SMALL INTESTINES. 



735 



Fig. 402. Two Villi magnified. 



They extend transversely across the cylinder of the intestine for about three- 

 fourths or five-sixths of its circumference. The larger folds are about two 

 inches in length, and two-thirds of an inch in depth at their broadest part ; but 

 the greater number are of smaller size. The larger and smaller folds alternate 

 with each other. They are not found at the commencement of the duodenum, 

 but begin to appear about one or two inches beyond the pylorus. In the lower 

 part of the descending portion, below the point where the common choledoch 

 and pancreatic ducts enter the intestine, they are very large and closely approxi- 

 mated. In the transverse portion of the duodenum and upper half of the jeju- 

 num, they are large and numerous ; and from this point, down to the middle of 

 they the ileum, they diminish considerably in size. In the lower part of the 

 ileum, almost entirely disappear; hence the comparative thinness of this portion 

 of the intestine, as compared with the duodenum and jejunum. The valvulaa 

 .^onniventes retard the passage of the food along the intestines, and afford a 

 more extensive surface for absorption. 



The villi are minute, highly vascular processes, projecting from the mucous 

 membrane of the small intestine throughout its whole extent, and giving to its 

 surface a velvety appearance. In shape, some are triangular and laminated, 

 others conical or cylindrical, with clubbed or filiform extremities. They are 

 largest and most numerous in the duodenum and jejunum, and become fewer 

 and smaller in the ileum. Krause estimates their number in the upper part of 

 the small intestine at from fifty to ninety in a square line ; and in the lower 

 part from forty to seventy ; the total number for the whole length of the intes- 

 tine being about four millions. 



In structure each villus consists of a network of capillary and lacteal vessels, 

 with nuclear corpuscles and fat-globules in their interstices, inclosed in a thin 

 prolongation of basement membrane covered , 

 by a single layer of columnar epithelium, 

 the particles of which are arranged per- 

 pendicular to the surface. A layer of or- 

 ganic muscular fibre has been described, 

 forming a thin hollow cone round the cen- 

 tral lacteal. It is possible that this assists 

 in the propulsion of the chyle along the 

 vessel. The mode of origin of the lacteals 

 within the villi is unknown. 



The simple follicles, or crypts of Lieber- 

 kuhn, are found in considerable numbers 

 over every part of the mucous membrane 

 of the small intestine. They consist of 

 minute tubular depressions of the mucous 



membrane, arranged perpendicularly to the surface, upon which they open by 

 small circular apertures. They may be seen with the aid of a lens, their orifices 

 appearing as minute dots, scattered between the villi. Their walls are thin, 

 consisting of a layer of basement membrane, lined by cylindrical epithelium, 

 and covered on their exterior by capillary vessels. Their contents vary, even 

 in health, and the purpose served by their secretion is still very doubtful. 



The duodenal, or Brunner's glands, are limited to the duodenum and com- 

 mencement of the jejunum. They are small, flattened, granular bodies, 

 imbedded in the submucous areolar tissue, and open upon the surface of the 

 mucous membrane by minute excretory ducts. They are most numerous and 

 largest near the pylorus. They may be compared to the elementary lobules 

 of a salivary gland, spread out over a broad surface, instead of being collected 

 in a mass. In structure they resemble the pancreas. 



The solitary glands (glandulse solitaries) are found scattered throughout the 

 mucous membrane of the small intestine, but are most numerous in the lower 

 part of the ileum. They are small, round, whitish bodies, from half a line to a 



Artery 



