394 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The pancreatic juice exerts a vigorous digestive action upon 

 proteids, fat, and starch. It contains trypsin, which acts on pro- 

 teids; amylopsin, which rapidly changes starch into sugar; and 

 steapsin, which emulsifies and splits fat. The proteids are con- 

 verted into crystallizable substances, such as leucin, tyrosin, 

 aspartic acid, etc., as well as albumoses and peptones. 



Intestinal juices also exert a digestive action, especially on 

 protein and starch. 



Bacteria increase in numbers as the food passes along the intes- 

 tines; fermentation and putrefaction gradually supersede the 

 action of the digestive juices. In herbivorous animals, digestion 

 is aided by the enormous number of bacteria present in the lower 

 portions of the intestines. These bacteria act upon the undigested 

 food, split up fats, change starch, and other carbohydrates into 

 lactic, butyric, and acetic acids, and exert considerable digestive 

 action on crude fiber. Three gases, carbon dioxide, marsh gas, 

 and hydrogen, are formed in the process. The crude fiber is 

 digested only by such fermentation. A quantity of substances 

 which would not be acted upon by the digestive juices are dis- 

 solved and made useful to the animal. 



Absorption. The dissolved nutrients are absorbed to some 

 extent by the walls of the stomach, but most largely by the intes- 

 tinal walls, and pass either directly into the blood, or first into the 

 chyle and then into the blood. 



The Proteids are taken up as albumoses, peptones, and, to some 

 extent, as leucin, tyrosin, and other crystallizable nitrogenous 

 bodies. But since these substances do not occur in the chyle or 

 in the blood, they must have been synthesized into animal pro- 

 teids in the membranes of the digestive organs. That is to say, 

 the proteids of the food are first split up, then converted into 

 necessary animal proteids. It is quite possible that some of the 

 products of the digestion of the various proteids, are much better 

 suited to the formation of animal proteids than others ; and some 

 products may be entirely unsuitable for the purpose of the 



