CHAPTER III 

 THE DIGESTION OF THE NUTRIENTS 



In order that the nutrients of the feedingstuffs may be 

 ma(h^ available for the nutrition of the animal body, they 

 first must undergo digestion. Digestion is the process by 

 which the digestive agents change into forms which are 

 soluble, diffusible, and available to the tissues such por- 

 tions of the feed as are capable of such changes in the diges- 

 tive tract. However, portions of the feed which are really 

 capable of digestion often are not digested. Their escape 

 may be merely a matter of chance. 



The Digestive System. — The digestive system consists 

 of the organs concerned in the reception and digestion of 

 the feed, in the passage of the feed through the animal body, 

 and in the excretion of the unabsorbed residue. For con- 

 venience in study, the digestive system may be divided into 

 the alimentary canal and the accessory organs of digestion. 

 The alimentary canal, or aUmentary tract as it is often called, 

 is the passage which begins with the mouth, includes the 

 esophagus, stomach, small intestine, large intestine, and ends 

 with the anus. These divisions are shown schematically in 

 Figure 3. 



The esophagus or gullet is the tube-like passage which 

 leads from the mouth to the stomach. In the horse and 

 hog the stomach consists of a single pear-shaped sac which 

 has a capacity in the horse of 3 to 4 gallons and in the hog of 



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