212 THE PHYSIOLOGY OF MUSCULAR WORK 



stituents coagulate ( 6 ). When an excised voluntary muscle of a 

 mammal is heated, it begins to shorten at 43, shortens more at 

 47, and again when the temperature reaches 58. The excita- 

 bility of the muscle is destroyed when the shortening occurs at 

 47, and this corresponds with the temperature at which para- 

 myosinogen is coagulated. The internal temperature of a healthy 

 mammal is about 36 to 37, and it is well known that life is 

 endangered when owing to fever or some other cause the tem- 

 perature of the body rises to 44. Birds are also warm-blooded 

 animals, but their internal temperature is several degrees above 

 that of mammals ; the temperature of a sparrow is 42, that of a 

 hen 41 to 43. There must therefore be some difference in the 

 heat-rigor of the proteins of their muscles ; experiments on this 

 point have proved that the coagulation of the protein does not 

 occur until the temperature is raised to 53. 



The temperatures at which coagulation begins are not rigidly 

 fixed, for prolonged heating at a lower temperature will produce 

 that change of state ; it is necessary also to remember that the 

 excised muscles are devoid of a circulation of blood, and no doubt 

 are in a condition which is not the same chemically as the normal 

 muscle. These criticisms do not invalidate the practical importance 

 of heat-rigor in connection with the effects of hyperpyrexia and 

 the pathological changes which occur in muscle during prolonged 

 fever. It is necessary, however, to exercise care in any argument 

 from the part to the whole body. 



The next constituents of muscle which must be considered are 

 the extractives ; these can be divided into two classes, the non- 

 nitrogenous and the nitrogenous. The former group is repre- 

 sented by glycogen, dextrin and sugars, inosite, fat, and lactic acid. 



Glycogen or animal starch (C (J H 1() 0,.) n is a polysaccharide which 

 appears to be intimately associated with the source of muscular 

 energy. The amount of glycogen which can be extracted from a 

 muscle varies according to its activity, and this fact will explain 

 the lack of agreement in the percentages found by different ob- 

 servers. Resting muscle contains from 0*1 to 2-5 per cent, of 

 glycogen. The amount not only varies in different animals, but 

 also in different muscles of the same animal : the more active 

 muscles contain less. By prolonged activity the glycogen of the 

 muscles can be removed, but it disappears from the liver first ; 

 poisonous doses of strychnine which cause violent convulsions 



