44 MANUAL OF FABM ANIMALS 



The ash or mineral matter taken in serves as the mineral matter 

 of the animal body. The protein acquired may serve a three- 

 fold purpose ; it must serve as the proteid supply of the animal 

 body, as, so far as known, there is no other source ; it may serve 

 in heat production; and it may serve as a fat-former. The 

 carbohydrates serve a double purpose, that of heat-production 

 and fat-forming. The fat serves a similar purpose as the carbohy- 

 drates — forming fat and producing heat — and is 2? times as 

 efficient as the carbohydrates. 



DIGESTIBILITY OF FOOD 



The value of a food depends on the digestibility of the 

 various materials of which it is composed, and not alone on its 

 composition as shown by a chemist. Because of the importance 

 of the digestibility of the various food materials, and in order 

 to get a clear conception of digestion, it will be necessary to 

 review briefly the process through which the food materials 

 must pass on their way from the food as such to the flesh or 

 energy of the animal body. 



Digestion. — The process of digestion is largely a process of 

 solution, aided to a certain extent by chemical changes that take 

 place through the influence of various ferments with which the 

 food comes in contact as it passes through the digestive tract. 

 The digestive organs form a canal through the body, and the 

 food in its passage through this tract or canal is acted upon by 

 various fluids. Each fluid has a special work to do in the pro- 

 cess of digestion. To begin, the food is taken into the mouth, 

 where it is reduced to fine particles in order that the digestive 

 juices may better do their work and to put it into condition to be 

 swallowed. During this mastication there is a digestive fluid, 

 called saliva, poured upon the food, which moistens it and changes 

 a part of the starch. After leaving the mouth the food passes 

 down the esophagus into the stomach, where the digestive 

 fluid, called gastric juice, is poured upon it and which changes a 



