1186 



SPLANCHNOLOGY 



SUPERIOR 



HEMORRHOIDAV. 

 VEIN 



SUPERIOR 



HEMORRHOIDAL 

 ARTERY 



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 epiploicae. They are 

 most numerous on the transverse 

 colon. 



The muscular coat (tunica mus- 

 cularis) consists of an external longi- 

 tudinal, 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 (tcsncei coli), 

 each of about 12 mm. in width; 

 one, the posterior, is placed along 

 the attached border of the intestine; 

 the anterior, the largest, corre- 

 sponds 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 sig- 

 moid 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 length- 

 ened, 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 por- 

 tion 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 vertebrae, 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 Eectococcygeal muscles. 



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 



1 See footnote, p. 1183. 



MIDDLE 



HEMORRHOIDAL 



ARTERY 



INFERIOR 



HEMORRHOIDAL 



ARTERY 



FIG. 1083. The bloodvessels of the rectum and anus, showing the dis- 

 tribution and anastomosis on the posterior surface near the termination 

 of the gut. (Poirier and Charpy) 



