PHYSIOLOGICAL ANATOMY OF THE SMALL INTESTINE. 237 



occupying nearly the whole of its structure. Between the tubules, are blood- 

 vessels, embedded in a dense stroma of fibrous tissue with non-striated mus- 

 cular fibres. In vertical sections of the mucous membrane, the only situa- 

 tions where the tubules are not seen are in that portion of the duodenum 

 occupied by the ducts of the glands of Brunner and immediately over the 

 centre of the larger solitary glands and some of the closed follicles which 

 are collected to form the patches of Peyer. The tubes are not entirely absent 

 in the patches of Peyer, but are here collected in rings, twenty or thirty tubes 

 deep, which surround each of the closed follicles. Microscopical examination 

 of the surface of the mucous membrane by reflected light shows that the 

 openings of the tubules are between the villi. 



The tubules usually are simple, though sometimes bifurcated, are com- 

 posed externally of a structureless basement-membrane, and are lined with a 

 layer of cylindrical epithelium like the cells which cover the villi, the only 

 difference being that in the tubes the cells are shorter. These cells never 

 contain fatty granules, even during the digestion of fat. The central cavity 

 which the cells enclose, which is about one-fourth of the diameter of the 

 tube, is filled with a clear, viscid fluid, which is the most important constitu- 

 ent of the intestinal juice. The length of the tubules is equal to the thick- 

 ness of the mucous membrane and is about ^ of an inch (0-33 mm.). Their 

 diameter is about ^ t an i ncn (0'07 mm.). In man they are cylindrical, 

 terminating in a single, rounded, blind extremity, which frequently is a little 

 larger than the rest of the tube. These tubules are the chief agents concerned 

 in the production of the fl,uid known as the intestinal juice. 



The intestinal villi, though chiefly concerned in absorption, are most con- 

 veniently considered in this connection. These exist throughout the whole 

 of the small intestine, but are not found beyond the ileo-caecal valve, although 

 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 char- 

 acteristic 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 duodenum and jejunum. Sappey estimated, as an average, 

 about 6,450 fo the square inch (1,000 in a square centimetre) and more than 

 ten millions (10,125,000) throughout the whole of the small intestine. In 

 the human subject the villi are flattened cylinders or cones. In the duode- 

 num, where they resemble somewhat the elevations found in the pyloric por- 

 tion 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 cylin- 

 drical form predominates in the lower portion of the intestine. In the jeju- 

 num they attain their greatest length, measuring here -fa to -fo of an inch 

 (0-83 to 1-25 mm.) in length by ^ to y^ of an inch (0-36 to 0'21 mm.) 

 in breadth at their base. 



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

 mucous membrane, provided with blood-vessels and with lacteals, or intestinal 

 lymphatics. Externally is found a single layer of long, cylindrical epithelial 

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