972 



THE ORGANS OF DIGESTION 



valves unite and iivo i)ri)l()n.-rrd at either end as a ridge partially surrounding the 

 intestine (fnena). Villi cover tliat surface of the folds looking towards the ileum. 

 Tile surface towards the large intestine is free from villi. In the formation of this 

 valve the longitudinal nuiscular fibres pass across from the ileum to the large 

 intestine without dipi)ing down between the two layers of each fold. The circular 

 muscular fibres, on the other hand, are contained between the layer of the mucous 

 mcmt)ran(^ and submucous tissue which form them. 



The colon. — In the adult the ascending and the descending parts of the colon 

 are i)laced vertically, while the direction of the transverse colon is i)ractically 

 horizontal. The average length in the adult of the ascending colon (as measured 

 from the tip of the ciecum" to the hepatic flexure) is eight inches, and of the 

 descending colon (as measured from the splenic bend to the connii en cement of the 

 sigmoid loop) is eight and a half inches. The descending colon is thejiart of the 

 large bowel that is least liable to variation. It is the only part of the gut except the 

 duodenum that retains its original position as a portion of the great primary 

 vertical loop. The transverse colon, on the other hand, is liable to considerable 

 variation in length, position, and arrangement. Its average length is twenty 



Fig. 57(}.— Section of the Ascending Colon. (Allen Thomson.) 



Crescentic ridge of mucous 



membrane which divides the sacculi Longitudinal muscle 



Serous coat 



Anterior band 



Mucous membrane 7^5y x^^S Posterior band 



- Mucous membrane 



Crescentic ridge of 

 mucous membrane 



Circular muscle 



Appendix epiploica 



Crescentic ridge of 

 mucous membrane 



Serous coat 



Inner band 



Circular muscle 



inches in the adult. It has been found to vaiy in adults from twelve to thirty- 

 three inches. 



The ascending colon. — The ascending colon extends from the ca?cum to the 

 inferior surface of tht; liver external to the gall bladder, forming there the hepatic 

 flexun!. It is covered by peritoneum in front and at the side, but in a certain 

 ])roiiortion of cases (twenty-six per cent, in one hundred dissections) this part of 

 the large intestine is connected with the posterior wall of the al)domen by a meso- 

 colon, that is to say, it is quite surrounded by peritoneum. Connected Avith the 

 ascending colon is sometimes found a fold of i)eritoneum, extending from the right 

 side of the gut to the parietes at a little above the level of the highest part of the 

 iliac crest. It forms a shelf upon which rests the extreme right margin of tlie 

 liver. It might be called the sustentacnlum he/path. 



The ascending colon is in relation behind with the right kidney, second portion 

 of the duodenum, and quadratus lumborum. In front are some of the coils of the 

 ileum (fig. 577). 



The transverse colon: — The transverse colon, smaller in diameter than the 

 ascending, (extends from tlie under surface of the liver to the s))leen. Being longer 

 than a straight Hue between these two ])oints, it lias to describe an arch with its con- 



