MUSCLES AND PHENOMENA OF CONTRACTION. 137 



other products free from the nitrogen. The main nitrogen- 

 free product resulting from this muscular explosion is car- 

 bon dioxide (CO 2 ) , which is then eliminated through the 

 lungs. The nitrogenous products are, however, retained by 

 the muscle and with nitrogen-free foods, such as fats and 

 sugars, may be re-combined to form new living albumen. 

 This albumen may then disintegrate a second time, the 

 nitrogen-free products such as carbon dioxide may be sent 

 out, while the nitrogenous products are again retained, again 

 to be united with fats and sugars and built into new muscle 

 substance. 



Observations made on working muscles are in marked 

 harmony with this view. Thus, every one knows that the 

 harder one works the more carbon dioxide does one breathe 

 out of the lungs ; but very careful experiments have shown 

 that the amount of nitrogen eliminated through the kidneys 

 does not all vary with the work done, a man working hard 

 not eliminating any more nitrogen than one idle. If, of 

 course, all these nitrogenous products formed in the disin- 

 tegration of muscle could be retained and thus used over 

 and over again, somewhat like a brass shell might be re- 

 loaded and so used over and over again in firing, if nothing 

 were lost, there would be no necessity in a full grown man 

 of introducing new albumens into his system. But in these 

 repeated disintegrations, although most of the nitrogenous 

 products are saved and used over and over again, a little 

 incidental loss, a kind of wear and tear incident to all 

 changes results, and it is this little bit which is then sent 

 to the kidneys finally be to eliminated. To replace this 

 wear and tear it is absolutely necessary to put into the blood 

 new nitrogen-containing foods such as albumens. This ex- 

 plains in an unusually clear way how such substances as 

 fats, sugars, and the starches may be gradually utilized in 

 building up muscle substance. It explains further, the im- 

 possibility of doing without albumens, that is, nitrogenous 

 foods, altogether. It explains why the carbon dioxide is in 

 a general way doubled in amount when the work is doubled. 



