NUTRITION. 457 



shows that it is, and that to a very large extent, even when 

 the work done is quite moderate and falls within the limits 

 which could be performed by the normal proteid degradation 

 of the Body. Quite easy muscular work doubles the carbon 

 dioxide excreted in twenty-four hours, and in a short period 

 of very hard work it may rise to five times the amount elimi- 

 nated during rest. Since the urea is not increased, or but 

 slightly increased, at the same time, this carbon dioxide can- 

 not be due to increased proteid metamorphosis; and it there- 

 fore indicates that a muscle works by the oxidation of car- 

 bonaceous non-nitrogenous compounds. Since all the carbon 

 compounds oxidized in the Body contain hydrogen this 

 element is also no doubt oxidized during muscular work; but 

 the estimation of the amount so used is difficult and has 

 not been satisfactorily made, because the Body contains so 

 much water ready formed that a large quantity is always 

 ready for increased evaporation from the lungs and skin, 

 whenever the respirations 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';, and that the proteid constituents of the living 

 muscle .substance are essentially the machinery determining 

 in what way the energy shall be spent: they may and do 

 suffer some wear and tear, but this bears no direct proportion 

 to the work done; as a steam-engine may rust, so muscle 

 proteid may and does oxidize, but not to supply the organ 

 with energy for use. This conclusion, arrived at by a study 

 of the excretions of the whole Body, is confirmed by the re- 

 sults 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 carbon 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 can be made to do so, be thrown into death rigor it 

 gives off more carbon dioxide; and if taken perfectly fresh 

 and sent into rigor mortis without contracting, it gives off 

 carbon dioxide also, in amount equal to the sum of that 



