CHAPTER X. 



THE GASTRO-INTESTINAL CANAL. 



THIS canal is a tube varying greatly in its calibre 

 in different parts, and continuous at either end with 

 the external surface of the body. In certain parts 

 of its course it is intimately connected with adjacent 

 structures ; but, for the most part, it is attached only 

 at one side by a structure the mesentery which 

 serves to convey to it its blood- and lymphatic-vessels 

 and nerves. The walls of the tube, although vary- 

 ing in structure in different sections, consist in gen- 

 eral of a muscular layer, a mucous layer lining the 

 tube, and, in those parts where it is suspended in 

 the abdominal cavity, a serous layer covering it. 



Confining our attention to the stomach and in- 

 testines, we find that these layers are not simple, 

 but have each a composite structure ; thus, we find 

 in the serosa, a layer consisting chiefly of dense 

 fibrillar connective tissue, subserosa, covered with a 

 layer of endothelium. The muscular tunic, or mtis- 

 culosa, consists of two layers of smooth muscular 

 tissue : an external, in which the cells lie longitudi- 

 nally, and an internal in which they lie transversely 



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