THE CONSTITUENTS AND USES OF FOOD. 9 



and parcel of the various bodily organs, in the other the losses 

 of the structure or mechanism itself are neither essential nor 

 large. In both structures, however, carbon and hydrogen are 

 burnt, yielding the same products ; in both water is heated 

 and vaporised ; in both stored up, or potential energy is con- 

 verted into actual energy and heat. From both structures 

 carbonic acid and steam escape ; from both structures incom- 

 bustible or unused waste is thrown out. From the points 

 of comparison and of contrast just indicated, it is evident 

 that food has to perform one function which is not demanded 

 of engine-coal, namely the repair and even the building of the 

 structure or apparatus in which it is consumed. Moreover, 

 there are but a few substances which can be used as food in 

 the human body, while the steam-engine may consume coal or 

 coke or petroleum substances which, to the organs of man, are 

 absolutely useless. 



Nutrients and Food- Adjuncts. We are now in a position to 

 classify the various useful constituents of human food according to 

 the functions they perform and their chemical constitution. But 

 there are many substances consumed by man which cannot be 

 regarded as actually nutritive, but which, by a universal instinct, 

 are associated with his daily food. We refer to the group of 

 vegetable substances which includes spices, condiments, tea, coffee, 

 etc. While the essential foods may be designated " nutrients," 

 these may be called "food-adjuncts." They are useful in stimu- 

 lating the flow of the various digestive secretions of the alimentary 

 tract, and in making food of palatable and varied flavour. . Many of 

 them have also a marked and favourable influence on the nervous 

 system. 



It will be seen, from what has been already stated, that the 

 nutrients fall into two divisions, incombustible and combustible. 

 To the former belong water and salts : to the latter, certain com- 

 pounds containing carbon, hydrogen, and oxygen, and another group 

 of compounds in which nitrogen also exists. All the combustible 

 nutrients serve, or may serve, as sources of heat and energy in the 

 system, but to those containing nitrogen a further office belongs 



