THE ORGANS OF DIGESTION. 275 



3. The artery of the yolk-sac, the terminal part of the 

 superior mesenteric, may persist as a fibrous band which 

 stretches from the mesentery at the situation of Meckel's 

 diverticulum to the umbilicus. Over it, the gut may become 

 strangulated. 



4. The U-shaped loop, instead of retreating within the 

 abdomen at the end of the second month, may remain within 

 the umbilical funnel of somatopleure. This gives rise to a 

 congenital umbilical hernia. Such hernias occur in all degrees ; 

 they may contain a piece of intestine, or almost the whole of 

 the abdominal contents. In such cases the somatopleure, or 

 belly wall, which forms the covering of the hernia, is commonly 

 thin and transparent. 



DEKIVATES OF THE HIND-GUT. 



The Rectum and cloaca are formed out of the posterior end 

 of the hind-gut. The manner in which they become sepa- 

 rated from the allantois and open in the proctodaeurn at the 

 anal depression has been described (page 118). 



The Descending Colon and Sigmoid Flexure are also formed out 

 of the hind-gut. The artery of the hind-gut is the inferior 

 mesenteric (Fig. 222). Hence it supplies the rectum, sigmoid, and 

 descending colon. In the fifth week the hind-gut is suspended 

 from the front of the aorta and spine by the dorsal mesentery of 

 the hind-gut. This becomes transformed into the meso-rectum, 

 meso-sigmoid and descending meso-colon. The angle between the 

 hind-gut and U-shaped loop becomes the splenic flexure (Fig. 

 222). At the commencement of the third month the U-shaped 

 loop has become twisted round on the axis of the superior 

 mesenteric artery (Fig. 223^4), so that the part of the hind-gut 

 which forms the splenic flexure is turned forwards and to the 

 left until it touches the spleen (Fig. 226). It carries its 

 artery, the left colic, with it. At this time the anterior limb 

 ■of the U-loop grows rapidly, and is produced into toils of small 

 intestine— the jejunum and ileum — which press the descending 

 meso-colon against the kidney and the parietal peritoneum cover- 

 ing the left kidney (Fig. 223 B). The left surface and layer 

 ■of the meso-colon adheres to the pre-renal layer of the peri- 



