THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 



j 9; 



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

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

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

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

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

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

 cavities ventrally (Fig. 40). 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 betweei 

 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 t( 

 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 mesenteria 

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



pleuro-pericardial 

 membrane 



neural tube 



notochord 



pleural 

 cavity 



lung 



body wall 



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

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

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

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

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

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

 the \Y, B. Saunders Company.) 



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

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

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

 tomes and thus becomes in large part muscular. 



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

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

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

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

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

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



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

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

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

 and visceral portions. 



