CHEMICAL CHANGES IN PLANTS AND ANIMALS. 423 



The various kinds of animal food are derived chiefly from three 

 groups of organic substances, viz., carbohydrates (sugars, starch, etc.), 

 fats, and albuminous or nitrogenous substances. The inorganic sub- 

 stances, such as phosphates, chlorides, etc., required by the animal in 

 the construction of bones, for the liberation of hydrochloric acid in 

 the gastric j uice, etc., are generally found as constituents of various 

 kinds of food or are derived from drinking-water. Milk contains all 

 the necessary organic or inorganic constituents ; bread is rich in phos- 

 phates, which latter are also found in smaller or larger quantities in 

 nearly all kinds of vegetable and animal food. 



Through the food are supplied those compounds which supply the 

 constituents that replace the exhausted material of the living cells, 

 and by chemical changes their inherent potential energy is converted 

 into the heat of the body and into the kinetic energy used in work- 

 ing the living mechanism. Whilst the nitrogenous substances have 

 primarily the task of continuously replacing the wear and tear of the 

 nitrogenous tissues, they also serve to keep up the animal heat and 

 consequently the involuntary or voluntary motion. 



To some extent, the animal body may be regarded as a complicated machine, 

 in which the potential energy, supplied by the food, is converted into actual 

 energy of heat and mechanical labor. The main difference is that in our 

 machines the fuel serves as the source of energy only, while in the body the 

 food is mainly changed first into tissue (thus building up and renewing the 

 body constantly), serving as fuel afterward. While in the best steam-engine 

 only one-tenth of the fuel is utilized as mechanical work, over one-fifth of the 

 energy of the food is realized in the human body. 



The relative proportions in which the two kinds of food are taken 

 by animals depend upon the nature of the animal and upon its par- 

 ticular condition of existence. 



Below are given in column A the daily quantities of dry food 

 required to maintain a grown person in good health, with neither 

 loss nor gain in weight, while the figures in column B refer to the 

 quantities of dry food for a working man of average height and weight. 



A. B. 



Proteids 100 grammes. 130 grammes. 



Fats 100 " 85 " 



Carbohydrates . . . .240 " 400 " 



Inorganic salts .... 25 ". 30 " 



Water 2600 " 2600 " 



The table below shows that 900 grammes (about 2 pounds) of 

 bread, 340 grammes (f pound) of lean meat, and 57 grammes (2 



