CHAPTER X. 

 THE DIGESTIVE ORGANS. 



The digestive tract consists of six hollow, and for the most part, 

 tubular organs, which successively open into each other and extend 

 from the pharynx to the anus. The food, after mastication and 

 admixture with saliva in the mouth, passes through (1) the eesoph- 

 agus into (2) the stomach. Here it undergoes digestive changes 

 under the influence of the gastric secretions. Thence it passes into 

 (3) the duodenum, where the secretions of the liver and pancreas and 

 other glands are mixed with it and still further fit it for absorption. 

 From the duodenum it enters (4) the small intestine, the walls of 

 which take up the available products of digestion, and thence 

 passes into (5) the colon. In the latter the fluid portions are 

 gradually absorbed and the relatively dry residue, the faeces, passes- 

 out of the body through (6) the rectum and the anal orifice. 



The walls of the digestive organs have a general similarity through- 

 out the whole of the digestive tract. They consist of four coats : 1, 

 an internal mucous membrane ; 2, a submucous coat ; 3, a muscular 

 coat ; and, 4, either a serous or a fibrous external coat. These coats 

 are, respectively, continuous with each other throughout the whole 

 tract. The internal coat, or mucous membrane, varies in both 

 structure and function in the different organs, and will, therefore, re- 

 quire closer study than the other coats. The latter have nearly the 

 same structure in all the organs. The submucous coat is made up 

 of areolar fibrous tissue, which permits some freedom of motion 

 between the mucous and muscular coats, and contains the larger 

 bloodvessels and lymphatics that supply all the coats. The mus- 

 cular coat consists, in general, of two layers of smooth muscular 

 tissue: an internal circular layer and an external longitudinal layer. 

 Its function is to produce those vermicular or peristaltic move- 

 ments which mix and gradually propel the food along the digestive 

 tract. The external coat is smooth and serous over those portions 

 of the tract which require the greatest freedom of motion. It is 

 nowhere complete, but, where present, is really a portion of the 



136 



