494 ANATOMY OF VERTEBRATES. 



middle and posterior thirds of that organ. 1 The long, narrow, 

 and flexuous ductus pneumaticua is continued from the forepart 

 of the posterior division of the air-bladder in the Cyprinoids, and 

 opens into the dorsal part of the oesophagus, fig. 229, su : the 

 short, straight, and wide ductus pneumaticus, in the Lepidosteus, 

 opeiis also into the dorsal part of the oesophagus, the orifice being 

 served by a sphincter: in the Erythrinus the air-duct commu- 

 nicates with the side of the oesophagus ; in Polypterus, as in 

 Lepidosiren, with the under or ventral part of the beginning of 

 the oesophagus. 2 



The principal seat of the vascular ramifications in the air- 

 bladder, like that in a true lung, is the mucous lining membrane ; 

 but the modes of ramification in the primitive piscine form of the 

 air-breathing organ are as variable as any of its other properties. 

 The arteries of the air-bladder are derived sometimes directly 

 from the abdominal aorta, sometimes from the cceliac artery, 

 sometimes from the last branchial vein ; and in the Lepidosiren 

 they are continued from the aortic termination of the two non- 

 ramified branchial arteries, fig. 312, I', and therefore convey 

 venous blood to the cellular, lung-like, double air-bladder. The 

 veins of the air-bladder return, in some fishes, to the portal vein ; 

 in some, to the hepatic vein ; in some, to the great cardinal vein ; 

 and, in the Lepidosiren, ib. p, they penetrate, by a common 

 trunk, the great post-caval vein, ib. e, formed by the confluence 

 of the visceral and vertebral veins of the trunk ; but instead of 

 terminating there, the pulmonary venous trunk passes forward, 

 through the sinus and auricle, to the entry into the ventricle, and 

 there terminates above the valvular cartilaginous tubercle. Thus 

 the aerated blood from the lungs enters the ventricle directly, 

 instead of being previously mixed with the venous blood in the 

 auricle. 



The vascular system of the lung-like air-bladders of the 

 Protopterous and Ganoid Fishes forms no ' retia mirabilia ' or 

 vaso-<xano;lions, but resolves itself into a generally diffused 

 reticular capillary system, which is much richer and closer in 

 the more subdivided and thicker cellular structure of the anterior 

 than of the posterior parts of the bladders in the Lepidosiren. 



In the Osseous Fishes the principal forms of the terminal 

 divisions of the arteries of the air-bladder are as follows: — 1. A 

 resolution of the smaller ramifications into fan-like tufts of 

 capillaries over almost every part of the inner surface (Carp). 

 2. The formation of similar, but larger and more localised, 



1 cxiv. vol. ii. pi. viii. fig. 1. - xxi. 1841, p. 194. 



