OHAPTEK XXXI 



THE ALIMENTARY CANAL; SECRETING GLANDS 



THE alimentary canal consists of a long muscular tube lined by 

 mucous membrane beginning at the mouth, and terminating at the 

 anus. It comprises the mouth, pharynx, oesophagus or gullet, stomach, 

 small intestine and large intestine. Opening into it are numerous 

 glands which pour juices into it; these bring about the digestion of 

 the food as it passes along. Some of the glands, such as the gastric 

 and intestinal glands, are situated in the mucous membrane which 

 lines the canal; others, such as the salivary glands, liver, and pan- 

 creas, are situated at a distance from the main canal, and pour their 

 secretion into it by means of side tubes or ducts. 



The two important coats in the wall of the canal are : 



(1) The muscular coat. This consists of two layers ; in the outer, 

 the fibres are arranged longitudinally, and in the inner, circularly. 

 In the stomach there is a third coat, in which the fibres have an 

 oblique direction. At the cardiac orifice of the stomach (that is, 

 where the oesophagus enters) and at its pyloric orifice (that is, where 

 the small intestine leaves) the circular fibres are increased in amount 

 to form a sphincter. The muscular fibres are of the plain variety, 

 except in the pharynx and upper part of the oesophagus where they 

 are striated. A nerve plexus (plexus of Auerbach, fig. 100, p. 79) 

 is situated between the two muscular coats. 



(2) The mucous membrane. This consists of an epithelium on its 

 surface; this is stratified in mouth, pharynx, and oesophagus, but 

 columnar in other parts. Beneath the epithelium is a corium of 

 connective tissue, in which there is a considerable quantity of 

 lymphoid tissue; in the intestine the lymphoid nodules are often 

 spoken of as solitary follicles, except in the lower part of the small 

 intestine (the ileum), where they are congregated together as Peyer's 

 patches. At the back of the mouth, the tonsils are masses of 

 lymphoid nodules covered with mucous membrane. In the deepest 

 part of the mucous membrane is a thin layer of involuntary muscle 

 called the muscularis mucosce. 



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