426 THE CANADIAN NATURALIST. [Dec. 



molecular fore. No one in this room believes, I presume, that the 

 result would have been at all different, had the motion been 

 supplied by a steam-engine or a water-wheel. Again, food, as we 

 have seen, is of value for the potential energy it contains, which 

 may become actual in the body. Liebig, in 1842, asserted that 

 for the production of muscular force, the food must first be con- 

 verted into muscular tissue,-^ a view until recently accepted by 

 physiologists.^^ It has been conclusively shown, however, within 

 a few years that muscular force cannot come from the oxidation of 

 its own substance, since the products of this metamorphosis are 

 not increased in amount by muscular exertion. ^^ Indeed, 

 reasoning from the whole amount of such products excreted, the 

 oxidation of the amount of muscle which they represent would 

 furnish scarcely one-fifth of the mechanical force of the body. 

 But while the products of tissue-oxidation 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.^ 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 interme- 

 diate stage of heat. For this reason, the muscle is the most econom- 

 ical producer of mechanical force known. While no machine what- 

 ever can transform all of the energy into motion (the most econom- 

 ical steam-engines utilizing only one- twentieth of the heat) the mus- 

 cle 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 contracts, then, four times as much energy appears as 

 heat as is converted into motion. Direct experiments by Heiden- 

 hain have confirmed this, by showing that an important rise of 

 temperature attends muscular contraction f^ 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 digestive process. 

 The latter refers to the useful work the body may perform. 

 Careful estimates place the entire work of the body 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 



