CHAPTEE IX. 



Fig. 56. 



ABSORPTION. 



THE mucous membrane of the small intestine, beside containing the 

 glandular follicles already described, is provided with a special appa- 

 ratus for the process of absorption. This apparatus consists of innu- 

 merable minute eminences or prolongations of its substance, so closely 

 set over its free surface that they give to it a characteristic velvety appear- 

 ance. These are the so-called villosities or villi of the small intestine. 

 They are found throughout this part of the alimentary canal, from the 

 pylorus to the free border of the ileo-csecal valve, most abundant in the 

 duodenum and jejunum, rather less so in theileum, but in general in the 

 proportion of from 20 to 40 to the square millimetre. In the upper 

 part of the intestine they are flattened, laminated, or leaf-like in form, 

 becoming cylindrical and filamentous in the middle and lower portions. 

 In the human subject they are about one-half a millimetre in length. 



Each villus consists of a mass of tissue continuous with that of the 

 mucous membrane beneath. It is covered 

 with a uniform layer of nucleated, finely 

 granular cylindrical epithelium cells, closely 

 united with each other by their lateral sur- 

 faces, and presenting at their outermost por- 

 tion a thin layer which is more transparent 

 than the rest of their substance, and is 

 marked, according to Kolliker, Frey, and 

 other observers, by fine vertical striations. 

 The villus is penetrated from below by blood- 

 vessels supplied from a terminal twig of the 

 mesenteric artery, which form by their fre- 

 quent division and inosculation an exceed- 

 ingly abundant capillary network, almost 

 immediately beneath the epithelial layer. 

 At its base they reunite to form a venous 

 branch, which is one of the commencing 

 rootlets of the mesenteric vein. 



In the deeper part of the villus, and lying 

 nearly in its longitudinal axis, there is also 

 the commencement of a lymphatic vessej, 

 which, after its emergence from the base of 

 the organ, joins the general system of the 

 abdominal lymphatic or lacteal vessels. The 

 lymphatic vessel is usually single in the fili- 



AN IHTESTTTTAL YlLLUS. 



a. Layer of cylindrical epithe- 

 lium, with its external trans- 

 parent striated portion, b b. 

 Bloodvessels entering and leav- 

 ing the villus. c. Lymphatic 

 vessel occupying its central 

 axis. (Leydig.) 



(189) 



