18 Feeds and Feeding. 



II. Digestion. 



27. Digestion. — The changes which food undergoes within the 

 digestive tract of the animal to prepare it for absorption and ulti- 

 mate use in building new tissues, repairing body waste, and as a 

 source of energy are collectively known as digestion. Digestion is 

 effected by enzymes or ferments elaborated by glands of the mouth, 

 stomach, pancreas, and small intestines. Bacteria inhabiting cer- 

 tain parts of the digestive tract attack the woody cellulose of the 

 food, breaking it down and thereby freeing nutrients. In addition 

 to the action of the secretions and bacteria, the food in its course 

 thru the digestive tract is subjected to mechanical processes which 

 tend to reduce it to a fine state of division, the object of the whole 

 process being to separate from the useless matter those constituents 

 which are to nourish the body. 



28. The alimentary tract. — The digestive tract is a long, tortuous 

 tube passing thru the animal from mouth to vent, enlarged in 

 places for the storage of food or waste. Within its linings are 

 secretory organs furnishing various fluids of digestion, and into it, 

 from other specific secretory organs located near by, pour still otlier 

 digestive fluids. Within its walls are nerves controlling its action, 

 arteries which nourish it with fresh blood, and veins and lymphat- 

 ics which absorb and carry from its interior the products of diges- 

 tion, as well as water, mineral matter, and gases. It should be 

 borne in mind that the contents of the stomach and intestines are 

 really outside the body proper. Only when a substance has passed 

 into or thru the walls of the digestive tract has it actually entered 

 the body of the animal. 



In young ruminants, or animals which chew the cud, the first 3 

 stomachs are less developed than in grown animals. Colin found 

 that the first stomach, or rumen, of a calf held 2.6 lbs. of water; 

 the second stomach, or reticulum, 0.22 lb.; the third stomach, or 

 manyplies, 0.35 lb.; and the true stomach, or abomasum, 7.7 lbs. 

 As the diet of the growing calf changes to more solid food, such as 

 grass, hay, and grains, the rumen or paunch gradually increases in 

 size, until in the grown ox it holds 9 times as much as the other 3 

 stomachs combined. 



The length and capacity of the intestines and the capacity of the 

 stomachs of different farm animals are as follows : 



