DIVISIONS OF THE DIGESTIVE SYSTEM 



103 



the latter crosses the body and is flexed backward as the descend- 

 ing colon. In the rabbit, however, that portion definable .as the 

 ascending colon, which is the shortest in man, is greatly elongated, 

 and is composed of five principal limbs, united by flexures. The 

 last two of these are concealed in dissection from the ventral 

 surface by the base of the superior mesenteric artery, since they 

 lie on'its right side. The descending colon is also only nominally 

 related to the left side of the body-wall in the rabbit, its supporting 

 peritoneum, the descending mesocolon, being fused with that of 





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a- 



ill 



Fig. 55. The caecum with its ventral wall removed to show the lumen 

 and the contained spiral valve: a, ileocolic aperture. 



the ascending limb of the duodenal loop so that it is restored to 

 an approximately median position. The caecum, like the colon, 

 is much enlarged and particularly elongated, its course as it lies 

 in the body being comparable to two turns of a left-hand spiral 

 (Fig. 54). Its blind terminal portfon, the thick-walled vermiform 

 process, is also relatively large and, until the beginning of this 

 portion is reached, its internal surface area is further increased 

 by the presence of a long spiral fold or spiral valve (Fig. 55) 

 comparable with that present in the intestines of sharks and skates. 



