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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The mucous membrane (tunica mucosa) is thick and highly vascular at the upper part of the small intestine, but somewhat paler and thinner below. It consists of the following structures: next the areolar or submucous coat is a double layer of unstriped muscular fibers, outer longitudinal and inner circular, the muscularis mucosæ internal to this is a quantity of retiform tissue, enclosing in its meshes lymph corpuscles, and in this the bloodvessels and nerves ramify; lastly, a basement membrane, supporting a single layer of epithelial cells, which throughout the intestine are columnar in character. The cells are granular in appearance, and each possesses a clear oval nucleus. At their superficial or unattached ends they present a distinct layer of highly refracting material, marked by vertical striæ, the striated border.


FIG. 1061– Villi of small intestine, showing bloodvessels and lymphatic vessels. (Cadiat.) (See enlarged image)

  The mucous membrane presents for examination the following structures, contained within it or belonging to it:
Circular folds.
Duodenal glands.
Villi.
Solitary lymphatic nodules.
Intestinal glands.
Aggregated lymphatic nodules.


FIG. 1062– An intestinal gland from the human intestine. (Flemming.) (See enlarged image)

  The circular folds (plicæ circulares [Kerkringi]; valvulæ conniventes; valves of Kerkring) are large valvular flaps projecting into the lumen of the bowel. They are composed of reduplications of the mucous membrane, the two layers of the fold being bound together by submucous tissue; unlike the folds in the stomach, they are permanent, and are not obliterated when the intestine is distended. The majority extend transversely around the cylinder of the intestine for about one-half or two-thirds of its circumference, but some form complete circles, and others have a spiral direction; the latter usually extend a little more than once around the bowel, but occasionally two or three times. The larger folds are about 8 mm. 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 2.5 or 5 cm. beyond the pylorus. In the lower part of the descending portion, below the point where the bile and pancreatic ducts enter the intestine, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous, but

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