost everyone. The 

 muscles of warm blooded animals can contract at a somewhat 

 higher temperature than can those of the cold blooded variety, 

 and it does not take so low a temperature to stop their action. 



Fatigue of Striped Muscles. When one is tired and it be- 

 comes more and more of an effort to make the muscles of the 

 body contract and accomplish tasks, it is not primarily due 

 to the fatigue of the muscles themselves but to that of the 

 nerves. The muscle itself, however, on account of changes 

 which take place in it after doing a large amount of work may 

 become fatigued. The factors which enter into the phe- 

 nomena of fatigue are not all thoroughly understood, but a 

 common supposition that weariness in muscles is relieved by 

 merely feeding them, by stopping for a meal, for instance, is 

 certainly erroneous. Not only is the food not at the dis- 

 posal of the muscle for a period of several hours after it is 

 eaten, but it has been proved that a muscle can recover to 

 a considerable extent from fatigue, even when no blood at all 

 is flowing through it. 



Main Voluntary Muscles in the Body. Figure 138 shows 

 the distribution of some of the principal muscles on the ex- 

 terior of the body. There are more than three hundred 

 voluntary muscles, some large, some small, some short and 

 ome long. They are commonly enlarged in the middle and 





