202 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



fixed position now being transverse instead of sagittal, the mesentery is known as 

 the transverse mesocolon. The mesentery of the ascending colon is flattened 

 against the dorsal body wall on the right and fuses with the peritoneum. Simi- 

 larly, the descending mesocolon fuses to the body wall of the left side (Fig. 196 

 A, B). There are thus left free (1) the transverse mesocolon; (2) the mesentery 

 proper of the jejunum and ileum with numerous folds corresponding to the loops 

 of the intestine; (3) the iliac mesocolon; (4) the mesorectum, which retains its 

 primitive relations. 



Anomalies of the diaphragm and mesenteries are not uncommon. The per- 



Lesser omentum 



Dorsal . 

 tnesogastn urn 



Stomach 



Greater 

 omentum 



Transverse 

 mesocolon 



Caecum 



Mesentery 

 Mesorectum 



Iliac mesocolon 

 A B 



Fig. 196. — Diagrams showing the development of the mesenteries in ventral view (modified from 

 Tourneux in Heisler). * Cut edge of greater omentum; a, area of ascending mesocolon fused to dorsal 

 body wall; b, area of descending mesocolon fused to dorsal body wall. 



sistence of a dorsal opening in the diaphragm, more commonly on the left side, 

 may be explained as due to the defective development of the pleuro-peritoneal 

 membrane. Such a defect may lead to diaphragmatic hernia, the abdominal 

 viscera projecting to a greater or less extent into the pleural cavity. 



The mesenteries also may show malformations due to the persistence of the 

 simpler embryonic conditions, usually correlated with the defective development 

 of the intestinal canal. The ascending and descending mesocolon may be free, 

 having failed to fuse with the dorsal peritoneum. The primary folds of the greater 

 omentum may fail also to unite so that the inferior recess extends to the caudal 

 end of the greater omentum. 



