198 THE FEEDING OF ANIMALS 



283. The digested food is used in two general directions, 

 (a) for the production of energy and (6) for constructive 

 purposes. 



(a) The food energy is made available through com- 

 bustion, i. e., the oxidation of the carbon compounds of 

 the food to simpler substances, carbon dioxid and water, 

 thus liberating the energy stored in the plant during its 

 growth. Protein is never fully oxidized, but carbohydrates 

 and fats may be. All the organic nutrients may be oxi- 

 dized to produce energy, the phycological energy values 

 of protein, carbohydrates, and fats being approximately 

 as 1, 1, 2.25. The larger part of the energy used by farm 

 animals comes from the carbohydrate portion of the food. 

 This liberated energy finds expression in the animal 

 organism in various ways, as heat, mechanical energy or 

 motion, and chemical transformations. The total energy 

 of food is never all available to the animal because of a 

 loss in the excreta and gases. Moreover, the productive 

 energy is much less than the available energy, because 

 much energy is used in the work of appropriation of the 

 food. 



(b) The food compounds are used for constructive 

 purposes, either without changing their general charac- 

 ter, as, for instance, the building of muscular tissue from 

 the plant proteins, or they may be reorganized into bodies 

 of a very different character, as in the formation of 

 animal fats from starch and sugar. Protein is used to 

 construct muscular tissue, in fact, all the nitrogenous 

 parts, and is a source of fat. Carbohydrates can only be 

 used constructively for the formation of fat, and the same 

 is true of food fats or oils. Mineral matter is needed for 

 the formation of bone, enters into the constitution of the 

 soft parts, and has important metabolic functions. 



