738 ORGANS OF DIGESTION. 



thefrsena, or retinacula of the valve. The left end of this aperture is rounded: 

 the right end is narrow and pointed. 



Each segment of the valve is formed by a reduplication of the mucous mem- 

 brane, and of the circular muscular fibres of the intestine, the longitudinal 

 fibres and peritoneum being continued uninterruptedly across from one intestine 

 to the 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 mem- 

 brane 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 the membrane in the large intestine. These differences in structure continue 

 as far as the free margin of the valve. 



When the caecum 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 contact with the posterior wall of the abdomen by the peri- 

 toneum, 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 convolu- 

 tions 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 down- 

 wards beneath the lower end of the spleen, forming the splenic flexure. In its 

 course it describes an arch, the concavity of which is directed backwards 

 towards the vertebral 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 dupli- 

 cature of that membrane, the transverse meso-colon. 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 meso- 

 colon. 



The descending colon passes almost vertically downwards through the left 

 hypochondriac 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 Quadratus Lumborum. It is smaller in calibre and more 

 deeply placed than the ascending 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 mar- 

 gin 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 verti- 

 cally, and to one or the other side, like the letter S hence its name. It is 

 retained in its place by a loose fold of peritoneum, the sigmoid meso-colon. It 

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

 with the iliac fossa. 



