THE ORGANS OP DIGESTION. 



277 



or only partially. When the sigmoid is lifted up a recess or 

 fossa may be apparent beneath the meso-sigmoid, to the outer 

 side of the left common iliac artery, which is due to a failure of 

 adhesion between the meso-sigmoid and parietal peritoneum. It 

 occurs opposite the convexity of the sigmoid loop. 



Development of the Colon and Caecum.— In the sixth week 

 ■a process or diverticulum is seen to grow out from the free border 

 of the posterior limb of the U-shaped loop (Fig. 225 A). The 



- asc. col. 



bloodless fold 



ileum 

 ileo-caec. pouch 



~posit. of left diuert. 

 mesent. 

 art. of appen. 



/position of\ 



[right diu. on, 

 \ caec. 



appendix 



Fig. 224.— Diagram of the Apex of the Caecum at the time of birth and the Diver- 

 ticula which may be produced in the Fundus of the Caecum afterwards. 



diverticulum forms the caecum and appendix. It continues to 

 grow outwards and downwards. The part of the posterior loop 

 above the caecal diverticulum becomes increased in diameter and 

 forms the ascending and transverse parts of the colon. As the 

 superior mesenteric (vitelline) artery descends in the loop, it 

 gives off three branches to the posterior limb — the middle colic, 

 right colic and ileo-colic arteries (Figs. 222 and 223 A). The 

 mesentery of the U-shaped loop may be divided into two parts, 

 the fate of the two parts being different : 



1. The mesentery of the anterior limb in front of the superior 

 mesenteric artery — the pre-arterial part. This forms the greater 

 part of the mesentery of small bowel. 



2. The mesentery of the posterior limb, behind the artery — the 



