CHAPTER XXVI 

 THE ALIMENTARY CANAL AND ITS APPENDAGES 



General Arrangement. The alimentary canal is essentially a 

 tube running through the Body (Fig. 2) and lined by a vascular 

 membrane, most of which is specially adapted for absorption; it 

 communicates with the exterior at three points (the nose, the 

 mouth, and the anal aperture), at which the lining mucous mem- 

 brane is continuous with the general outer integument. Support- 

 ing the absorbent membrane are layers which strengthen the tube, 

 and are in part muscular and, by their contractions, serve to pass 

 materials along it from one end to ' the other. In the walls of the 

 canal are numerous blood and lymphatic vessels which carry off 

 the matters absorbed from its cavity; and there also exist in con- 

 nection with it numerous glands, whose function it is to pour into 

 it various secretions by which the chemical act of digestion is 

 carried on. Some of these glands are minute and embedded in the 

 walls of the alimentary tube itself, but others (such as the salivary 

 glands) are larger and lie away from the main channel, into which 

 their products are carried by ducts of various lengths. 



The alimentary tube is not uniform but presents several dilata- 

 tions on its course ; nor is it straight, since, being much longer than 

 the Body, a large part of it is packed away by being coiled up in 

 the abdominal cavity. 



Subdivisions of the Alimentary Canal. The mouth-opening 

 leads into a chamber containing the teeth and tongue, the mouth- 

 chamber or buccal cavity. This is succeeded by the pharynx or 

 throat-cavity, which narrows at the top of the neck into the gullet or 

 esophagus; this runs down through the thorax and, passing 

 through the diaphragm, dilates in the upper part of the abdominal 

 cavity into the stomach. Beyond the stomach the channel again 

 narrows to form a long and greatly coiled tube, the small intestine, 

 which terminates by opening into the large intestine, much shorter 

 although wider than the small, and terminating by an opening on 

 the exterior. 



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