DIGESTIBILITY OF FOODS. 283 



Hice polish, a fine powder, is a still more highly nitrogenous 

 food, and is very rich in phosphoric acid. 



In addition to those mentioned, certain other bye-products 

 are sometimes used as food. Their composition will be seen 

 from the table. 



The value of a food depends partly upon its composition, 

 partly upon its digestibility. 



In order to determine the latter, which varies considerably 

 with different races of animals and also in different individuals 

 of the same race, food of known composition is supplied in 

 known quantities to animals and their solid excrements col- 

 lected, weighed, and analysed. In this way the amounts of the 

 various constituents which have been assimilated can be ascer- 

 tained ; errors, which are generally slight, but which in certain 

 cases may be considerable, are introduced by the admixture 

 of intestinal secretions, bile, &c., with the faeces. These secre- 

 tions increase the nitrogenous and fatty constituents of the ex- 

 crements, and therefore tend to give too low values for the 

 digestibility of the proteids and fats. 



The proportion of each food constituent digested out of 100 

 supplied is known as the " digestion coefficient," 



In the tables are given the digestion coefficients of the con- 

 stituents of a large number of food stuffs, chiefly from the re- 

 sults of American and German experiments. It is to be noted 

 that by "digestibility" of a food or constituent of a food no 

 reference is paid to the rapidity or ease with which it is assimi- 

 lated, nor to the effect upon the health or comfort of the 

 animal consuming it. These are points on which the taste and 

 individuality of the animal have even more influence than on 

 the true digestibility, i.e., the actual chemical transformations 

 leading to the assimilation. 



In America attempts are being made to distinguish between 

 digestibility and availability of the 'constituents of food. As 

 denned by Atwater,* these are 



Digestibility : Measured by the difference between the total 

 food and the undigested residue. 



* Rep. of Storr's Agric. Expt. Station, 1899, 60. 



