FOOD CONSTITUENTS. 83 



The carbo-hydrates and fat are quite incapable of 

 adding to the nitrogenous tissues of the body. They may, 

 however, have this effect indirectly by protecting the 

 albuminoids of the food from oxidation. A moderate 

 quantity of albuminoids supplied to a growing animal 

 will thus produce a larger increase of muscle when ac- 

 companied by a supply of carbo-hydrates or fat than if 

 consumed alone. In the former case the non-nitroge- 

 nous ingredients of the food supply the heat and force 

 demanded by the animal body, in the latter case the 

 albuminoids have to meet every requirement. 



If an adult animal receives the small quantity of 

 albuminoids and salts necessary to supply the waste of 

 tissue, the whole of its remaining wants may be met by 

 supplies of carbo-hydrates and of fat. 



The relative value of food constituents for the produc- 

 tion of heat and force depends on the amount of heat 

 evolved during their oxidation. It has been found that 

 100 parts of fat when burnt give the same amount of 

 heat as 213 parts of the albuminoids of muscle, 469 parts 

 of asparagine (the heat yielded by the matter excreted as 

 urine when these substances are consumed in the body 

 is in both cases deducted), 229 parts of starch, 235 parts 

 of cane sugar, or arabic acid (gum), and 255 parts of 

 glucose, or crystallised milk sugar. Digested cellulose 

 has a smaller value than the carbo-hydrates just named, 

 267 parts are probably equivalent to 100 of fat. 



The ash constituents present in the food are the same 

 as those found in the animal body; all that is accom- 

 plished by the animal is to select from the supply those 

 of which it is in want. 



2. Digestion. The object of digestion is to bring 



