286 FOODS AND FEEDING. XIII. 



influence on the proportion digested unless the diet is very rich 

 and generous ; the digestibility of the food then diminishes. 



From the laborious investigations of Grandeau and his asso- 

 ciates it appears that gentle exercise increases the proportion 

 of food digested by horses, while work slightly diminishes it. 



Fodder is subject to the greatest variation in digestibility, 

 being almost always most digestible when young. This is 

 true, manifestly, with reference to the " fibre," which becomes 

 less and less digestible as the plant becomes more lignified, 

 but also applies to the proteids, fat, and starch. This is well 

 seen by reference to the digestion coefficients for hay and green 

 grass, clover, &c., in the table just given. 



It is found that the digestibility of some constituents may be 

 altered by the addition to the food of an increased quantity of 

 one' constituent. This is only true under certain circum- 

 stances, viz., when the albuminoid ratio of the food is changed 

 so as to fall outside certain limits, which differ with the 

 particular animal considered. By albuminoid ratio or nutritive, 

 ratio is meant the ratio of the digestible albuminoids to the 

 digestible non-albuminoids expressed in equivalent of starch. 

 The calculation of the starch equivalent of fat, sugar, &c., is 

 based upon the results of calorimetric experiments, i.e., the 

 quantities which will produce by their combustion an equal 

 amount of heat. 



By placing an animal in a respiration calorimeter, so 

 arranged that everything which enters and leaves may be 

 measured, the quantities of various dry food stuffs which will 

 produce in the body of the animal as much heat as 100 parts 

 of fat has been determined. The following table gives the 

 results as compared with those obtained by direct oxidation of 

 the food in a combustion calorimeter": 



As determined As determined in com- 

 rith animals. bustion calorimeter 



* Rubner, quoted by Atwater, Bull. 21 U.S. Dept. of Agriculture. Atwater found, 

 as a mean, that the heats of combustion of the available fat, protein, and carbo- 

 hydrates of foods were represented by 9400, 4400, and 4100 thermal units (Rep. of the 

 Storrs Agric. Expt. Station, 1899). These numbers agree well with the older numbers 

 obtained by Rubner in 1885. 



