CHAPTER XIII 

 THE ALIMENTARY CANAL. DIGESTION 



In Chapter II some general statements were made in 

 regard to the origin and service of food. The term 

 was used to include things which may enter into the 

 body as constructive material or as fuels. By far 

 the largest share of our food, especially after the com- 

 pletion of growth, is valuable purely as fuel. We have 

 now to trace the preparation of food for absorption, 

 its distribution and storage, and the final utilization 

 of it by the tissues. First of all some attention must 

 be given to the anatomy of the human alimentary 

 tract. 



The Alimentary Canal. In all the higher animals 

 this is a passage leading from a mouth to a vent or anus. 

 The contents of the canal are not to be considered as 

 within the body but only in contact with a part of its 

 surface. The canal in man is 25 or 30 feet long, the 

 great length being made possible by the coiling of the 

 intestine in the abdominal cavity. A long canal is not 

 any more capacious than one shorter and wider, but it 

 has more surface and this is important since through the 

 lining of the tract the useful part of the food is received 

 into the blood. 



The successive parts of the canal are the mouth, the 

 pharynx, the esophagus, the stomach, the small intestine, 

 and the large intestine or colon. The large intestine 

 terminates in the rectum leading to the anal outlet. 

 The arrangement of the structures about the mouth is 

 sufficiently familiar. The pharynx is what we often call 

 the throat, a short section common to the digestive and 

 the respiratory systems. In it the course taken by the 



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