CHEMICAL CHANGES IN PLANTS AND ANIMALS. 351 



organic or inorganic constituents ; bread is rich in phosphates, 

 which latter are also found in smaller or larger quantities in 

 nearly all kinds of vegetable and animal food. 



Food has a twofold office, viz., to build up or nourish the 

 whole body, and to maintain the animal heat. The latter action 

 is chiefly accomplished by the oxidation of carbohydrates and 

 fats, which are therefore also spoken of as heat-giving or respir- 

 atory food, whilst nitrogenous or albuminous substances are 

 those which serve to build up and nourish the animal frame, 

 and are termed plastic or nutritive food. 



Of the heat produced by the oxidation of fats and carbo- 

 hydrates, a certain amount escapes, whilst another quantity is 

 employed in working the involuntary machinery of the body 

 (motion of the heart, lungs, intestinal canal, etc.), and the rest 

 is available for conversion into voluntary muscular actions. 

 Whilst the nitrogenous substances have primarily the task of 

 continuously replacing the wear and tear of the nitrogenous 

 tissues, any excess of them may serve to keep up the animal 

 heat and consequently also the involuntary or voluntary motion. 



The relative proportions in which the two kinds of food are 

 taken by animals depend upon the nature of the animal and 

 upon its particular condition of existence. A young, growing 

 animal, for instance, requires a relatively larger quantity of 

 nitrogenous food than an animal whose growth has ceased, and 

 an animal exposed to a low temperatfire requires more of heat- 

 giving food than one living in a warmer locality. 



The relative proportions of nitrogenous and non-nitrogenous 

 matter in various kinds of food are shown in the following 

 table : 



Non-ni- 

 trogenous. 



17 

 12 

 11 

 10 



5.0-6.8 

 5.0-6.5 

 6 

 6 

 5.5 



Nitro- 

 genous. 



Pork, 



Fat mutton, 



Peas (dried), 



White beans, 



Milk, 



Beef, 



Cheese, 



Meat, 



Veal, 



"White of egg, 



Non-ni- 

 trogenous. 



3 



2.7 

 2.6 

 2.3 

 2.2 

 1.7 

 0.7 



0.5-1.5 

 0.1 

 



