36 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 proportion of only 12 to 15 per cent. The last ob- 

 server points out that this proportion must vary greatly for dif- 

 ferent muscles and for muscles in different animals, while for the 

 same muscle it will vary with the extent and duration of the con- 

 tractions and other conditions. From experiments made upon dogs 

 in which a measured amount of work was done and in which 

 the energy changes were estimated from the oxygen absorbed 

 and carbon dioxid eliminated, Zuntzf calculates that somewhat 

 more than -j- of the total chemical energy liberated in the muscles 

 may be applied to external work, the other taking the form of 

 heat. Similar experiments made by the same observer^ 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 yielded by the material oxidized in the body may 

 take the form of external work. When the muscular work per- 

 formed was effected by the muscles of the arms and upper part 

 of the body, as in turning a wheel, a smaller yield (25 per cent.) 

 was obtained. It appears from these figures that the muscular 

 machine is an especially efficient one as regards the amount of 

 external work that can be obtained from the oxidation of a given 

 amount of material. Steam engines are said to be capable of 

 yielding only 10 to 15 per cent, of the heat energy of the fuel in 

 the form of mechanical or useful work. 



The Curve of Work and the Absolute Power of a Muscle. 

 The statements in the preceding paragraph prove that the muscle, 

 judged from the standpoint of a machine to do work, compares most 

 favorably in its efficiency with machinery of human construction. 

 But it should be borne in mind that in this as in other respects the 

 properties of cross-striated muscular tissues vary greatly. In some 

 animals or individuals it is a much more efficient machine than in 

 others. This fact is indicated by our general experience regarding 

 variations in muscular strength in different individuals, and is proved 



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



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



j Zuntz and Schumberg, "Physiologie des Marsches," Berlin, 1901. 



