CHAPTER II. 



FIG. 37. 



ABSOEPTION. 



THE absorption of the digested food, which is the main office per- 

 formed by the small intestine, is provided for by a special struct- 

 ure of its mucous membrane. The apparatus consists in an abundance 

 of minute eminences or prolongations, the so-called viUi of the small 

 intestine, so closely set over its surface as to give it a characteristic velvety 

 appearance. They are found throughout this part of the alimentary 

 canal, from the pylorus to the free border of the ileo-caBcal valve, most 

 abundant in the duodenum and jejunum, rather less so in the ileum, but 

 averaging in number from 20 to 40 to the square millimetre. In the 

 upper part of the intestine they are flattened and leaf-life, cylindrical or 

 filamentous in its middle and lower portions In man 

 they are about one-half a millimetre in length. 



Each villus is covered with nucleated, finely gran- 

 ular cylindrical epithelium cells, closely united with 

 each other by their lateral surfaces, and presenting 

 at their outermost portion a transparent layer, 

 marked, according to Kolliker, Frey, and other 

 observers, by fine vertical striations. It is pene- 

 trated below by blood-vessels from a terminal twig- 

 of the mesenteric artery, which form by their divi- 

 sion and inosculation a capillary net-work beneath 

 the epithelial layer. At its base they reunite to 

 form a venous branch, one of the rootlets of the 

 mesenteric vein. 



In the deeper part of the villus, and nearly in its 

 longitudinal axis, there is the commencement of a 

 lymphatic vessel, which, after its emergence, joins 

 the general abdominal system of lymphatic or / 

 lacteal vessels. It is usually single in the filiform a 

 and cylindrical villi, double or triple in those of 

 more flattened form. It has exceedingly thin walls, 

 consisting of a single layer of flattened epithelium 

 cells. 



Closed Follicles of the Small Intestine. In 

 addition to the follicles of Lieberkiiha, the intestine 

 presents two sets of glandular-looking organs, 

 known as the glandulae solitarise and the glandulae 

 agminatse. The first of these, or the solitary glandules, are found in 

 the upper part of the intestine, scattered over its surface, as minute 



195 



AN INTESTINAL YILLVS. 

 a. Layer of cylindrical 

 epithelium, with its ex- 

 ternal transparent stri- 

 ated portion. b, b. 

 Bloodvessels entering 

 and leaving the villas. 

 c. Lymphatic vessel oc- 

 cupying its central 

 axis. (Leydig.) 



