CHAPTER XXI. 



THE ALIMENTAKY CANAL AND ITS APPEN- 

 DAGES. 



General Arrangement. The alimentary canal is essen- 

 tially an involuted portion of the skin, specially set apart 

 for absorption, and forming a tube which runs through 

 the Body (Fig. 2); it communicates with the exterior at 

 three points (the nose, the mouth, and the anal aperture), 

 at which this modified skin, or mucous membrane, is con- 

 tinuous with the general outer integument. Supporting 

 the lining absorptive membrane are other layers which 

 strengthen the tube, and are also 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 

 connection with it numerous glands, whose function it is to 

 pour into it various secretions which exert a solvent influ- 

 ence on such foodstuffs as would otherwise escape absorp- 

 tion. Some of these glands are minute and imbedded 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 

 dilatations on its course; nor is it straight, since, being much 

 longer than the Body, 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 sue- 



