16 HUMAN PHYSIOLOGY 



expenditure of its energy, which we may define as the 

 power of doing work. Obviously, if the machine's 

 work is prolonged to a certain extent, it will require 

 to be supplied with additional material out of which 

 it will repair its own waste, and from which it will 

 derive a new store of working power. In the case 

 of an ordinary engine, the power of doing work is 

 derived from the combustion of coal and the con- 

 version of water into steam, whilst there is also to 

 be considered a certain amount of actual wear and 

 tear of the engine itself. Both sources of waste 

 have to be reckoned with, and have to be made good 

 by repairs to the engine and by a constant supply of 

 coal out of which its energy is developed. Now, our 

 bodies are closely related to the engine in respect of 

 their work and repair. Our frames do not merely 

 demand food in order that the waste of the body may 

 be repaired and replaced by new material, but they 

 also require nutriment that they may develop new 

 supplies of energy for the work they have to perform. 

 In these considerations is found the answer to the 

 question, "Why do we eat our dinner?" We take 

 food in other words because out of the material 

 thus supplied we repair our bodily waste, and we 

 develop new stores of working power. 



ABOUT FOODS. Bearing the comparison with the 

 engine in mind, we naturally divide the foods on 

 which we exist into two chief classes. These may 

 be described as, first, body-building or tissue-forming 

 foods, otherwise called nitrogenous foods ; and second, 

 energy -producing foods these last being also termed 

 non-nitrogenous articles of diet. This division is a 

 perfectly natural one. A " nitrogenous " food is 

 one which includes the element nitrogen in its 

 composition. The presence or absence of this 



