THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 



197 



as described above, and the pericardial sac is free on all sides from the body wall. That part 

 of the former pleuroperitoneal cavity which lies dorsal and lateral to the heart is separated from 

 the posterior part of the same cavity by a muscular partition, the diaphragm, which extends 

 across the body cavity in a transverse plane. The two pleural cavities lie anterior to the 

 diaphragm, one on either side of the pericardial cavity which is inclosed between their medial 

 walls. The pericardial cavity is free on all sides, owing to the extension of the two pleural 

 cavities ventrally (Fig. 49). The two pleural cavities, at first dorsal in position in mammals, 

 grow ventrally and split the pericardium from the ventral body wall. They push in between 

 the pericardial sac and the ventral body wall and their medial walls meet ventral to the peri- 

 cardial sac to form the mediastinal septum seen in the dissection. The cavity posterior to 

 the diaphragm is the peritoneal cavity. 



The diaphragm corresponds in large part to the oblique septum of birds but contains addi- 

 tional components. It is a structure peculiar to mammals. It is formed in part by mesenterial 

 folds, the pleuroperitoneal membranes, which grow down from the dorsal body wall and fuse 



pleuro-pericardial 

 membrane 



neural tube 

 notochord 



parietal 

 pleura 



body wall 



parietal 

 pericardium 



FIG. 49. Diagrams to show the separation of the pericardial sac in mammals. A, early stage in 

 which the parietal pericardium forms the lining of the body wall; the pericardial and pleural cavities 

 are separated by the pleuropericardial membrane, which is the anterior continuation of the transverse 

 septum. B, later stage, showing how the ventral extension of the two pleural cavities splits the parietal 

 pericardium from the body wall, and gives rise to the mediastinal septum; the parietal pericardium 

 then becomes the pericardial sac. (B from Prentiss and Arey's Textbook of Embryology, courtesy of 

 the W. B. Saunders Company.) 



with the posterior wall of the transverse septum, much as in birds (Fig. 45!) and E, p. 160). To 

 these components there are also added some portions of the dorsal body wall and a portion 

 of the dorsal mesentery. Finally the diaphragm is penetrated by buds from adjacent myo- 

 tomes and thus becomes in large part muscular. 



The derivatives and relations of the transverse septum in mammals are similar to those 

 previously described. Its anterior face forms part of the pericardial sac. The great veins 

 have, however, gradually been drawn out of it (as also in birds) and now all of the veins enter 

 the anterior end of the heart. The posterior face of the septum forms part of the diaphragm, 

 that part to which the liver is attached by the coronary ligament. Coronary and falciform 

 ligaments of the liver are present as previously and originate in the same way. 



The coelomic linings, pericardium, pleuroperitoneum, or pleura and peritoneum, present 

 the same relations in all of the vertebrates. Each has a parietal portion lining the body wall, 

 a visceral portion covering the viscera, and mesenteries or ligaments connecting the parietal 

 and visceral portions. 



