672 ORGANS OF DIGESTION. 



other. When these are divided or removed, the ileum may be drawn outwards, 

 and all traces of the valve will be lost, the ileum appearing to open into the large 

 intestine by a funnel-shaped orifice of large size. 



The surface of each segment of the valve directed towards the ileum is covered 

 with villi, and presents the characteristic structure of the mucous membrane of 

 the small intestine ; whilst that turned towards the large intestine is destitute of 

 villi, and marked with the orifices of the numerous tubuli peculiar to this mem- 

 brane. These differences in structure continue as far as the free margin of the 

 valve. 



When the cascum is distended, the margins of the opening are approximated, so 

 as to prevent any reflux into the ileum. 



The colon is divided into four parts, the ascending, transverse, descending, and 

 the sigmoid flexure. 



The ascending colon is smaller than the caecum. It passes upwards from the 

 right iliac fossa, to the under surface of the liver, on the right of the gall-bladder, 

 where it bends abruptly inwards to the left, forming the hepatic flexure. It is 

 retained in position with the posterior wall of the abdomen by the peritoneum, 

 which covers its anterior surface and sides, its posterior surface being connected by 

 loose areolar tissue with the Quadratus lumborum and right kidney ; sometimes the 

 peritoneum almost completely invests it, and forms a distinct but narrow meso- 

 colon. It is in relation, in front, with the convolutions of the ileum and the 

 abdominal parietes ; behind, it lies on the Quadratus lumborum muscle, and right 

 kidney. 



The transverse colon, the longest part of the large intestine, passes transversely 

 from right to left across the abdomen, opposite the confines of the epigastric and 

 umbilical zones, into the left hypochondriac region, where it curves downwards 

 beneath the lower end of the spleen, forming its splenic flexure. ' In its course it 

 describes an arch, the concavity of which is directed backwards towards the ver- 

 tebral column ; hence the name, transverse arch of the colon. This is the most 

 movable part of the colon, being almost completely invested by peritoneum and 

 connected to the spine behind by a large and wide duplicature of this membrane, 

 the transverse mesocolon. It is in relation, by its upper surface, with the liver and 

 gall-bladder, the great curvature of the stomach, and the lower end of the spleen ; 

 by its under surface, with the small intestines; by its anterior surface, with the 

 anterior layers of the great omentum and the abdominal parietes ; by its posterior 

 surface, with the transverse mesocolon. 



The descending colon passes almost vertically downwards through the left hypo- 

 chondriac and lumbar regions to the upper part of the left iliac fossa, where it 

 terminates in the sigmoid flexure. It is retained in position by the peritoneum, 

 which covers its anterior surface and sides, its posterior surface being connected 

 by areolar tissue with the left crus of the Diaphragm, the left kidney, and the Quad- 

 ratus lumborum. It is smaller in calibre, and more deeply placed than the ascend- 

 ing colon. 



The sigmoid flexure is the narrowest part of the colon ; it is situated in the left 

 iliac fossa, commencing at the termination of the descending colon, at the margin 

 of the crest of the ilium, and ending in the rectum, opposite the left sacro-iliac 

 symphysis. It curves in the first place upwards, and then descends vertically, 

 and to one or the other side like the letter/, hence its name; and is retained in 

 its place by a loose fold of peritoneum, the sigmoid mesocolon. It is in relation, 

 in front, with the small intestines and abdominal parietes; behind, with the iliac 

 fossa. 



The Rectum is the terminal part of the large intestine, and extends from the 

 sigmoid flexure to the anus ; it varies in length from six to eight inches, and has 

 received its name from being somewhat less flexuous than any other part of the 

 intestinal canal. It commences opposite the left sacro-iliac symphysis, passes 

 obliquely downwards from left to right to the middle of the sacrum, forming a 

 gentle curve to the right side. Eegaining the middle line, it descends in front of 



