CHAPTER XXII. 

 THE ALIMENTARY CANAL AND ITS APPENDAGES. 



General Arrangement. The alimentary canal is essen- 

 tially 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 membrane is continuous with the general outer 

 integument. Supporting 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 ab- 

 sorbed from its cavity; and there also exist in connection with 

 it numerous glands, whose function it is to pour into it various 

 secretions which exert a solvent influence on such foodstuffs as 

 would otherwise escape absorption. Some of these glands are 

 minute and imbedded in the walls of the alimentary tube it- 

 self, 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 

 dilatations 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 suc- 

 ceeded by the pharynx or throat-cavity, which narrows at 

 the top of the neck into the gullet or oesophagus; this runs 

 down through the thorax and, passing through the dia- 

 phragm, 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 intes- 



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