DIAPHRAGM 



241 



another, and bound the pleuro-pericardial opening. The folds in which they lie 

 (the original anterior edges of the septum transversum) become expanded owing 

 to the enlargement of the pleural clefts and pericardial cavity, and their free 

 margins fuse with the free ventral border of the mesenteric septum so as to shut 

 off the pericardium from the pleural cavities (fig. 293, p. 239). The septum thus 

 formed is known as the pleuro-periczrdial membrane. 



The closure of the pleuro-peritoneal openings is effected by a very complicated 

 series of changes, which result in the formation of the diaphragm. The 

 chief factor in the development of the septum is the extension of the liver 

 trabeculse into the septum transversum, into certain folds in connexion with it, 

 and nto the mesenteric septum, until the liver occupies the whole depth of the 

 body-cavity. The connective-tissue sheet covering it, and derived from the several 

 parts into which the trabeculas extend, becomes freed from the liver-substance, 



pleuro-pericardial openings 



. duct of Cuvier 



dorsal pilln r 

 right lung 



liver 



mesentery 



peritoneal 



recexx 



floor of 

 pleural space 



cceliac artery in 

 cut mesentery 



FIG. 296. MODEL OP A HUMAN EMBRYO OF 6'8 MM. (After Piper.) 



The coelom is opened, and the dorsal wall removed by cutting through the dorsal mesentery, 

 lungs and liver are thus exposed from the dorsal aspect. 



The 



and forms a divisional plane which by a further series of modifications is developed 

 into the diaphragm. 



We have already seen that the liver at first consists of a mass of epithelial 

 trabeculse occupying the septum transversum, and that the mesenteric septum 

 is attached to its dorsal aspect. On each side of this the pleural spaces are con- 

 tinuous with the peritoneal cavity. The openings are in part constricted by folds 

 named the pleuro-peritoneal membranes (figs. 294-296). These lie at first in a 

 nearly sagittal plane, with their free edges directed backwards. The dorsal pillar 

 of each fold is continued on to the dorsal body- wall to be attached to the mesial 

 side of the Wolffian ridge, and the ventral end is prolonged on the dorsal aspect of 

 the liver, running behind into the attachment of the mesenteric septum to that 

 organ. The pleuro-peritoneal membranes are in reality the anterior ends of the 

 Wolffian ridges, here reduced to membranous folds owing to the atrophy of the 

 head ends of the Wolffian bodies (Bertelli, Keith, Brachet, Wolfel) (fig. 297). 

 They separate the anterior end of the pleuro-peritoneal cavity into mesial recesses 



VOL. I. R 



