DIGESTION AND RESORPTION 89 



2. THE CHEMISTRY OF DIGESTION 



127. Digestion a chemical process. The foregoing para- 

 graphs have dealt chiefly with those more general facts regard- 

 ing the organs of digestion which are necessary for an under- 

 standing of their functions and only incidentally and in outline 

 with the chemical processes involved. It is now time to revert 

 to the statement made at the beginning of the chapter, namely, 

 that digestion is the first step in the conversion of feed sub- 

 stances into body substances, and specifically in the case of farm 

 animals the conversion of vegetable into animal substances. 

 These, however, are chemical transformations and from this 

 point of view a knowledge of the structure of the digestive 

 apparatus is of significance chiefly as an aid to the understanding 

 of these processes. Tn taking up this aspect of the subject, it 

 will be convenient to consider the three chief groups of nutrients 

 separately. 



The digestion of carbohydrates 



By far the larger proportion of the carbohydrates contained 

 in the feed of farm animals consists of polysaccharids, especially 

 starch, cellulose and the various pentosans and hexo-pentosans. 

 The disaccharids, especially sucrose and lactose, probably stand 

 next in importance, while comparatively small amounts of 

 monosaccharids are consumed. 



128. Cellulose. The cellulose of feeding stuffs was long 

 assumed to be indigestible. Haubner was the first to show the 

 incorrectness of this assumption and to prove that even the 

 cellulose of such substances as paper pulp and sawdust, as well 

 as that of ordinary feeds, was digested by cattle. The subse- 

 quent investigations of Henneberg and Stohmann (158, 707) 

 showed that the crude fiber of feeding stuffs was digested to a 

 considerable extent by cattle, and sheep, and later digestion ex- 

 periments have proved this to be true not only of ruminants 

 but to a varying degree of other animals, both herbivora and 

 omnivora, including domestic fowls. Even man is capable of 

 digesting the tenderer forms of cellulose to a considerable 

 extent. 



None of the digestive enzyms of the higher animals, however, 

 have been shown to have any action upon cellulose and the small 



