io2 THE CHEMISTRY OF CATTLE FEEDING 



acts upon carbohydrates which have escaped the action of the 

 saliva, upon fats which have not been completely saponified 

 by the bile, and upon proteins which have not been completely 

 peptonised by the gastric juice. The action of trypsin, the 

 chief proteolytic ferment of the pancreatic juice, is not exactly 

 the same as that of pepsin. Like the latter, it forms proteoses 

 and peptones, but the action proceeds further and results 

 ultimately in the formation of amino-acid compounds ; whereas 

 very little, if any, amino-acids are formed even by prolonged 

 peptic digestion. 



It is not known whether any of the digestive juices contain 

 an active ferment of the cytase type. It is certain that a 

 variable but usually considerable amount of the cellulose of 

 the food disappears in the process of digestion by herbivorous 

 animals, but this may be due to the action of bacteria. 

 Probably the intestinal gases are mainly formed in this way. 



As a result of all these processes, the food is reduced 

 finally to the fluid condition. This fluid is of milky consist- 

 ency and is called chyme. Digestion is then complete. The 

 chyme passes downwards, and the soluble and diffusible com- 

 pounds which it contains are absorbed by hair-like structures, 

 called villi, which project from the walls of the intestines. 



It appears, then, that the process of digestion as a whole 

 consists in the reduction of the solid constituents of the food 

 to the fluid condition. Any substance or portion which is not 

 acted on by the digestive juices, or which is not rendered 

 soluble and diffusible, cannot be absorbed, and is said to be 

 indigestible. This indigestible matter passes downwards 

 through the intestines, and, together with any excess of 

 digestive juices, effete membranes, etc., is excreted as faeces. 



Digestibility. The term "digestibility" refers to the 

 thoroughness and rapidity with which the constituents of the 

 food are transformed into chyme. If one of two different 

 foods or portions of food undergo this transformation more 

 rapidly or more thoroughly than another, under similar con- 

 ditions, it is said to be more digestible. 



The digestibility of a food can be estimated from the 

 amount excreted as faeces. For example, in an experiment of 



