88 HEALTH AND DISEASE 



The Functions of Foods. — The fact that food contains albuminoids, 



which correspond to the constituents of flesh, and hydrocarbons, which 

 represent fats, and also mineral constituents, which have been referred to, 

 has led physiologists to adopt certain theories in reference to the function 

 of each class of food in the animal body. The nitrogenous constituents and 

 albuminoids have had ascribed to them the function of flesh-formers. 

 They are distinguished from other nitrogenous constituents by the presence 

 of a small quantity of sulphur and phosphorus, which constituents are 

 absent from gelatine. Fats and also carbohydrates, such as starch ami 

 sugar, are considered to represent the combustible materials or heat-forming 

 substances which are consumed in the process of respiration, the excess 

 being deposited in the form of fat. 



It is stated that animals cannot subsist on a diet composed exclusively 

 of flesh-forming materials, nor on one from which carbohydrates are 

 entirely excluded. On the other side there are instances recorded of 

 animals having lived in perfect health for a considerable time on nitro- 

 genous and also on non-nitrogenous foods. These exceptional cases need 

 not be taken into account in practice, as there is sufficient evidence that 

 the animal to-day requires for its support a food in which are combined 

 all the constituents which are found in the organism. 



Experiments have shown that nitrogenous food can be used for the 

 formation of fat, and it is admitted that all the fat which is found in the 

 body could not have been obtained from the amount of fatty matters 

 which have been consumed as food. And the fact of animals having lived 

 and been maintained in health on purely nitrogenous diet, is sufficient in 

 itself to show that a considerable proportion of the material must have 

 been consumed in the process of respiration. 



Assuming that albuminoids are chiefly concerned in the formation of 

 flesh and the development of force, only contributing in a minor degree 

 to the formation of fat, it must also be allowed that fats have for their 

 principal function the production of heat as a result of oxidation, and in 

 this they are assisted by other carbohydrates — starch, sugar, and gums. 

 But it is certain that they do not in any way contribute to the formation 

 of nitrogenous constituents. 



An important point is to be considered in reference to the respective 

 value of each article of diet and its digestibility. A food may contain a 

 considerable proportion of albuminoids which are comparatively useless 

 to certain animals, on account of the digestive organs being incapable 

 of appropriating the nutriment they afford. Bean straw, for example, 

 contains 8 per cent of nitrogenous substances, but its structure renders it 

 comparatively indigestible as food for the horse, in which animal digestion 



