Chap. XIV] THE DIGESTIVE SYSTEM 



267 



Fig. 152. — Portion of Small Intes- 

 tine LAID OPEN TO SHOW VALVULE CONNI- 



VENTES. (Collins.) 



folds extends part of the way around the circumference of the in- 

 testine. Unlike the rugfe of the stomach, the valvulae conniventes 

 do not disappear when the 

 intestine is distended. About 

 the middle of the jejunum 

 they begin to decrease in 

 size, and in the lower part of 

 the ileum they almost en- 

 tirely disappear. The pur- 

 pose of the circular folds is : 

 (1) to prevent the food from 

 passing through the intes- 

 tines too quickly, and (2) to present a greater surface for the 

 absorption of digested food. 



J^ilH. — Throughout the whole length of the small intestine the 

 mucous membrane presents a velvety appear- 

 ance due to minute finger-like projections 

 called villi. Each villus consists of a central 

 lymph channel called a lacteal, surrounded 

 by a network of blood capillaries, held to- 

 gether by lymphoid tissue. This in turn is 

 surrounded by a layer of columnar cells and 

 covered by the mucous coat of the intestine. 

 After the food has been digested it passes into 

 the lacteals and capillaries of the villi, so that 

 this arrangement increases the surface for 

 absorption. 



Glands and nodes of the small intestine. 

 — Besides these projections formed for ab- 

 sorption the mucous membrane is thickly 

 studded with secretory glands and nodes. 

 These are known as — 



1. Simple follicles or crypts of Lieber- 

 kuhn. 

 2. Duodenal or Brunner's glands. 



[ (a) Solitary lymph nodules. 

 [ (h) Aggregated lymph nodules. 

 (1) Simple follicles. — These glands are found all over the 

 surface of the small and large intestine. They are simply tubular 



Fig. 153. — An Intes- 

 tinal Villus, a, a, a, 

 columnar epithelium ; 

 b, b, capillary network ; 

 d, lacteal vessel. 



3. Lymph nodules 



