Chap. XV.] 



ALIMENTATION. 



183 



into the interior of the tube. The onward course of the food 

 is delayed by being caught in the hollows formed hy these 

 folds, and thus more thoroughly subjected to the action of the 

 digestive juices : this arrangement also affords a larger surface 

 for absorption. The valvulse conniventes are not found in the 

 beginning of the duodenum, but begin to appear one or two 

 inches from the pylorus ; about the middle of the jejunum they 

 begin to decrease in size, and in the lower part of the ileum 

 they almost entirely disappear. 



Again, the surface of the mucous membrane is increased by 

 the linger-like projections which are so close set as to give a 



Fig. 116. — Sectiox through the Lymphoh) Tissue of a Solitary Glanb. 

 (Cadiat.) a, centre of the gland, with the lymphoid tissue fallen away; b, epithe- 

 lium of mucous memhrane ; c, c, villi, with epithelium partly broken away ; d, crypts, 

 or glands, of Lieberkiihu. 



shaggy or velvety appearance to the membrane. These projec- 

 tions or villi, as they are termed, extend throughout the whole 

 length of the small intestine, and are especially provided for 

 purposes of absorption. Each villus is a portion of the mucous 

 membrane, and consists of an external layer of columnar cells 

 attached to a basement membrane, and of a central mass of lym- 

 phoid tissue. In the centre of each villus is the rootlet of a 

 lacteal vessel, while under the basement membrane is a network 

 of capillaries. The blood-vessels and lymphatics of the villi 

 communicate with networks of both vessels in the sub-mucous 



