THE LARGE INTESTINE 



1173 



development. It is sometimes small and insignificant; in other cases it reaches a large size. 

 It may be so rotated that the ileum passes behind the colon and opens on the right side. The 

 posterior part has been seen much more developed than the anterior, so that the ileum has 

 entered from the front, and the appendix has come ofT from the anterior wall. The csecum 

 may remain undescended, and be found just under the liver or in the vicinity of the umbiUcus. 

 In case the rotation of the embryonic intestinal loop fails to occur (which rarely happens) 

 the csecum may remain permanently upon the right side. If the normal process of adhesion 

 fails to occur, the caecum and colon, along with the small intestine, may remain suspended from 

 the mid-dorsal line by the primitive niesenterium. commune. Or any of the intermediate stages 

 of partial adhesion may persist. 



The vermiform process.— Attached to what was originally the apex of the 

 caecum is a narrow, Wind tube, the vermiform process [processus vermiformis] or 

 appendix. It comes off at a variable distance (usually about 2.5 cm.) below the 

 ileo-caecal valve on the postero-medial aspect of the csecum, though sometimes 

 from the lower end of the caecum, or elsewhere. On the interior, at the point 

 where it joins the csecum (fig. 932), there is a shght inconstant valve [valvula pro- 

 cessus vermiformis]. The appendix joins the caecum at the point where the three 



Fig. 935. — Cross-section of the Ascending Colon. (Allen Thomson.) 



Longitudinal muscle 



Crescentic ridge of mucous mem- 

 brane which divides the sacculi 



Serous coat 



Tsenia libera 

 Mucous membrane 



Crescentic ridge of mucous 

 membrane 



Circular muscle 



Appendix epiploica 



Taenia mesocolica 

 Mucous membrane 



Crescentic ridge of mucous 

 membrane 



Serous coat 



Taenia omentalis Circular muscle 



taeniae meet, and the anterior taenia forms the best guide to this point. In the 

 adult, the average length of the appendix is between 8 cm. and 10 cm., the extremes 

 being 2 cm. to 25 cm. It is usually much twisted and coiled upon itself. Its direc- 

 tion is most frequently downward toward the pelvic cavity, or upward and medial- 

 ward behind the ileum in the direction of the spleen. It occasionally turns lateral- 

 ward, or more rarely upward behind the caecum. 



The vermiform process does not have a true mesentery, but usually (in about 

 90 per cent, of cases) is provided with a falciform fold [mesenteriolum] of periton- 

 eum, continuous with the left (lower) layer of the mesentery of the ileum (figs. 

 906, 934). 



In general outline this fold of peritoneum is triangular. In the adult it does not extend 

 along the whole length of the tube. It is, in fact, too short for the appendix, and it is this 

 that accounts for the twisted condition of this process. Along the free margin of the fold runs 

 a branch of the ileo-colic artery (fig. 934). 



The ascending colon. — The ascending colon [colon ascendens] (figs. 906, 914) 

 extends in the right lumbar (lateral abdominal) region from the csecum to the infe- 

 rior surface of the liver, lateral to the gall-bladder, forming there the right colic 

 [flexura coli dextra] or hepatic flexure. Its average length is about 20 cm. (or 

 somewhat less when measured in sihi) . It is covered by peritoneum in front and 

 on the side (fig. 902), but in a certain proportion of cases (26 per cent, according 

 to Treves) this part of the large intestine is connected with the posterior wall of 

 the abdomen by a meso-colon (usually very short) and is therefore surrounded by 

 peritoneum. Connected with the ascending colon is sometimes found a fold 'of 



