DIGESTION AND RESORPTJON^ ' IOI 



The digestion of ash 



The various digestive enzyms whose action has been con- 

 sidered in the foregoing pages bring about extensive chemical 

 changes in the organic nutrients of feeding stuffs by means of 

 which they are prepared to enter into the nutritive processes 

 in the tissues. At the same time, the so-called " inorganic " 

 ingredients of feed are also prepared for resorption, but the di- 

 gestion of these substances has been less extensively studied 

 than that of the organic nutrients. 



142. Sulphur and phosphorus. As regards the sulphur of 

 the proteins, it does not appear that this element is separated 

 from its union with carbon, nitrogen and hydrogen in the pro- 

 cesses of protein digestion. The sulphur of the proteins is con- 

 tained in the amino-acid cystin, which, so far as known, is 

 resorbed without further change. As regards the phosphorus of 

 the nucleo-proteins, opinions still differ as to whether it is split 

 off as phosphoric acid in the course of digestion or resorbed, still 

 in "organic" combination, as a nucleosid. To what extent 

 other ash ingredients are taken up, like sulphur and phosphorus, 

 in organic combination, it is difficult to say, but that such re- 

 sorption takes place is to be regarded as probable. 



143. Electrolytes. As regards those ash ingredients of 

 feeds which are present as electrolytes, it may be assumed that 

 they are brought into solution to a greater or less extent by the 

 hydrochloric acid of the gastric juice, but how much reprecipi- 

 tation may occur in the more or less alkaline contents of the 

 intestine it is difficult to say. The whole subject of the diges- 

 tion, of the ash ingredients of feeding stuffs, however, is so in- 

 timately related to the question of the paths of excretion and to 

 the problems of ash metabolism that it can be more profitably 

 considered in that connection. 



Summary of changes in digestion 



144. Solution of nutrients. The substances which make 

 up the larger part of the feed of domestic animals (and of man 

 as well) are comparatively insoluble in water. Some of them, 

 such as cellulose and the fats, may be regarded as practically 

 entirely insoluble. Others, like starch and the proteins, are 



