194 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



tery proper of the jejunum and ileum, with numerous folds corresponding to the 

 loops of the intestine; (3) the iHac mesocolon; (4) the mesorectum, which retains 

 its primitive relations. 



Anomalies of the diaphragm and mesenteries are not uncommon. The persistence of 

 a dorsal opening in the diaphragm, more commonly on the left side, finds its explanation in 

 the imperfect 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. In about 30 per cent, of cases the ascending and descending mesocolon is more or less 

 free, having failed to fuse with the dorsal peritoneum. The primary sheets of the greater 

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

 greater omentum. 



A striking anomaly is situs mscerum inversus, in which the various visceral organs are 

 transposed right for left and left for right as in a mirror image. An independent transposi- 

 tion of the thoracic or abdominal viscera alone may occur. The larger left great venous 

 trunks are thought to be chiefly responsible for the usual positions of the viscera. 



