MUCOUS MEMBRANE OF THE SMALL INTESTINE 



211 



larger in children than in the adult and are larger in the colon than in 

 the small intestine. 



The intestinal villi, although concerned chiefly in absorption, are 

 most conveniently considered in this connection. These exist through- 

 out the small intestine, but they are not found beyond the ileo-csecal 

 valve, although they cover that portion of the valve which looks toward 

 the ileum. Their number is very large, and they give to the membrane 

 its peculiar and characteristic 

 velvety appearance. They 

 are found on the valvulae 

 conniventes as well as on 

 the general surface of the 

 mucous membrane. They are 

 most abundant in the duode- 

 num and jejunum. Sappey 

 estimated, as an average, 

 about 6450 to the square 

 inch (1000 in a square centi- 

 meter), and more than ten 

 millions (10,125,000) through- 

 out the small intestine. In 

 the human subject the villi 

 are in the form of flattened 

 cylinders or cones. In the 

 duodenum, where they resem- 

 ble somewhat the elevations 

 found in the pyloric portion 

 of the stomach, they are 

 shorter and broader than in 

 other situations and are more 

 like flattened conical folds. 

 In the jejunum and ileum, 

 they are in the form of long, 

 flattened cones and cylinders. 

 As a rule the cylindrical form predominates in the lower portion of the 

 intestine. In the jejunum they attain their greatest length, measuring 

 here -fa to ^ of an inch (0.83 to 1.25 millimeter) in length by -^ to T |^ 

 of an inch (0.36 to 0.21 millimeter) in breadth at their base. 



The structure of the villi shows them to be simple elevations of the 

 mucous membrane, provided with bloodvessels and with lacteals, or 

 intestinal lymphatics, Externally is found a single layer of cylindrical 

 epithelial cells resting on a nucleated basement-membrane (see Plate 



Fig. 44. Axial section of a villus of a dog 

 (Kultschitzky). 



a, layer of cuticularized epithelium ; b, goblet-cells ; 



c, space between the attached ends of the epithelium ; 



d, nucleated cell of the basement membrane ; e, non- 

 striated muscular fibres; f, reticulum from the tunica 

 propria; g, lumen of the central lymphatic. (Most of 

 lymphoid cells have been removed and the bloodvessels 

 are not shown.) 



