THE DIGESTIVE ORGANS. 137 



The deepest layer of the mucous membrane is the muscularis 

 mucosse, made up of two or three strata of smooth muscular tissue 

 in which the fibres run in different directions. 



The submucous coat of the stomach consists of loose areolar tissue, 

 which allows considerable freedom of motion between the mucous 

 membrane and the muscular coat. When, therefore, the organ is 

 empty the contraction of the muscular coat throws the mucous 

 membrane into coarse folds (rugae). The large arteries, veins, and 

 lymphatics course in this submucous tissue, and thence send branches 

 into both the mucous and muscular coats. The nerves also form a 

 ganglionated plexus in this coat. 



The muscular coat consists of an external longitudinal layer, 

 inside of which is another layer encircling the organ. The external 

 layer is continuous with the outer muscular layer of the oesophagus. 

 The internal muscular layer of the latter organ is continued into the 

 wall of the stomach as a scattered set of oblique fibres lying internal 

 to the encircling fibres already mentioned. The muscular coat of the 

 stomach may, therefore, be considered as composed of three layers, 

 the innermost of which is incomplete. At the pylorus the encircling 

 muscular layer is thickened. 



Aside from the fibrous tissue that more or less completely sepa- 

 rates its layers, the muscular coat contains ganglionated nerve- 

 plexuses. 



The external surface of the stomach is covered with a serous in- 

 vestment of peritoneum, except along the curvatures, where the 

 peritoneum is reflected from the organ, permitting the passage of 

 its vessels and nerves. 



5. The Duodenum. The structures characteristic of the small 

 intestine first make their appearance in the duodenum. We shall 

 first consider those features which are found throughout the small 

 intestine, and then describe those which are peculiar to the duod- 

 enum (Fig. 119). 



The mucous membrane presents thin, transverse folds, the val- 

 vulse conniventes, which are not obliterated when the intestinal wall 

 is stretched. They are made up of a thin layer of areolar tissue, 

 extending from the submucous coat of the intestine, which is cov- 

 ered on both surfaces with mucous membrane. This arrangement 

 serves greatly to increase the surface of mucous membrane coming 

 in contact with the contents of the intestine, a provision facilitating 

 absorption of the products of digestion. 



