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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
its posterior border or root, about 15 cm. long, is attached to the posterior abdominal wall from the left side of the body of the second lumbar vertebra to the right sacroiliac articulation, crossing successively the horizontal part of the duodenum, the aorta, the inferior vena cava, the ureter, and right Psoas muscle (Fig. 1040). Its breadth between its vertebral and intestinal borders averages about 20 cm., and is greater in the middle than at its upper and lower ends. According to Lockwood it tends to increase in breadth as age advances. Between the two layers of which it is composed are contained bloodvessels, nerves, lacteals, and lymph glands, together with a variable amount of fat.


FIG. 1059– Vertical section of a villus from the dog’s small intestine. X 80. (See enlarged image)



FIG. 1060– Transverse section of a villus, from the human intestine. (v. Ebner.) X 350. a. Basement membrane, here somewhat shrunken away from the epithelium. b. Lacteal. c. Columnar epithelium. d. Its striated border. e. Goblet cells. f. Leucocytes in epithelium. f’. Leucocytes below epithelium. g. Bloodvessels. h. Muscle cells cut across. (See enlarged image)


Meckel’s Diverticulum (diverticulum ilei).—This consists of a pouch which projects from the lower part of the ileum in about 2 per cent. of subjects. Its average position is about 1 meter above the colic valve, and its average length about 5 cm. Its caliber is generally similar to that of the ileum, and its blind extremity may be free or may be connected with the abdominal wall or with some other portion of the intestine by a fibrous band. It represents the remains of the proximal part of the vitelline duct, the duct of communication between the yolk-sac and the primitive digestive tube in early fetal life.

Structure.—The wall of the small intestine (Fig. 1058) is composed of four coats: serous, muscular, areolar, and mucous.
  The serous coat (tunica serosa) is derived from the peritoneum. The superior portion of the duodenum is almost completely surrounded by this membrane near its pyloric end, but is only covered in front at the other extremity; the descending portion is covered by it in front, except where it is carried off by the transverse colon; and the inferior portion lies behind the peritoneum which passes over it without being closely incorporated with the other coats of this part of the intestine, and is separated from it in and near the middle line by the superior mesenteric vessels. The rest of the small intestine is surrounded by the peritoneum, excepting along its attached or mesenteric border; here a space is left for the vessels and nerves to pass to the gut.
  The muscular coat (tunica muscularis) consists of two layers of unstriped fibers: an external, longitudinal, and an internal, circular layer. The longitudinal fibers are thinly scattered over the surface of the intestine, and are more distinct along its free border. The circular fibers form a thick, uniform layer, and are composed of plain muscle cells of considerable length. The muscular coat is thicker at the upper than at the lower part of the small intestine.
  The areolar or submucous coat (tela submucosa) connects together the mucous and muscular layers. It consists of loose, filamentous areolar tissue containing bloodvessels, lymphatics, and nerves. It is the strongest layer of the intestine.

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