396 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The time required for the passage of food through the body 

 varies with different kinds of animals. Residues of the food 

 begin to appear in 12 hours with the dog, 36 hours with the pig, 

 and three or four days in the excrement of the cow, sheep, goat, 

 or horse. The residues are usually completely excreted in 7 to 

 8 days by cows, sheep, and similar animals, though bulky food 

 may continue to appear for 14 days if followed by easily digested 

 food, such as young grass, etc. 



Metabolic Products. The transformation which food under- 

 goes within the animal is termed metabolism, and the products of 

 the life action are termed metabolic products. The metabolic 

 products in the excrement are residues of digestive juices and 

 other animal products. A portion of the intestine, isolated but 

 left within the body, has been found to collect a certain amount 

 of waste material, which, under normal conditions, passes into 

 the excrement. In ordinary digestion experiments, the metabolic 

 products in the excrement may, for all practical purposes, be 

 regarded as a portion of the undigested food, since they represent 

 so much material lost from the body in the excrement. In 

 other experiments, however, the metabolic products must be 

 taken into account. The metabolic products contain protein, or 

 fat and ash, but no carbohydrates or crude fiber. In some 

 digestion experiments with food poor in fat, or with materials very 

 poor in protein, more fat or protein has been found in the excre- 

 ment than was present in the food eaten. The quantity of 

 metabolic fat is, however, small, and of little importance. 



The metabolic nitrogenous substances are of more importance. 

 There are two ways of estimating the quantity of metabolic 

 nitrogen. One method consists in feeding the animal on 

 materials nearly free of nitrogen, and estimating the nitrogen in 

 the excrement. For example, Pfeiffer fed hogs on potato starch, 

 cane sugar, olive oil, and salts, a ration almost free of nitrogen, 

 and found the excrement to contain 4.4 per cent protein. Kell- 

 ner obtained similar results with sheep which were fed sawdust, 

 sugar, and starch. 



The other method does not give us the exact quantity of 



