THE LARGE INTESTINE. 515 



short distance above the commencement of the latter, which forms 

 below the point of entrance, a pouch-like portion, about two inches in 

 length, constituting the cceeum, so named because it is a blind pouch 

 or cul-de-sac, Fig. 92, c. 



Projecting from the lower and back part of the caecum, is a narrow, 

 coiled, and tapering tube, about 4 inches in length, and about as thick 

 as a worm, hence named the vermiform or worm-like appendix^ Fig. 

 92, a. It communicates with the caecum by an opening, protected by 

 a membranous ridge ; its outer end is closed. It may be regarded as 

 a part of the caecum arrested in its growth, and is the homologue of 

 the long caecum found in Mammalia generally, the orang-outang, 

 chimpanzee, and wombat being, however, exceptions. 



The caecum, and the ascending, transverse, and descending colon, 

 with its sigmoid flexure, are distinguished from the small intestine, 

 and also from the rectum, by their peculiar sacculated form. The 

 sacculi of these parts, are arranged in three longitudinal rows, sepa- 

 rated from each other by three intermediate bands. Their presence 

 depends upon a peculiar arrangement of the coats of the intestine. 

 These, as in the small intestine, are four in number, viz., proceeding 

 from without inwards, the serous, the muscular, the areolar, and the 

 mucous coats. The serous or peritoneal coat, is complete in only cer- 

 tain portions of the great intestine, viz., in the transverse part of the 

 colon, the sigmoid flexure, and the upper part of the rectum ; whilst 

 the caecum, the ascending and descending colon, and the lower part 

 of the rectum, are closely fixed behind, and therefore receive only a 

 partial covering from the peritoneum. The muscular coat consists, as 

 usual, of external longitudinal and internal circular fibres. On the 

 vermiform appendix, both sets of fibres form uniform layers. On the 

 sacculated pouch of the caecum, and throughout the whole length of the 

 colon, however, the longitudinal fibres, thinly scattered over the sac- 

 culi, are chiefly collected into three long bundles, which form the three 

 longitudinal bands between the sacculi. These bands, indeed, are 

 shorter, from end to end, by nearly one-half, than the intermediate part 

 of the intestine, which accordingly is puckered, and projects inwards 

 in the form of sharp crescentic ridges between the dilated parts which 

 form the sacculi. These sacculi become smaller and more scattered on 

 the sigmoid flexure of the colon. On the rectum, the longitudinal 

 fibres speedily form a thick stratum, evenly distributed over the whole 

 circumference of the intestine, so that the sacculi disappear. The cir- 

 cular fibres cover the whole surface, but are accumulated in greater 

 numbers on the ridges between the sacculi. Upon the rectum, how- 

 ever, they soon form a thick and uniform layer ; the lower portion of 

 this is particularly well developed, constituting the internal sphincter 

 muscle, which constricts the lower part of the bowel, and assists the 

 external sphincter muscle, situated beneath the skin, around the aper- 

 ture of the intestine, in keeping the bowel closed. The areolar or sub- 

 mucous coat of the large intestine is attached loosely to the muscular 

 coat, but more intimately to the mucous membrane ; it is sacculated, 

 and helps to maintain the form of the intestine ; it supports the tender 

 mucous coat, and furnishes a stratum, in which the bloodvessels, lym- 



