HEAT IN MUSCULAR CONTRACTION. 397 



certain limits the Shortening power increases with the load to be lifted. 1 

 This he attributes also to the fact that extension in all these substances 

 increases the property of double refraction. He concludes, therefore, that the 

 cause of the development of contractile stress in living muscle is to be sought 

 for in the warming of its doubly refractive particles. Heat sufficient for this 

 purpose he thinks can be produced by the transformation of the chemical 

 potential energy of the constituents of muscle, which follows excitation. 

 Having regard to the " infinitesimally small number of particles concerned 

 in a contraction," he can see no reason why the difference of temperature 

 between the heat-producing and the work-producing particles should not be 

 sufficient to account for the existence of such a relation between the amounts 

 of work heat and of translated heat as would be required by the second 

 thermodynamic law. He does not maintain that his catgut and indiarubber 

 models represent all that happens in living muscle, but points out that they 

 present phenomena which are among the most important of those of muscular 

 contraction. 



From the preceding considerations it will be seen that, in regarding 

 the origin of muscular force as a thermodynamic process, Engelmann 

 recognises that the development of high temperature at the foci of 

 origin of mechanical energy in muscle is a necessary link in the process. 

 Before we accept Tick's conclusion that no such link is wanted, 

 let us remember that the view usually entertained by physiologists, 

 that at the moment of excitation the attraction of carbon and oxygen 

 atoms is directly converted into axial tension, involves difficulties of 

 which the character can be best judged by reading the pages in which 

 Fick himself sets forth what he regards as the simplest molecular 

 mechanism by which the transformation of chemical into mechanical 

 energy can be effected. 2 The question how the former is converted into 

 the latter is rather physical than physiological. The physiological 

 chemist may proceed with his inquiries as to the sources of chemical 

 energy, and the experimental physiologist pursue the lines of investiga- 

 tion which will be sketched in the next section, without waiting for the 

 decision of the question whether muscular contraction is or is not a 

 thermodynamic process. It may be surmised that, whenever that 

 question is answered, it will be by following the indications given by 

 Engelmann in his Croonian Lecture, namely, by the investigation of 

 an analogous process in a non-living structure. 



THE PRODUCTION OF HEAT IN MUSCULAR CONTRACTION. 



The reasons which compel the physiologist, in studying the mechanical 

 phenomena of muscular contraction, to have recourse, in the first instance, 

 to the muscles of the frog, have even greater force when his purpose is 

 to investigate the changes of temperature which are associated with 

 these phenomena. For here the quantities to be measured are very 

 small, and the experiments require an even greater degree of precision 

 than those which relate to the mechanical effects of stimulation. This 

 can only be secured by repeating the observations a sufficient number of 

 times under constant conditions. In the muscles of a homceothermic 

 animal, such constancy is scarcely attainable; for the necessity of 

 maintaining the circulation introduces sources of error which it is hard 

 to guard against. 



1 Croonian Lecture, p. 422. 



2 Fiok, Arch.f. d. ges. Physiol., Bonn, 1893, Bd. liii. S. 611. 



