THE INTESTINAL CANAL. 



1029 



which separate the pouches, cellulte, or haustra. The whole projection is made 

 up of all the coats of the intestine and is called the plica or valvula sigmoidea 

 (Fig. 649). A valve passes between two trenire Avhere otherwise a transverse fold 

 would exist. Only rarely do two or three valves lie in the same plane, so that 

 they would be in position to effect a scissor-like motion and cut a mass of faecal 

 matter into scybalse or round balls. 



The circular fibres form a thin continuous layer and are especially collected 

 in the constrictions between the sacculi. In the rectum they form the Internal 

 sphincter muscle. 



The xubmiu-ous coat is in the same position and serves the same purpose as in 

 the small intestine. 



The mucous membrane is pale, smooth, destitute of villi and raised into cres- 

 centic folds, separating the pouches and corresponding to the external constric- 

 tions separating the sacculi. 



As in the small intestine, the mucous membrane consists of a muscular layer, 

 the muscularis mucostie ; of a quantity of retiform tissue, in which the vessels 

 ramify ; of a basement membrane and epithelium, which is of the columnar 

 variety and exactly resembles the epithelium found in the small intestine. The 

 mucous membrane of this portion of the bowel presents for examination crypts 

 of Lieberkiihu and solitary glands. 



The crypts of Lieberkiikn are tubular prolongations of the mucous mem- 

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

 longer, more numerous, and placed in much closer apposition than those of the 

 small intestine, and they open by minute rounded orifices upon the surface, giving 

 it a cribriform appearance. 



The solitary glands (Fig. 650) in the large intestine are most abundant in 



Surface of mucous membrane, 

 with openings of Lieberkuhn't 

 follicle*. 



Lieberkiihn'i foUidet. 



Muscularis mucoste (two layers). 

 Submucous connective tissue. 



Solitary gland. 

 FIG. 650. Minute structure of large intestine. 



the cfKcum and appendix vermiformis, but are irregularly scattered also over the 

 rest of the intestine. They are similar to those of the small intestine. 



Vessels and Nerves. The arteries supplying the large intestine give off large 

 branches, which ramify between the muscular coats, supplying them, and, after 

 dividing into small vessels in the submucous tissue, pass to the mucous membrane. 

 Those arteries are the ileo-colic, colica dextra, colica media from the superior 

 mesenteric, colica sinistra and sigmoidea from the inferior mesenteric. 



The lymphatic vessels consist of two layers ; the deep set lie under the glands 

 of Lieberkuhn, and the superficial forms a wide-meshed network which penetrates 

 the submucosa in all directions. The lymphatics of the ascending, transverse, and 

 descending colons open into the mesenteric glands, those of the sigmoid flexure 

 into the lumbar glands. 



