CHAPTER XIV. 



ALIMENTATION CONTINUED : THE DIGESTIVE APPARATUS ; ALI~ 

 MENTARY CANAL, AND ACCESSORY ORGANS. 



THE digestive apparatus consists of the alimentary canal, and 

 the accessory organs, the teeth, salivary glands, pancreas, and 

 liver. By some authorities the spleen is also now included 

 among the digestive organs. 1 



Alimentary canal. The alimentary canal is a musculo-mem- 

 branous tube extending from the mouth to the anus. It is 

 about six times the length of the body, and the greater part of 

 it is coiled up in the cavity of the abdomen. The diameter of 

 the tube is by no means uniform, being considerably dilated in 

 certain parts of its course. It is composed of three coats from 

 the mouth to where it passes through the diaphragm, and of 

 four coats in the abdominal cavity. These coats are : (1) the 

 mucous ; (2) the sub-mucous (described in our last chapter) ; 

 (3) the muscular ; (4) the serous. The muscular coat is com- 

 posed for the most part of unstriped muscular fibres, the layers 

 of which are disposed in various ways, the most general arrange- 

 ment being in a longitudinal and circular direction. By the 

 alternate contraction and relaxation of fibres arranged in this 

 fashion (the contractions starting from above), the contents of 

 the tube are propelled from above downwards. The serous coat 

 is derived from the peritoneum, which is the serous membrane 

 lining the walls, and covering the viscera, of the abdomen. 



Into the interior of the alimentary canal are poured secre- 

 tions from the glands in the mucous membrane, with which it is 

 lined, and also secretions from the accessory glands, which lie 

 outside the canal and are connected with its interior by ducts. 



The alimentary canal for convenience of description may be 

 divided into : 



1 Plate VII. shows relative position of digestive organs in abdominal cavity. 



153 



