288 THE DIGESTIVE SYSTEM 



dinal sections of the organ the same fibres are seen in longitudinal 

 section. 



The submucosa, of areolar connective tissue, is identical with 

 that of the stomach except in the duodenum where it is penetrated 

 by the branched tubulo-acinar glands of Brunner. The muscularis 

 mucosae forms a complete muscular layer and, except in the duo- 

 denum, is not penetrated by the secreting glands. 



The Mucous Membrane of the small intestine 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 por- 

 tion between the muscularis mucosae and the bases of the intesti- 

 nal villi. It is almost completely occupied by the simple tubular 

 glands or crypts of Lieberkuhn. 



The corium of the small intestine, in which the crypts of Lie- 

 berkuhn are embedded, and which forms the substance of the 

 intestinal villi, consists of a fibro-reticular stroma which is so infil- 

 trated with leucocytes as to form a diffuse lymphoid tissue. In 

 many parts of the mucosa the lymphoid tissue forms isolated 

 nodules, the solitary follicles, or aggregations of such nodules, 

 which are known as the agminated follicles or Peyer's patches. 

 Solitary follicles 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. 



The structure of the solitary follicles does not differ from that 

 of other lymphoid 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 crypts of Lie- 

 berkuhn 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 follicle. 



The largest of the solitary follicles not only produce a distinct 

 elevation of the surface of the mucous membrane but may even 

 break through the muscularis mucosse and project into the con- 

 nective tissue of the submucosa. The solitary follicles, like other 

 lymphoid nodules, usually contain a germinal center. 



