SMALL INTESTINE. 



669 



Fig. 336. Two Villi magnified. 



Artery 



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 approximated. In the 

 transverse portion of the duodenum and upper half pf the jejunum, they are large 

 and numerous ; and from this point, as far as the middle of the ileum, they 

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

 entirely disappear ; hence the comparative thinness of this portion of the intes- 

 tine, as compared with the duodenum and jejunum. The valvulae conniventes 

 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 mem- 

 brane of the small intestine throughout its 

 whole extent, and giving to its surface a 

 beautiful velvety appearance. In shape, 

 some are triangular and laminated, others 

 conical or cylindrical, with clubbed or fili- 

 form extremities. They are largest and 

 most numerous in the duodenum and jeju- 

 num, 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 intestine 

 being 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 perpendicularly to the surface. A 

 layer of organic muscular fibre has been described forming a thin hollow cone 

 round the central 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 un- 

 known. 



The simple follicles or crypts of Lieberkiihn 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 conix 

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

 in the submucous areolar tissue, and open upon the surface of the mucous mem. 

 brane 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 (glandulae solitarise) 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 

 line in diameter, consisting of a closed saccular cavity, having no excretory duct, 

 and containing an opaque white secretion. Their free surface is covered with 

 villi, and each gland is surrounded by openings like those of the follicles of 

 Lieberkiihn. Their use is not known. * 



