MUCOUS MEMBRANE OF THE SMALL INTESTINE. 311 



the tube, running, for the most part, at right angles to the 

 external layer, but some of them having rather an oblique 

 direction. The circular layer is thickest in the duodenum ; 

 diminishing gradually in thickness to the middle of the 

 jejunum, but after that maintaining about a uniform thick- 

 ness throughout the canal, to the ileo-caecal valve. 



The jejunum, the second division of the small intestine, is 

 continuous with the duodenum. It presents no well-marked 

 line of separation from the third division, but is generally 

 considered to include the upper two-fifths of the small in- 

 testine, the lower three-fifths being called the ileum. It has 

 received its name from the fact that it is almost always 

 found empty after death. This portion of the intestine pre- 

 sents no important peculiarities as regards its peritoneal and 

 muscular coat. 



The ileum is somewhat narrower and thinner than the 

 jejunum ; otherwise possessing no marked peculiarities except 

 in the structure of its mucous membrane. This opens into 

 the commencement of the colon, and is the termination of 

 the small intestine. 



Mucous Membrane of the Small Intestine. The mucous 

 coat of the small intestine is somewhat thinner than the 

 lining membrane of the stomach. It is thickest in the duode- 

 num, and gradually becomes thinner till we reach the ileum. 

 It is highly vascular, presenting, like the mucous membrane 

 of the stomach, a great increase in the quantity of blood 

 during the process of digestion. It has a peculiar soft and 

 velvety appearance, and during digestion, is of a vivid red 

 color, being pale pink during the intervals. It presents for 

 anatomical description the following parts : 1, folds of the 

 membrane, called valvulse conniventes ; 2, duodenal race- 

 mose glands, or the glands of Brunn ; 3, intestinal tubules, 

 or the follicles of Lieberkuhn ; 4, intestinal villi ; 5, solitary 

 glands or follicles ; 6, agminated glands, or the patches of 

 Peyer. 



