HEAT-PRODUCTION IN ACTIVE MUSCLE. 577 



mated contractions are, accordingly, attended with the generation of a 

 smaller amount of heat than corresponds to that developed by two single 

 successive contractions. 



As the stimulus becomes stronger, heat-production increases, in the case of 

 isometric contractions proportionately to the degree of tension; that of isotonic 

 contractions at first more rapidly than the height of the lift, but with strong 

 stimuli proportionately to the latter. Even if the height of the lift, the strength 

 of the stimulus, and the tension of the contracting muscle remain the same during 

 successive contractions, the muscle nevertheless generates more heat during the 

 first than during the following contractions. The amount of heat generated also 

 depends upon the character of the stimulus employed; thus, a muscle tetanized 

 by slow shocks generates more heat than one contracted by rapid shocks. 



2. The development of heat depends upon the tension of the muscle; it 

 increases with increase in tension. If the muscle be prevented from con- 

 tracting by fixation of its extremities, the maximum of heat-production 

 takes place during stimulation, and the more quickly the more rapidly 

 the stimuli succeed one another. Such a condition arises during tetanus, 

 in which the violently contracted muscles mutually oppose each other. 

 Therefore, a marked development of heat has been observed in con- 

 nection with this disease. Dogs thrown into a state of continuous 

 tetanus by electrical stimulation or by the induction of spasm die in 

 consequence of elevation of their bodily temperature to a fatal height 

 (44 or 45 C.). This large production of heat is attended with a con- 

 siderable degree of acidity and the formation of alcoholic extractives 

 in the muscular tissue. 



In the case of isometric tetanus the metabolism and heat-production increase 

 more rapidly than the tension as the stimulus becomes stronger. The continuous 

 maintenance of tension in the muscle on the one hand, as well as the contraction 

 of the muscle with a small amount of work without considerable tension, never- 

 theless requires only relatively little metabolism for the generation of heat, as 

 compared with the work, which is essentially proportional to the consumption of 

 combustible material. If the stimulated muscle be so fixed that it cannot con- 

 tract, and if it then by releasing its lower extremity be permitted to contract 

 and lift a weight, an additional amount of chemical potential energy will be trans- 

 formed for the performance of this latter task. 



3. Heat -production diminishes as fatigue increases, and it again in- 

 creases during recovery. The muscle becomes fatigued earlier in its 

 production of heat than in its performance of work. 



4. In a muscle normally supplied with circulating blood the produc- 

 tion of heat, and also the mechanical performance of work, takes place 

 much more energetically than in a muscle whose circulation is inter- 

 rupted. Recovery following fatigue also takes place under such con- 

 ditions more rapidly and completely. 



The total amount of work and heat in a muscle must always be equivalent to 

 the transformation of a corresponding amount of chemical potential energy. Of this 

 the portion that is transformed into work will be the larger the greater the force that 

 is opposed as a result of the contraction of the muscle. In the latter event this 

 equals about one-fourth of the transformed potential energy. If the resistance be 

 less, the work performed is a smaller fraction of the transformed potential energy. 



At a high temperature, therefore probably in the febrile state, muscle exhibits 

 greater metabolism for the generation of increased amounts of heat, without 

 increase in the work performed. 



In man, the production of heat in muscles made to contract by electrical 

 stimulation can be appreciated through the skin. It was observed by Landois 

 also when voluntary movements were executed. Venous blood flowing from 

 a contracting muscle acquires a higher temperature than the arterial blood by 

 as much as 0.6 C. as a result of energetic action. 

 37 



