Undigested Residue Why Digestibility Varies 121 



play a part in transferring the nutrients into the blood 

 circulation, and that this absorption can no longer be 

 explained wholly on physical grounds. 



FECES 



The soluble and insoluble portions of the intestinal 

 contents become separated gradually, and the undissolved 

 part arrives finally at the last stage of its journey along 

 the alimentary canal and is expelled as the solid excre- 

 ment or feces. This is made up of the undigested food* 

 and a small proportion of other matter, such as residues 

 from the bile and other digestive juices, mucus and 

 more or less of the epithelial cells, which have become 

 detached from the walls of the stomach and intestines. 

 Very small quantities of fermentation products are 

 present also, which give to the feces its offensive odor. 

 The incidental or waste products may properly be con- 

 sidered as belonging to the wear and tear of digestion 



THE RELATION OF THE DIFFERENT FEEDING STUFF 

 COMPOUNDS TO THE DIGESTIVE PROCESSES 



Numerous digestion experiments with a large variety 

 of feeding stuffs have abundantly established the fact 

 that these materials differ greatly in their solubility in 

 the digestive juices. This is an important matter, and 

 one which should be well understood, for we must con- 

 sider both the weight of a ration and its availability 

 in determining its nutritive value. Variations in diges- 

 tibility are caused primarily by variations in composi- 



* See note, p. 125. 



