DIGESTION AND RESORPTION III 



intestinal walls or from lack of time, and so be found in the 

 feces. 



156. Composition of feces. Evidently the feces are a very 

 complex and variable mixture, including, on the one hand, the 

 various excretory products just enumerated and, on the other 

 hand, indigestible feed substances 'and digestible materials 

 which have for one reason or another escaped actual digestion 

 or which, having been digested, have failed of resorption. 

 Among the latter may be included unresorbed products of the 

 putrefaction of the proteins, especially skatol, which impart 

 to the feces their offensive odor. 



The proportions of these two groups the excretory products 

 and the feed residues in the feces vary widely with the 

 nature of the feed consumed. In the carnivora the body wastes 

 predominate, so that the feces of these animals are to be regarded 

 as primarily an excretory product. To a considerable degree 

 the same thing is true of man, especially when living on a con- 

 centrated diet. With the herbivora, on the contrary, the in- 

 digestible or undigested feed residues constitute the bulk of 

 the feces, although the amount of true excretory products is 

 by no means insignificant. Omnivora like the hog occupy an 

 intermediate position in this respect. 



4. THE DETERMINATION or DIGESTIBILITY 



157. Definition of digestibility. The words digestible and 

 digestibility are used in more than one sense. Sometimes, for 

 example, a food is said to be digestible because it is easily di- 

 gested that is, causes no unpleasant sensation after it is 

 eaten while by an indigestible food is meant one that is apt 

 to cause gastric or intestinal disturbances. Again, it is not un- 

 common to judge of the digestibility of stock feeds by their 

 effects and to regard that one as the more digestible which causes 

 or seems to cause the greater gain in weight. 



The word digestibility as used in the study of animal nutrition, 

 however, has a definite and limited meaning. It denotes the 

 percentage of the feed, or of any single ingredient of the feed, 

 which is dissolved or otherwise acted on in the digestive canal 

 so that it can be resorbed and thus put at the disposal of the 

 body cells. For example, the digestion experiment with a steer 



