674 



ORGANS OF DIGESTION. 



the largest, becomes inferior along the arch of the colon, where it corresponds to 

 the attachment of the great omentum, but is in front in the ascending and 

 descending colon and sigrnoid flexure ; the third or lateral band is found on the 

 inner side of the ascending and descending colon, and on the under border of the 

 transverse colon. These bands are nearly one-half shorter than the other parts 

 of the intestine, and serve to produce those sacculi characteristic of the caacum 

 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, and upon its lower part, as well as round the rectum, 

 they spread out, and form a thick uniform layer. 



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 form the 

 Internal sphincter. 



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



The 'mucous membrane, in the caecum and colon, is pale, and of a grayish or pale 

 yellow color. It is quite 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 distension of 

 the gut. Besides these, there are three or four permanent folds, of a semilunar 

 shape, described by Mr. Houston. 1 They are usually three in number ; sometimes 

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

 the commencement of the rectum, on the right side ; another extends inwards from 

 the left side of the tube opposite the middle of the sacrum ; the largest and most 

 constant one projects backwards from the fore part of the rectum, opposite the 

 base of the bladder. When a fourth is present, it is situated about an inch above 

 the anus, on the back of the rectum. These folds are about half an inch in width, 

 and contain some of the circular fibres of the gut. In the empty state of the 

 intestine they overlap each other, as Mr. Houston remarks, so effectually as to 

 require considerable manoeuvring to conduct a bougie or the finger along the canal 

 of the intestine. Their use seems to be, " to support the weight of fecal matter, 

 and prevent its urging towards the anus, where its presence always excites a 

 sensation demanding its discharge." The mucous membrane of the large intestine 

 presents for examination, epithelium, simple follicles, and solitary glands. 



Fig. 340. Minute Structure of Large Intestine. 



Apertures 



Aperture of 

 Solitary f,, 



Stnf> 



The epithelium is of the columnar kind. 



The simple follicles are minute tubular prolongations of the mucous membrane, 

 arranged perpendicularly, side by side, over its entire surface ; they are longer, 



Dub. Hosp. Reports, vol. v. p. 163. 



