30 ADVANCED PHYSIOLOGY 



their digestibility and final values. Human beings have been 

 used in these experiments so that the results are of great value. 



Proteids as Fuel. To keep an engine running it is not 

 enough that it be kept in repair; there must be a fire in 

 the fire-box. In one respect the body differs from an engine : 

 while the iron of which a locomotive is made cannot be used 

 as fuel, the proteids of which the body is made can be 

 burned. By being burned we mean here, united with oxygen, 

 a phenomenon which chemists call oxidation. Foods in the 

 body unite with oxygen and this may be called " burning," 

 though the oxidation is not so rapid in the body as in an 

 actual fire, and there is, of course, no flame: but the union 

 with oxygen is similar, heat is developed in a similar manner 

 and the final results are much alike. Proteids have a double 

 value: (1) they are burned in the body and (2) they build up 

 tissue. When all the proteid eaten is not needed to build up 

 or to repair the tissue, the rest may be burned to furnish heat 

 and force. 



In using proteids for fuel there is, however, one disadvan- 

 tage which limits their value as food. After any fuel is 

 burned, certain waste products always remain. When a fire 

 burns in a locomotive, a vast amount of smoke and gas 

 arises and passes off through the smoke-stack, while ashes are 

 left to be raked down through the grate and thrown away. 

 The fire will not continue to burn unless these waste ashes 

 and gases are removed. In the body, too, as a result of the 

 burning, gases and other wastes arise. The gases pass off 

 readily enough through the lungs. But there is a more 

 troublesome residue that corresponds to the ashes. We have 

 noticed that proteids contain some of the chemical element, 

 nitrogen. After the burning of the proteid in the body, 

 this nitrogen becomes a waste product and can be disposed 



