THK BODY CAVITIES, DIAPHRAGM AND MESENTERIES 195 



There are thus left free: (i) 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 : 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 pro- 

 jecting to a greater or less extent into the pleural cavity. Similarly, faulty development 

 of the left pleuro-pericardial membrane sometimes causes the heart and left lung to occupy 

 a common cavity. 



The mesenteries also may show malformations, due to the persistent 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 are 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 viscerum invcrsus, in which the various visceral organs 

 are transposed right for left and left for right, as in a mirror image. An independent 

 transposition 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. 



