430 THE HUMAN BODY. 



evaporation from the lungs and skin, whenever the respira- 

 tions are quickened, as they are by exercise. It, thus, is very 

 difficult to say how much of the extra water eliminated, 

 from the Body during work is due merely to this cause and 

 how much to increased hydrogen oxidation. 



The conclusion we are led to is, then, that a muscle works 

 by the oxidation mainly, if not entirely, of carbon and 

 hydrogen; much as a steam-engine does: the proteid con- 

 stituents of the muscle answer roughly to the metallic parts 

 of the engine, to the machinery using the energy liberated 

 by the oxidations, but itself only suffering wear and tear 

 bearing no direct proportion to the work done; as an engine 

 may rust, so muscle proteid may and does oxidize, but not 

 to supply the organ with energy for use. Tins conclusion, 

 arrived at by a study of the excretions of the whole Body, is 

 confirmed by the results obtained by the chemical study of 

 a single muscle. A fresh frog's muscle (which agrees in all 

 essential points with a man's) contains practically no car- 

 bon dioxide, yet, made to work in a vacuum, gives off that 

 gas, and more the more it works. Some carbon dioxide is. 

 therefore formed in the working muscle. If the muscle, 

 after contracting as long as it will, be thrown into death 

 rigor it gives off more carbon dioxide; and if taken per- 

 fectly fresh and sent into rigor mortis without contracting 

 it gives off carbon dioxide also, in amount exactly equal to 

 the sum of that which it would have given off in two stages, 

 if first worked and then sent into rigor. The muscle must, 

 therefore, contain a certain store of a carbon-dioxide-yield- 

 ing body, and the decomposition of this is associated with 

 the occurrence both of muscular activity and death stiffen- 

 ing. Similar things are true of the acid simultaneously 

 developed ; the muscle when it works produces some sarco- 

 lactic acid, and when thrown into rigor mortis still more. 

 No increase of urea or kreatin or any similar product of 

 nitrogenous decomposition is found in a worked muscle 

 when compared with a rested one, but the total carbohy- 

 drates are rather less in the former. These facts make it 

 clear that muscular work is not done at the expense of 

 proteid oxidation; and we have already seen (p. 387) that 



