Reference > Anatomy of the Human Body > Page 1186
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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The serous coat (tunica serosa) is derived from the peritoneum, and invests the different portions of the large intestine to a variable extent. The cecum is completely covered by the serous membrane, except in about 5 per cent. of cases where the upper part of the posterior surface is uncovered. The ascending, descending, and iliac parts of the colon are usually covered only in front and at the sides; a variable amount of the posterior surface is uncovered. 1 The transverse colon is almost completely invested, the parts corresponding to the attachment of the greater omentum and transverse mesocolon being alone excepted. The sigmoid colon is entirely surrounded. The rectum is covered above on its anterior surface and sides; below, on its anterior aspect only; the anal canal is entirely devoid of any serous covering. In the course of the colon the peritoneal coat is thrown into a number of small pouches filled with fat, called appendices epiploicæ. They are most numerous on the transverse colon.
  The muscular coat (tunica muscularis) consists of an external longitudinal, and an internal circular, layer of non-striped muscular fibers:
  The longitudinal fibers do not form a continuous layer over the whole surface of the large intestine. In the cecum and colon they are especially collected into three flat longitudinal bands (tænæi coli), each of about 12 mm. in width; one, the posterior, is placed along the attached border of the intestine; the anterior, the largest, corresponds along the arch of the colon to the attachment of the greater omentum, but is in front in the ascending, descending, and iliac parts of the colon, and in the sigmoid colon; the third, or lateral band, is found on the medial side of the ascending and descending parts of the colon, and on the under aspect of the transverse colon. These bands are shorter than the other coats of the intestine, and serve to produce the sacculi which are characteristic of the cecum and colon; accordingly, when they are dissected off, the tube can be lengthened, and its sacculated character disappears. In the sigmoid colon the longitudinal fibers become more scattered; and around the rectum they spread out and form a layer, which completely encircles this portion of the gut, but is thicker on the anterior and posterior surfaces, where it forms two bands, than on the lateral surfaces. In addition, two bands of plain muscular tissue arise from the second and third coccygeal vertebræ, and pass downward and forward to blend with the longitudinal muscular fibers on the posterior wall of the anal canal. These are known as the Rectococcygeal muscles.


FIG. 1083– The bloodvessels of the rectum and anus, showing the distribution and anastomosis on the posterior surface near the termination of the gut. (Poirier and Charpy) (See enlarged image)

  The circular fibers form a thin layer over the cecum and colon, being especially accumulated in the intervals between the sacculi; in the rectum they form a thick layer, and in the anal canal they become numerous, and constitute the Sphincter ani internus.
  The areolar coat (tela submucosa; submucous coat) connects the muscular and mucous layers closely together.
  The mucous membrane (tunica mucosa) in the cecum and colon, is pale, smooth, destitute of villi, and raised into numerous crescentic folds which correspond to the intervals between
Note 1.  See footnote, p. 1183. [back]

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