92 THE HUMAN MECHANISM 



and then burned as fuel, giving power to an engine. The 

 occasional use of Indian corn or wheat for fuel, in the West, 

 the employment of hams and bacon as fuel by steamers short 

 of coal, the explosion of flour dust in mills, and similar phe- 

 nomena further illustrate by the teachings of experience the 

 fact that these foods are rich in energy, or power. 



When we say that the food must supply power to the 

 body, we mean that the power which it contains must be 

 available to the body. A lump of coal may be a source of 

 power, as is shown by its use in a locomotive ; but a lump 

 of coal would be of no use as food, because the body has 

 no such means of burning it as has the engine. Again, 

 nitroglycerin contains chemical elements needed in the food; 

 but although when exploded in a dynamite cartridge it 

 may furnish power enough to shatter heavy armor plate, 

 its energy is not available to the body. 



Thus, to recapitulate, (a) food makes good the loss of 

 living substance in the body; (&) it supplies material for 

 growth and for the manufacture of the secretions of the 

 body ; and (c?) it supplies power for the work which the body 

 is to do. It also performs one more important function, 

 which will be more clearly understood hereafter; for (d) by 

 its oxidation food provides the heat usually required to keep 

 up the body temperature. The detailed consideration of this 

 subject, however, must be postponed to Chapter XII. 



4. Chemical composition of foods ; nutrients. The human 

 race has learned by long experience that certain things meet 

 the demands of the body for food, and that other things 

 do not. Perhaps no animal uses so many different materials 

 as man in satisfying sensations of hunger and thirst. Some 

 foods are taken from the animal and some from the vegetable 

 kingdom, and their variety is greatly increased by special 

 modes of preparation. But however numerous the foods 

 from which we prepare the dishes served at different meals, 

 chemical analysis shows that the essential constituents of all 



