366 THE DIGESTIVE SYSTEM 



The Mucous Membrane. The mucous membrane of the small in- 

 testine is divisible into an inner and an outer zone. In the inner zone 

 the corium forms finger-like projections, the intestinal villi, which are 

 covered with tall columnar epithelium containing many mucus-secreting 

 goblet cells. The villi are characteristic of the small intestine, in which 

 alone they occur. They serve to increase the area of the lining epi- 

 thelium of the intestine, whose chief function is that of absorption. 



The outer zone of the mucous membrane includes all that portion 

 between the muscularis mucosae and the bases of the intestinal villi. It 

 is almost completely occupied by the simple tubular intestinal glands 

 (or crypts of Lieberkiihn). 



THE CORIUM. The corium of the small intestine, in which the 

 intestinal glands are embedded, and which forms the substance of the 

 intestinal villi, consists of a fibroreticular stroma which is so infiltrated 

 with lymphocytes as to form a diffuse lymphoid tissue. In many parts 

 of the mucosa the lymphoid tissue forms isolated nodules, the solitary 

 nodules, or aggregations of such nodules, which are known as the agmi- 

 nate nodules or Peyer's patches. Solitary nodules occur throughout 

 both the large and the small intestine. Peyer's patches are found only 

 in the small intestine and are most numerous in the upper portion of the 

 ileum. 



Structure of tlie Solitary Nodules. The structure of the solitary 

 nodules does not differ from that of other lymph nodules. They vary 

 much in size, most of them being of sufficient diameter to occupy 

 the entire thickness of the mucous membrane. They push aside the 

 adjacent intestinal glands by which they are encircled, and few or no 

 villi project from their free surface. The adjacent villi are so inclined 

 that their free ends often hide all but the projecting apex of the ovoid 

 solitary nodule. 



The largest of the solitary nodules not only produce a distinct ele- 

 vation of the surface of the mucous membrane but may even break 

 through the muscularis mucosae and project into the connective tissue 

 of the submucosa. The solitary nodules, like other lymph nodules, 

 usually contain a germinal center. 



Agminate Nodules (Aggregate Nodules; Peyer's Patches). Agmin- 

 ate nodules are formed by accumulations of lymph nodules, usually 

 occurring in that portion of the intestinal mucosa which is farthest 

 removed from the attachment of the mesentery. They frequently 

 form oval areas of macroscopic size. They usually number about thirty, 

 though there may be fewer, and frequently many more. The number 



