METABOLISM AND FOOD 59 



part of the ration. A ration can be complete or 

 one-sided according to whether it brings to the 

 animal all the nutrients which it requires, or whether 

 some are present in insufficient quantities, or not at 

 all. A food which contained too little protein or 

 mineral matter would be termed one-sided. An 

 animal receiving such a ration, even in large quanti- 

 ties, would be obliged to draw protein, or mineral 

 substances, from its own body, and would suffer 

 from " protein " or " mineral " hunger. 



In the following sections the action of the 

 nutrients, either alone or mixed with one another, 

 will be discussed, and then the complete food-stuffs 

 themselves. 



(a) The effect of protein. 



1. The metabolism of protein and fat when only 

 protein is given. 



Through the action of ferments and bacteria in 

 the digestive organs the food proteins are changed 

 for the most part into simpler substances — albu- 

 moses, peptones (pp. 23, 24). These are of a non- 

 protein character, and are easily taken up by the 

 cells of the alimentary canal. A small portion ol 

 the food protein is, however, absorbed without 

 undergoing this preliminary change. Most prob- 

 ably the simpler substances which are absorbed 

 are changed again into protein in the walls of the 

 stomach and intestines, for although the intestine 



