298 COMPARATIVE ANATOMY 



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 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 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 the pleura 

 which renders the movements of the lungs smooth and easy. 



ABDOMINAL PORES. 



By the term abdominal pore is understood 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. 1 



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 never have this function, and are better described as genital 

 pores. 



In the Holocephali and Elasmobranchii the abdominal pores are 

 usually paired and are situated posteriorly to the cloaca (Figs. 206, 

 289, 290), and may be enclosed within its lips. They are wanting in 

 the NotidanidaB, Cestracionidse, and Rhinidse, and are not con- 

 stantly present in the Scylliidse, even in individuals of the same 

 species. In Ganoids, they open between the urinogenital aperture 

 and anus, but are apparently wanting in Amia. Amongst Teleosts, 

 they are said to be present only in the SalmonidaB and Mormyridse, 

 right and left of the anus ; but even in these, the pore of one or 

 of both sides may be absent. In the Mursenidse, there is a single 

 genital pore, which is apparently more nearly comparable to the 

 similarly named structure in other Teleosts (see under Generative 

 Organs) and to the genital pores of Cyclostomes. In Ceratodus the 

 abdominal pores are paired, and open behind the cloaca, while in 

 Protopterus a single, apparently blind, canal is present on the same 

 side of the ventral fin as the vent (Fig. 209), sometimes to the 

 right and sometimes to the left of the middle line, either within or 

 without the sphincter of the cloaca. 



Abdominal pores are not known to occur in the Amphibia and 

 Mammalia, but amongst Reptiles they are perhaps represented by the 

 peritoneal canals of the Chelonia and Crocodilia, which in the 

 former are in close relation with the penis or clitoris, and usually 

 end blindly. 



1 The abdominal pores may possibly correspond to the remains of segmental 

 ducts. Other connections of the crelome with the exterior (by means of the 

 nephrostomes of Anamnia and the ostia of the oviducts in the majority of 

 Vertebrata) will be mentioned in a subsequent chapter. 



