242 Transactions of the American Institute. 



mnscular force cannot come from the oxydation of its own substance, 

 since the products of this metamorphosis are not increased in ainount 

 by muscuUir exertion. * Indeed, reasoning from tlie whole amount 

 of sucli products excreted, the oxydation of the amount of muscle 

 which they represent would furnish scarcely one-fifth of the mechani- 

 cal force of the body. But while the products of tissue oxydation do 

 not increase with the increase of muscular exertion, the amount of 

 carbonic gas exhaled by the lungs is increased in the exact ratio of 

 the work done.f No doubt can be entertained, therefore, that the 

 actual energy of the muscle is simply the converted potential energy 

 of the carbon of the food. A muscle, therefore, like a steam engine, 

 is a machine for converting the potential energy of carbon into motion. 

 But unlike a steam engine, the muscle accomplishes this conversion 

 directly, the energy not passing through the intermediate stage of 

 heat. For this reason, the muscle is the most economical producer of 

 mechanical force known. While no machine whatever can trans- 

 form all of the energy into motion, the most economical steam 

 engines utilizing only one-twentieth of the heat, the muscle is -able 

 to convert one-fifth of the energy of the food into work.:}: The other 

 four-fifths must, therefore, appear as heat. Whenever a muscle con- 

 tracts, then four times as much energy appears as heat as is con- 

 verted into motion. Direct experiments by Heidenhain have con- 

 firmed this, by showing that an important rise of temperature attends 

 muscular contraction ; § a fact, however, apparent to any one who has 

 ever taken active exercise. The work done by the animal body is of 

 two sorts, internal and external. The former includes the action of 

 the heart, of the respiratory muscles, and of those assisting the diges- 

 tive process. The latter refers to the useful work the body may per- 

 form. Careful estimates place the entire work of the bod}' at about 

 800 foot-tons daily; of which 450 foot-tons is internal, 350 foot-tons 

 external work. And since the internal work ultimately appears as 



* Voit, E., Untcrsiichun;:;en iiber den Einfluss dee Kochsalzes, dcs Knffees, und der Mnskelbewe- 

 guncen auf den StofTwechBcl ; Munich, 1860. Smith, E., PhiloBophical Transactions, 1861, 747. 

 Fick, A., and Wisliccnus, J., Thil. Mag., IV., xsxi., 485. Frankland, E.. toe. cil. Noyca, T. R., 

 American Journal Medical Sciences, Oct., 18C7. Parkus, E. A., Proceedings Jloyal Societj', xv., 339 ; 

 xvi., 44. 



+ Smith, Edward, Pliilosopliical Transactionp, 1859, 709. 



X Authorities differ as to the amount of energy converted by the steam engine. (See Note 16.) 

 Compare Marshall, op. cit., p. 918. " Whilst, therefore, in an engine one-twentieth part only of the 

 fuel consumed is utilized as mechanical power, one-flfth of the food absorbed by man is so appropria- 

 ted." 



§ ncidenhaia, Mechanische Leistung Warmccutwickclung und Stoffumstaz bei der Muskclthatig- 

 keit. Brcslan, 18G4. See also Haughton, Samuel, on the relation of food to work, published in 

 "Medicine in Modern Times," London, 1869, Macmillan & Co. 



