THE DIGESTION OF FEEDS 33 
amount of urea excreted in the urine, say during a day, we are 
able to ascertain the amount of protein substances in the feed or 
of body tissues that have been decomposed during the day (see 
p. 44). 
Metabolism.—tThe chemical changes that occur within the body inci- 
dent to the exercise of vital functions and to growth are included under the 
general term metabolism. Metabolic processes in the animal body are of 
two kinds: Katabolic or destructive, those by which the food materials 
are broken into compounds of simpler structure, and anabolic or construc- 
tive, by which these simpler compounds are again built up into complex 
substances. The formation of peptones and amino acids from the proteins 
is a katabolic process, while the reverse change, the building up of these 
simpler compounds into body protein, albumen, globulin, ete., is a con- 
structive process. “Both kinds of processes take place continuously in the 
living body, as we have seen; they are essential to life, and are discontinued 
only when life ceases. 
QUESTIONS 
1. Give the various groups of substances found in the animal body and 
state their main characteristics. 
2. Why is it necessary to give salt to farm animals? 
3. Name the various digestive fluids in the animal body, and state the 
changes which different components of feed undergo in the process of 
digestion. : 
4. What is the difference between ruminants and non-ruminants? 
‘5. Name the American farm animals that belong to each group. 
6. Describe the difference between the digestive apparatus of ruminants and 
non-ruminants. . 
7. Describe the process of absorption of the digested protein substances, 
carbohydrates and fat. 
