36 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



The Proportion of the Total Energy Liberated that may 

 be Utilized in Work. All of the energy liberated in the muscle 

 has its origin in the chemical changes that follow upon stimulation. 

 We assume that these changes are such that complex molecules 

 are broken down, with the formation of simpler ones, and that 

 some of the so-called chemical or internal energy that holds together 

 the atoms in the complex molecule is liberated and takes the three 

 forms described above. The chemical changes occurring in the 

 muscle during contraction are complex and not entirely understood, 

 but the significant ones from our present standpoint are oxidations 

 which destroy some of ^ the material in the muscle, with the forma- 

 tion of carbon dioxid and water and the liberation of heat. It is 

 a matter of interest to inquire as to the proportion of the total 

 heat energy which may be converted into useful work and the 

 conditions under which the optimum amount of work may be 

 realized. Regarded from this standpoint, the muscle may be 

 considered as a piece of machinery comparable, let us say, to a gas 

 engine. In the latter the heat generated by the explosive chemical 

 change is converted partially into external work by a properly 

 adapted mechanism and in a well-constructed engine as much 

 as 15 to 25 per cent, of the total energy may be obtained as work. 

 In the muscle there is also a mechanism of some kind, not as yet 

 understood, by means of which a part of the energy liberated 

 may be converted into work. Experiments made by Fick with 

 frogs' muscles indicate that the proportion of the total energy 

 which under optimum conditions may be utilized as work is, in 

 round numbers, from 25 to 30 per cent. Chauveau,* in experiments 

 made upon the elevator of the upper lip in the horse, found a pro- 

 portion of only 12 to 15 per cent. The last observer points out 

 that this proportion must vary greatly for different muscles and 

 for muscles in different animals, while for the same muscle it will 

 vary with the extent and duration of the contractions and other 

 conditions. From experiments made upon dogs in which a meas- 

 ured amount of work was done and in which the energy changes 

 were estimated from the oxygen absorbed and carbon dioxid 

 eliminated, Zuntz f calculates that somewhat more than ^ of the 

 total chemical energy liberated in the muscles may be applied to 

 external work, the other taking the form of heat. Similar ex- 

 periments made by the same observer J upon men have indicated 

 that the muscles work most economically in lifting the weight 

 of the body, as in mountain-climbing. In this form of muscular 

 work he estimates that from 35 to 40 per cent, of the heat energy 



* Chauveau, "Le travail musculaire, etc.," Paris, 1891. 



t Zuntz, "Archiv f. d. gesammte Physiologic," 68, 191, 1897. 



J Zuntz and Schumberg, "Physiologic des Marsches," Berlin, 1901. 



