928 THE ORGANS OF DIGESTION. 



Structure. The large intestine has four coats serous, muscular, areolar, and 

 mucous. 



The serous coat is derived from the peritoneum, and invests the different por- 

 tions of the large intestine to a variable extent. The caecum is completely covered 

 by the serous membrane, except in a small percentage of cases (5 or 6 per cen^.), 

 where a small portion of the upper end of the posterior surface is uncovered. The 

 ascending and descending 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 great 

 omentum and transverse mesocolon being alone excepted. The sigmoid flexure 

 is completely surrounded, except along the line to which the sigmoid mesocolon is 

 attached. The upper part of the rectum is completely invested by the peritoneum, 

 except along the attachment of the mesorectum ; the middle portion is covered 

 only on its anterior surface, and part of its sides in the upper portion ; and the 

 lower portion is entirely devoid of any serous covering. In the course of the colon 

 and upper part of the rectum the peritoneal coat is thrown into a number of small 

 pouches filled with fat, called appendices epiploicce. They are chiefly appended to 

 the transverse colon. 



The muscular coat consists of an external longitudinal and an internal circular 

 layer of muscular fibres. 



The longitudinal fibres, although found to a certain extent all round the intes- 

 tine, do not form a uniform layer over the whole surface of the large intestine. 

 In the caecum and colon they are especially collected into three flat longitudinal 

 bands or tsenise, each being about half an inch in width. These bands commence 

 at the attachment of the vermiform appendix, which is surrounded by a uniform 

 layer of longitudinal muscular fibres, to the caecum : 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 great omentum, but is in front 

 in the ascending and descending colon and sigmoid flexure ; the third, or lateral 

 band, is found on the inner side of the ascending and descending colon, and on 

 the under aspect of the transverse colon. These bands are nearly one-half shorter 

 than the other coats of the intestine, and serve to produce the sacculi which are 

 characteristic of the caecum and colon ; accordingly, when they are dissected off, 

 the tube can be lengthened, and its sacculated character becomes lost. In the 

 sigmoid flexure the longitudinal fibres become more scattered ; but upon its lower 

 part, and round 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 sur- 

 faces, where it forms two bands, than on the lateral surfaces. In addition to the 

 muscular fibres of the bowels, 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 fibres on the posterior wall of the anal canal. These are 

 known as the recto-coccygeal muscles. 



The circular fibres form a thin layer over the caecum and colon, being especially 

 accumulated in the intervals between the sacculi ; in the rectum they form a 

 thick layer, especially at its lower end, where they become numerous, and consti- 

 tute the Internal sphincter. 



The areolar coat connects the muscular and mucous layers closely together. 



The mucous membrane, in the caecum and colon, is pale, smooth, destitute of 

 villi, and raised into numerous crescentic folds which correspond to the intervals 

 between the sacculi. In the rectum it is thicker, of a darker color, more vascular, 

 and connected loosely to the muscular coat, as in the oesophagus. When the lower 

 part of the rectum is contracted, its mucous membrane is thrown into a number of 

 folds, some of which, near the anus, are longitudinal in direction, and are effaced 

 by the distention of the gut. Besides these there are certain permanent folds, of a 

 semilunar shape, known as Houston's valves. 2 They are usually three in number ; 

 sometimes a fourth is found, and occasionally only two are present. One is situated 



1 See foot-note, p. 924. 2 Dublin Hosp. Reports, vol. v. p. 163. 



