ABDOMINAL PORES. 



265 



(Fig. 212, P, P 1 ) : the latter is spoken of as the pulmonary 

 pleura, the former as the costal pleura. Towards the middle 

 line, the pulmonary pleura of either side is reflected so as to form 

 a septum between the right and left thoracic cavities. This septum 



tn - 



FIG. 212. DIAGRAM OF THE PLETJRAL AND PERICARDIAL CAVITIES OF MAMMALS, 



FOUNDED ON THE RELATIONS OF THESE PARTS IN MAN. (A, horizontal 



section ; B, transverse section. ) 



Tr, trachea ; Br, bronchi ; L, L, lungs ; H, heart ; W, vertebral column ; P, parietal, 

 and P 1 , visceral layer of the pleura ; t t, points at which these pass into one 

 another at the hilum pulraonalis (Hi) ; m, mediastinum ; PC, Ps l , parietal and 

 visceral layers of the pericardium ; _ff, ribs (wall of thorax) ; S, sternum. 



is called the mediastinum (Fig. 212, m), and the space between 

 its two layers the mediastinal space: through this, the aorta, 

 oesophagus, and postcaval vein run, and in the region of the heart, 

 the mediastinum is reflected over the parietal layer of the peri- 

 cardium (see p. 268, and Fig. 212, PC). 



There is a lymphatic fluid between the two layers of the pleura, 

 which renders the movements of the lungs smooth and easy. 



ABDOMINAL PORES. 



By the term abdominal pores is understood a perforation 

 almost always paired of the posterior end of the peritoneal 

 cavity, which puts the coelome into direct communication with the 

 exterior. 1 These pores are present in Cyclostomi, Elasmo- 

 branchii, certain Teleostei, Dipnoi, Chelonia, and Croco- 

 dilia. The perforations always take place through the ectoderm, 

 close to the apertures of the urinogenital organs and intestine ; each 

 is either situated on a papilla, or in the walls of the cloaca. 



It can only be stated positively as regards the function of the 

 abdominal pores, that in Cyclostomes and a few other Fishes they 

 serve to conduct the generative products to the exterior. As in the 

 rest of the Vertebrata special ducts are present for this purpose, 



1 Other connections of the ccelome with the exterior (nephrostomes of Anamnia 

 and oviducts of all Vertebrata) will be mentioned later on (see pp. 297, 300, and 302). 



