CHAPTER IV. 



FOODS. 



IN studying respiration we saw in some of its bearings the status of 

 oxygen as one of the indispensable supplies of the organism. The dis- 

 cussion demonstrated that living protoplasm requires oxygen for its 

 metabolism, and it showed furthermore how the protoplasm which makes 

 up the human body gets its oxygen, and what finally becomes of it. In 

 the present chapter we begin the description of the same sort of acquiring 

 and disposing process for the remainder of the material provisions on 

 which the body subsists and in the metabolism of which it lives. Oxygen 

 is a sort of food, but as it is a gas it requires special means for receiving 

 it and for excreting the products of its use. All food other than oxygen 

 is either solid or liquid in its density, and because of this difference has 

 required the evolution and action of a mechanism peculiar to itself. 



In describing the natural history of food proper and its organic use 

 we need to understand the nature of the various classes of alimentary 

 substances and their general relations to the organism. 



THE GENERAL NATURE OF A FOOD. 



A food or nutrient may be defined as anything which, taken into an 

 organism, is capable of supplying it with tissue or of producing energy. 

 Let us as a preliminary to our study of the nutrients see of what materials 

 an average animal body is made up, in order that we may know in 

 advance what substances will be required in the food for supplying tissue, 

 whether in the growing young animal, or in the replacement of the nor- 

 mal loss by wear and tear. We have already learned what chemical 

 elements combine to make up an animal organism. 



The Animal Organism's Proximate Principles. The table on page 30 

 is given partly as a place for reference to the names and classification 

 of the chief constituent compounds so far known to exist in an average 

 animal body of high development, and partly to suggest the need of very 

 various materials in the food which is to reproduce all these substances 

 as they bit by bit wear away. The iron, manganese, ammonia, and 

 carbon dioxide mentioned, and probably the nitrogen and hydrogen, exist 

 only to a minute extent in free uncombined form in the body. The others, 

 and doubtless many more proteid forms, and especially enzymes and 

 salts of the alkaline metals, seem to be present as such in large or small 

 quantities. The proportion of water given, 65 per cent., is constantly 



