MUCOUS MEMBBANE OF THE SMALL INTESTINE. 315 



agents concerned in the production of the fluid known as the 

 intestinal juice. 



The intestinal villi, though chiefly concerned in absorp- 

 tion, are most conveniently considered in this connection. 

 These exist throughout the whole of the small intestine, but 

 are not found beyond the ileo-csecal valve, though they 

 cover that portion of the valve which looks toward the ileum. 

 Their number is very great, and they give to the membrane 

 its peculiar and characteristic velvety appearance. They are 

 found on the valvulse conniventes as well as the attached por- 

 tions of the mucous membrane. In the duodenum and jeju- 

 num, they are most numerous. In these parts, there are from 

 fifty to ninety villi to a square line, and in the ileum, from 

 forty to seventy to a square line. 1 Sappey estimates, on an 

 average, about fifty to the square line, and more than ten 

 millions (10,125,000) throughout the whole of the small in- 

 testine. 8 The form of the villi varies somewhat in different 

 animals. In the human subject, they are flattened cylinders 

 or cones. In the duodenum, where they resemble 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 from s \ to j\ of an inch in 

 length, by T V to T 7 of an inch in breadth at their base. 



The structure of the villi shows them to be simple eleva- 

 tions of the mucous membrane, provided with blood-vessels, 

 and probably also with lacteals, or intestinal lymphatics. 

 Externally is found a single layer of long columnar epithe- 

 lial cells, resting on a structureless basement membrane. 



1 KOLLIKER, Manual of Human Microscopic Anatomy, London, 1860, p. 325. 



8 SAPPEY, Traite d'Anatomie Descriptive, Paris, 1857, tome iii., p. 140. Sap- 

 pey estimated twelve to fourteen villi to a square millimetre, which gives, ap 

 proximatively, about fifty to a square line. 



