THE PHENOMENON OF CONTRACTION. 37 



0.001° to 0.005° C. by a single contraction. The amount of heat 

 Uberated is estimated in terms of calories, the accepted heat-unit. 

 By a calorie (gram-calorie) is meant the quantity of heat necessary 

 to raise the temperature of 1 gram of water 1° C. If we know the 

 weight of a muscle, its specific heat as compared with water, and 

 the extent to which its temperature is raised by the contraction, 

 the calories of heat evolved are easily calculated by multiplying 

 together these three factors. For example, if a frog's muscle 

 weighing 2 grams shows a rise of temperature of 0.005° C. from a 

 single contraction, and if we take the specific heat of muscle as 0.83, 

 then the calories produced are equal to 2 X 0.83 X 0.005 or 0.0083 

 cal. Multiplying the weight of the muscle by its specific heat gives 

 its equivalent weight in water so far as capacity for heat is con- 

 cerned. The rise of temperature in the muscle cannot well be de- 

 termined by an ordinary thermometer. It is customary to use for 

 this purpose " some form of thermopile which is applied closely 

 to or surrounds the muscle. The heating of the junctions of the 

 thermopile gives rise to an electrical current which is measured by 

 a suitable galvanometer. The deflections of the galvanometer in 

 turn are valued in degrees of temperature by previous calibration 

 with known sources of heat. Careful observations upon the time 

 relations of the heat production* show that this occurs mainly at 

 the beginning of the contraction and after the contraction is over. 

 The first evolution of heat is sudden and is coincident with the 

 initial period of shortening. It is interesting to note that this out- 

 burst of heat takes place when the muscle is stimulated in the 

 absence of oxygen. Probably, therefore, the chemical reaction 

 through which it is produced is not of the nature of an oxidation. 

 The second or delayed heat production occurs after the contrac- 

 tion and relaxation are over and takes place only in the presence 

 of oxygen. It is assumed, therefore, to be due to an oxidation 

 reaction in which probably some of the nutritive material of the 

 muscle is oxidized. The energy liberated by this change appears 

 partly as heat, but in part is converted into some form of potential 

 energy whereby the muscle is restored to its previous condition. 

 It would appear, therefore, that the stimulus applied to a muscle 

 is responsible directly for a chemical change of a non-oxidative 

 character in which energy for the contraction is liberated together 

 with a large by-product of heat, and indirectly for a later oxidative 

 reaction in which energy is restored to the muscle, but in which 

 there is also a by-product of heat. In neither case is the heat 

 directly useful to the muscle so far as contraction is concerned, but 



* Hill, "Journal of Physiology," 46, 28, 1913, and Hill and Hartree, ibid., 

 54, 84, 1920. 



