THE PHENOMENON OF CONTRACTION. 37 



produced, but only the temperature of the muscle. Heat is esti- 

 mated quantitatively in terms of calories. By a calorie is meant 

 the quantity of heat necessary to raise 1 gm. of water 1 C. 

 Knowing the specific heat and weight of muscle, we can readily 

 calculate the number of calories produced. Thus, if a frog's 

 muscle weighing 2 gms. shows a rise of temperature of 0.005 C. 

 from a single contraction the production of heat in calories is given 

 by multiplying the weight of the muscle by its specific heat, 

 0.83, to reduce it to an equivalent weight of water, and this 

 product by the rise in temperature: 2 X 0.83 X 0.005 = 0.0083 

 calorie. The fact that muscular exercise increases the produc- 

 tion of heat in the body is a matter of general observation. Making 

 use of a very sensitive thermo-couple, Hill* has been able to 

 register the production of heat in an excised frog's muscle. In the 

 case of a simple contraction or twitch, the production of the heat 

 is practically instantaneous, indicating an underlying chemical 

 change of explosive suddenness. Analyses of the galvanometric 

 records obtained in these experiments show that there is also a 

 delayed heat production which occurs after the contraction is 

 over. We may say, in fact, that the result of a stimulus causing a 

 contraction is to set up two processes, each of which leads to the 

 development of heat. The heat produced in the first process is 

 detectible during the contraction, while that caused by the second 

 process is subsequent to the act of shortening and, therefore, is 

 not directly connected with the changes leading to contraction. 

 The significance of this delayed heat production will be referred to 

 in connection with the chemical changes of contraction and the 

 theories of muscle contraction. Second. Some electrical energy 

 is developed during the contraction. The means of detecting 

 and measuring this energy will be described in a subsequent chapter. 

 Considered quantitatively, the amount is small. Third. Work 

 is done if the muscle is allowed to shorten during the contraction. 

 By work is meant external or useful work that is, the muscle lifts 

 a weight or overcomes an opposing resistance. If a muscle con- 

 tracts against a weight too heavy to be lifted, or a resistance too 

 strong to be overcome, it does no external work, although, of 

 course, much energy is liberated as heat or, as it is sometimes called, 

 internal work. The work done by a muscle during contraction is 

 measured in the usual mechanical units, by the product of the 

 load into the lift. That is, if a muscle lifts a weight of 40 grams 

 to a height of 10 millimeters, the work done is 40 X 10 = 400 

 gram-millimeters, or 0.4 grammeter. We can in calculations 

 convert external work into heat or internal work by making use 

 of the ascertained mechanical equivalent of heat, according to 

 * Hill, "Journal of Physiology," 40, 389, 1910; 46, 28, 1913. 



