THE FOOD AND DIGESTION 313 



supplied in the food, it is retained in the tissues, and hence 

 animals can, when necessary, live with a comparatively small 

 supply. 



3. Salts of organic acids. The sodium and potassium 

 salts of citric, malic, and tartaric acid, which are found 

 abundantly in various vegetables, are, when taken into the 

 tissues, oxidised into carbonates which are strongly alkaline 

 salts. The proteids which are decomposed in the body 

 contain sulphur and phosphorus, and these are oxidised into 

 sulphuric and phosphoric acids. In herbivorous animals 

 the prejudicial effect of such acids is counteracted by the for- 

 mation of these alkaline carbonates, which neutralise the acids. 

 In carnivorous animals these salts are not so necessary, since 

 ammonium is formed from the nitrogen of the proteids in 

 sufficient quantity to neutralise the acids produced. Man 

 occupies a position midway between the herbivora and the 

 carnivora. The amount of energy yielded by the breaking 

 down of these salts into carbonates is so small that it is of no 

 importance. 



B. Food-stuffs yielding Energy. These are complex 

 combinations of carbon, hydrogen, and oxygen, with or 

 without nitrogen, sulphur, phosphorus, and iron. They are 

 of the same nature as the materials which are found on 

 analysis of dead protoplasm. They are commonly spoken of 

 as the Proximate Principles of the food, and they may be 

 classified as follows : 



1. Nitrogen-containing Proteids and Albuminoids or 

 Modified Proteids. 



2. Non-nitrogenous Fats and Carbohydrates. 



In studying the value of these food-stuffs it is necessary to 

 consider their Energy Yalue that is, the amount of energy 

 which can be yielded by the decomposition of a definite 

 quantity of each in the body. 



The fats and carbohydrates leave the body as carbon 

 dioxide and water, the proteids leave it partly as carbon 

 dioxide and water, partly as urea. 



Such a body as glucose, C 6 H 12 6 , by being oxidised to C0 2 

 and H 2 0, gives off' a certain amount of kinetic energy, and 

 the amount of energy liberated is the same whether the 



