ANIMALS. 119 



udditiuiial quantity in order to obtain enough of those 

 elements which supply heat and force. The nitrogen of 

 this second quantity is wasted, since the animal has no 

 further use for it. Heat and force may be more econom- 

 ically supplied with foods containing less nitrogen. 



Amides are another class of substances containins: 

 nitrogen. They exist to some extent in various kinds 

 of food, particularly in green or immature fodder })lants, 

 and in vegetables. They are less valuable than albumi- 

 noids, since their nitrogen cannot be used to form the 

 tissues of the body. They can only serve, like fats and 

 carbo-hydrates, to produce heat and force. As they are 

 found only in small quantities, and are comparatively of 

 little importance, they have not generally been distin- 

 guished from other elements in making an analysis of 

 foods. 



Protein is a term used to include both the alljumi- 

 noids and the amides, or all parts of food which contain 

 nitrogen. 



3. Fat. — The fatty parts of food correspond to the 

 fats of animals. They are either oxidized to produce 

 heat and mechanical energy, or are stored up in the sys- 

 tem for future use. They are the most valuable class of 

 sul)stances for producing heat and energy. For these 

 purposes they are worth al)out twice as much as the same 

 amount of allxuninoids. As they do not contain nitro- 

 gen, they cannot be used in forming the tissues of the 

 body. 



4. Carbo-hydrates. — The carbo-hydrates of food com- 

 prise such sul)stances as starch, sugar, cellulose, etc. 

 They are composed of carbon, hydrogen, and oxygen. 

 The term hjdrates is applied to them from the Greek 

 word hudor, meaning water, because their hydrogen and 



