96 THE AMINO ACIDS 



carbohydrates. "The chief reason why the nitro- 

 genous splitting products produced by the digestive 

 enzymes are universally assumed to be reconverted 

 into albumin is the teleological one. The food proteins 

 are tissue builders and the organism must not waste 

 them. The fact that the muscle tissues of normal men 

 do not increase when the protein of food is increased, 

 but that all of the nitrogen of such protein is at once 

 eliminated, has not been sufficiently considered in this 

 connection. The only adequate teleological explana- 

 tion of this fact is that this nitrogen is not needed for 

 the building of new tissues. It is not needed because 

 the organism cannot enlarge indefinitely, and because 

 after it has attained its full growth the daily waste of 

 tissue is small. Yet when more nitrogen than the 

 organism needs is furnished with the food, we find 

 that the protein containing it is still absorbed up to 

 the limit of the digestive capacity." "The greater part 

 of the protein furnished with standard diets like Voit's, 

 i.e., that part representing the exogenous metabolism, 

 is not needed, or to be more specific, its nitrogen is not 

 needed. The organism has developed special facilities 

 for getting rid of such excess of nitrogen so as to get 

 the use of the carbonaceous part of the protein con- 

 taining it. The first step in this process is the decom- 

 position of protein in the digestive tract into proteoses, 

 amido acids, ammonia, and possibly urea. The hydro- 

 lytic decompositions are carried further in the mucous 

 membrane of the intestines, and are completed in the 



