CHAPTER XVIII 

 FOOD AND DIGESTION 



One of the principal differences between plants and 

 animals is the manner of food absorption. Plants must 

 have their food dissolved on the outside before they can take 

 it into their circulatory system. Animals can take in in- 

 soluble food and make it soluble within themselves before 

 absorbing it into their circulatory system. For a proper 

 understanding of the principles of feeding it is necessary to 

 know something of the chemical processes by which foods 

 are made soluble in the animal and of their functions after 

 absorption. 



235. Food. — Sherman defines food as "those substances 

 which supply the body either with material needed for its 

 substance, or with energy for its activities." Sometimes a 

 distinction is made between the food of human beings and 

 the food of animals. This is a distinction in terminology 

 only, food of domestic animals being called feed as distinct 

 from food which is applied to human foods only. Since the 

 processes of digestion and the main constituents of food 

 are the same for man as for domestic animals, no distinction 

 will be made in this chapter, all the material considered 

 being called food. 



The tissues of the animal bodv are in a constant state of 

 change, new tissue being formed, old tissue being broken 

 down. These chemical processes of building up and tearing 

 down are called metabolism. Food supplies the material 

 for constructive metabolism. For the chemical changes of 

 metabolism in general, for the production of heat and work 

 in the animal, energy is necessary. Food supplies the 

 necessary material for this energy, which is a result of 

 destructive metabolism. A comparatively small part of the 

 food required for animals is necessary in repairing and build- 



