xi] MILK AS A FOOD 165 



functions. To the members of each of these different 

 groups the name nutrient is given. Of these 

 nutrients there are a large number; but they can 

 all be classified under a few groups, this classification 

 being based upon their chemical composition. Thus 

 we have the protein group, the characteristic feature 

 of which is the possession of nitrogen in its com- 

 position ; the carbohydrates, of which starch and 

 sugar are typical members ; and the fats. The first 

 group may be subdivided into several smaller groups, 

 such as albuminoids, gelatinoids, amides, and nitro- 

 genous extractive substances. Lastly, we have another 

 class of nutrients, which are also, in their way, of 

 importance, viz. mineral nutrients. Now, while all 

 of these groups of different nutrients perform 

 important functions, they cannot be described as of 

 equal importance. 



Functions of Food. The functions of food are 

 various. Inasmuch as the body is constantly 

 experiencing change, is always undergoing, as we 

 say, a certain amount of "wear and tear," and is 

 also, at certain periods of life, viz. in youth, under- 

 going a process of growth, one obvious function of 

 food is to make good this wear and tear, and to build 

 up the tissue, bones, etc., out of which the body is 

 formed. Since animal life, in building up its tissue, 

 is only able to make use of foods more or less similar 

 in their nature to its tissue, it follows that the 



