CCELOME 



389 



pleuroperitoneal cavity is divided into two main sections, an 

 anterior pleural and a posterior peritoneal chamber. 



In each of these three serous membranes (pericardial, pleural, 

 and peritoneal), a parietal and a visceral layer can be distinguished, 

 the former lining the outer wall of the chamber in question, and 

 the latter being involuted so as to invest closely and to suspend 

 its contained organ or organs. 1 



Towards the middle line, the parietal layer of the pleura of 

 either side is reflected so as to form a septum between the right 

 and left thoracic cavities. This septum is called the mediastinum, 

 and the space between its two layers the mediastinal space : 

 through this, the aorta, oesophagus, and postcaval vein run, and in 



FIG. 295. DIAGRAM OF THE PLEURAL AND PERICARDIAL CAVITIES OF MAMMALS, 



BASED OX THE RELATIONS IN THESE PARTS IN MAN. (A, horizontal 



section ; B, transverse section. ) 



Br, bronchi ; H, heart ; m, mediastinum ; L, lung ; P, parietal, and P 1 , 

 visceral layer of the pleura ; PC, P* 1 , parietal and visceral layers of the 

 pericardium ; R, ribs (wall of thorax) ; S, sternum ; TV, trachea ; W, 

 vertebral column ; ft, points at which the parietal and visceral layers of 

 the pleura pass into one another at the hilum pulmonalis (Hi). 



the region of the heart the mediastinum is reflected over the 

 parietal layer of the pericardium. There is a lymphatic fluid 

 between the two layers of all these membranes, which renders 

 the movements of the contained organs smooth and easy. 



Abdominal pores. By the term abdominal pore is under- 

 stood a perforation usually paired of the posterior end of the 

 wall of the peritoneal cavity which puts the coelome into direct 

 communication with the exterior. 



In Cyclostomes a pair of pores opens into the urinogenital 

 sinus, serving to conduct the generative products to the exterior : 

 they probably do not correspond to the abdominal pores of other 

 forms, which normally do not have this function, and are better 



1 In connection with the suspensory mesenteric structures in the peritoneal 

 cavity, mention must be made of the omental folds extending between the 

 abdominal viscera. 



