AIR-ULAPDEK OF FISIIKS. 275 



abdominal aorta, sometimes from the cocliac artery, sometimes from the 

 last branchial vein ; and in the Lepidosiren they ai'e continued from 

 the aortic termination of the two non-ramified branchial arteries 

 \fig. 71. l'\ and, therefore, convey venous blood to tlie 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 pene- 

 trate, 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 

 forwards, through the sinus and auricle, to the entry into the ven- 

 tricle, 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 Proto- 

 pterous and Ganoid fishes forms no ' retia mirabilia ' or vaso-gan- 

 glions, but resolves itself into a generally diffused reticular ca])illary 

 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 ai'e as follows : — 1st. A resolution 

 of the smaller ramifications into fan-like tufts of capillaries over 

 almost every part of the inner surface (Carp). 2d. The formation 

 of similar, but larger and more localised, radiating tufts (Pike) ; 

 in both without any special aggregation of the capillaries to form 

 a ' vaso-ganglion.' 3d. The conversion of the tufts by rapid subdi- 

 vision into capillaries aggregated so as to form red gland-like bodies; 

 the capillaries reuniting into larger vessels, which again ramify richly 

 round the border of the gland-like body ; the rest of the inner sur- 

 face of the air-bladder having the ordinary simple capillary system 

 (Perch and Cod). In the Cod-fish, a large artery, a branch of the 

 Cfeliac, and a still larger vein, which empties itself into the mesen- 

 teric, perforate together the fibrous tunic of the swim-bladder. 

 Before they reach the inner surface, they divide into some branches, 

 which then radiate and subdivide upon the mucous membrane : the 

 arterioles frequently anastomose together ; and the venules as fre- 

 quently anastomose witli each other : both are inextricably inter- 

 woven and form the basis of the so-called ' air-gland,' which is 

 essentially a large ' bipolar rete mirabile ' of Miiller, or vaso- 



* XXI. 1841, p. 19-1. 



T 2 



