GREA T BRITAIN. 19 



eliminated from the blood-vessels lining the interior of the 

 organ. 



The air-bladder has usually two coats an external 

 fibrous, tough, and glistening, and an internal vascular and 

 mucous one. Between these two coats is often seen 

 (especially in the Physoclisti) a red glandular body, most 

 frequently in its inferior region, and compared by some 

 anatomists to the thymus. This gland appears to have 

 the character of a rete mirabile, consisting of a double 

 plexus of arteries and veins. Perhaps it may serve the 

 purpose of removing superfluous gas or any deleterious 

 substance, while the pneumatic tube may possibly not be 

 employed to admit air, but acts as a safety-valve when the 

 organ is too tightly distended. 



The air-bladder must be refused the title of " lung " in 

 most fishes, because it is usually supplied with blood from 

 the adjacent arteries of the body, not direct from the heart, 

 and returns venous blood into the general circulation. It 

 is, however, homologous with the lung in its position and 

 function in some of the higher orders ; and as a gradation 

 can be traced, it becomes no less clear that this homology 

 (leaving its functions out of consideration) exists through- 

 out every variety and condition of air-bladder in the 

 piscine tribes. 



The air-bladder is not only absent in many families of 

 fishes, but may be present or deficient among species of 

 the same genus. The common form of British mackerel 

 {Scomber scomber] does not possess it, but it is present in 

 the rarer Spanish mackerel (S. colias\ also found in our 

 seas. In such forms as swim near the surface, it is generally 

 of comparatively a small size, while in some of those which 

 live near the bottom, as flat fishes, it is absent. In species 

 possessing this organ, should it become ruptured from any 



C 2 



