THE LARGE INTESTINE. 923 



tube, but leaves the distal third free and completely covered by peritoneum. 

 Between its two layers lies a considerable branch of the ileo-colic artery, the artery 

 of the appendix. Its canal is small, extends throughout the whole length of the 

 tube, and communicates with the caecum by an orifice which is placed below and 

 behind the ileo-caecal opening. It is sometimes guarded, according to Gerlach, 

 by a semilunar valve formed by a fold of mucous membrane, but this is by no 

 means constant. Its coats are the same as those of the intestine : serous, muscu- 

 lar, submucous, and mucous, the latter containing an abundant supply of retiform 

 tissue, especially in young subjects. 



It is stated that the vermiform appendix tends to undergo obliteration as an 

 involution change of a functionless organ. 



The Ileo-csecal Valve (Valvula BauMni). The lower end of the ileum termi- 

 nates by opening into the inner and back part of the large intestine, at the point 

 of junction of the caecum with the colon. The opening is guarded by a valve, 

 consisting of two semilunar segments, an upper or colic and lower or caecal, which 

 project into the lumen of the large intestine. The upper one, nearly horizontal in 

 direction, is attached by its convex border to the point of junction of the ileum 

 with the colon ; the lower segment, which is more concave and longer, is attached 

 to the point of junction of the ileum with the caecum. At each end of the aper- 

 ture the two segments of the valve coalesce, and are continued as a narrow mem- 

 branous ridge around the canal for a short distance, forming the frcena or retinacula 

 of the valve. The left or anterior end of the aperture is rounded ; the right or 

 posterior 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 portion of the 

 intestine to the other. When these are divided or removed, the ileum may be 

 drawn outward, 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 toward the ileum is covered 

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

 small intestine; while that turned toward the large intestine is destitute of villi, 

 and marked with the orifices of the numerous tubular glands peculiar to the mucous 

 membrane of 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, with which it is continuous. 

 It passes upward, from its commencement at the caecum, opposite the ileo-caecal 

 valve, to the under surface of the right lobe of the liver, on the right of the gall- 

 bladder, where it is lodged in a shallow depression, the impressio colica ; here it 

 bends abruptly inward to the left, forming the hepatic flexure. It is retained in 

 contact 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 Transversalis muscles, and with the front 

 of the lower and outer part of the right kidney (Figs. 507 and 508). Sometimes 

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

 colon. 1 It is in relation, in front, with the convolutions of the ileum and the 

 abdominal parietes. 



1 Treves states that, after a careful examination of one hundred subjects, he found that in fifty- 

 two there was neither an ascending nor a descending mesocolon. In twenty-two there was a descend- 

 ing mesocolon, but no trace of a corresponding fold on the other side. In fourteen subjects there was 

 a mesocolon to both the ascending and the descending segments of the bowel ; while in the remain- 

 ing twelve there was an ascending mesocolon, but no corresponding fold on the left side. It follows, 

 therefore, that in performing lumbar colotomy a mesocolon may be expected on the left side in 36 

 per cent, of all cases, and on the right in 26 per cent. (The Anatomy of the Intestinal Canal and Peri- 

 toneum in Man, 1885, p. 55.) 



