THE ILEO-COLIC VALVE. THE COLON. 



853 



inwards towards the caecum and colon. The lower fold is the larger of 

 the two ; the upper is placed more horizontally. At each end of the 

 aperture these folds coalesce, and are then prolonged as a single ridge 



Fig. 598. VIEW OF THE ILEO-COLIO VALVE Fig. 598. 



FROM THE LARGE INTESTINE. 



The figure shows the lowest part of the 

 ileum, i, joining the caecum, c, and the 

 ascending colon, o, which have been opened 

 anteriorly so as to display the ileo-colic 

 valve ; a, the lower, and c, the upper seg- 

 ment of the valve. 



for a short distance round the cavity 

 of the intestine, forming the /rcena 

 or relinacula of the valve. The op- 

 posed surfaces of the marginal folds 

 which look towards the ileum, and 

 are continuous with its mucous sur- 

 face, are covered like it with villi ; 

 while their other surfaces, turned to- 

 ward the large intestine, are smooth 

 and destitute of villi. When the 

 caecum is distended, the freena of the 

 valve are stretched, and the mar- 

 ginal folds brought into apposition, 



so as completely to close the aperture and prevent any reflux into the ileum, 

 while at the same time no hindrance is offered to the passage of additional 

 matters from thence into the great intestine. 



Each segment of the valve consists of two layers of mucous membrane, 

 continuous with each other along the free margin, and including between 

 them, besides the submucous areolar tissue, a number of muscular fibres, con- 

 tinued from the circular fibres of the ileum and from those of the large intes- 

 tine also. The longitudinal muscular fibres, and the peritoneal coat take no 

 part in the formation of the valve, but are stretched across it uninterruptedly 

 from one intestine to the other. 



The ASCENDING COLON, situated in the right lumbar and hypochondriac 

 regions, commencing at the ccecum opposite to the ileo-csscal valve, ascends 

 vertically to the under surface of the liver, near the gall-bladder, where it 

 proceeds forwards and then turns abruptly to the left, forming what is 

 named the. hepatic flexure of the colon. The ascending colon is smaller than 

 the crecum, but larger than the transverse colon. It is overlaid in front by 

 some convolutions of the ileum, and is bound down firmly by the peritoneum, 

 which passes over its anterior surface and its sides, and generally leaves an 

 interval in which its posterior surface is connected by areolar tissue with the 

 fascia covering the quadratus lumborum muscle, and with the front of the 

 right kidney. In some cases, however, the peritoneum passes nearly round 

 it, and forms a distinct though very short right meso-colon. 



The TRANSVERSE COLON passes across from the right hypochondrium, 

 through the upper part of the umbilical region, into the left hypochondrium. 

 Sometimes it is found as low as the umbilicus or even lower. At each 

 extremity it is situated deeply towards the back part of the abdominal 

 cavity, but in the middle it curves forwards, and lies close to the anterior 

 wall of the abdomen. Hence it describes an arch, the concavity of which is 



