AIR-BLADDER. 147 



Some Siluroids possess a peculiar apparatus for voluntarily 

 exercising a pressure upon the air-bladder. From the first 

 vertebra a process takes its origin on each side, expanding at 

 its end into a large round plate ; this is applied to the side of 

 the air-bladder, and by pressing upon it expels the air through 

 the duct ; the small muscle moving the plate rises from the 

 skull. 



The connection of the air-bladder with the organ of hear- 

 ing in some Physostomes has been described above, p. 117. 



In the modifications of the air-bladder, hitherto mentioned, 

 the chief and most general function is a mechanical one ; this 

 organ serves to regulate the specific gravity of the fish, to aid 

 it in maintaining a particular level in the water, in rising or 

 sinking, in raising or depressing the front part of its body 

 as occasion may serve. Yet a secretion of gas from the blood 

 into its cavity must take place; and if this be so, it is not at 

 all impossible that also an exchange of gases between the 

 two kinds of blood is efiected by means of the extraordinary 

 development of retia mirabilia in many air-bladders. 



In all fishes the arteries of the air-bladder take their origin 

 from the aorta or the system of the aorta, and its veins return 

 either to the portal, or vertebral, or hepatic veins; like the other 

 organs of the abdominal cavity it receives arterial blood and 

 returns venous blood. However, in many fishes the arteries 

 as well as veins break up below the inner membrane into 

 retia mirabilia in various ways. The terminal ramifications 

 of the arteries may dissolve into fan-like tufts of capillaries 

 over almost every part of the inner surface, as in Cypri- 

 noids. Or these tufts of radiating capillaries are more 

 localised at various places, as in Esocidm ; or the tufts are so 

 aggregated as to form gland-like, red bodies, the capillaries 

 reuniting into larger vessels, which again ramify freely round 

 the border of the red body ; the red bodies are formed not 

 only by minute arteries but also by minute veins, both freely 

 anastomosing with its kind, and being inextricably inter- 



