xlii AIR-BLADDER. 



they have 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 seems to have the character of a rete mirabile, consisting of a double 

 plexus of arteries and veins. 



It has long been known that the gas contained in the air-bladder is 

 a mixture of oxygen, azote, and carbonic acid, in variable proportions, 

 in accordance with species, and even with individuals. M. Moreau has 

 proved that among the fishes in which the air-bladder is closed (Physoclisti) 

 this organ always contains a greater part of the oxygen whenever the animal 

 is in a normal condition, that the oxygen disappears little by little if the 

 animal cannot any longer derive it from its surroundings, and that finally 

 it perishes asphyxiated. 



The air-bladder, excluding those forms which respire air, is generally 

 found after death tightly distended with gas, and this consists chiefly 

 of nitrogen in the fresh-water forms, and oxygen in marine,, genera, this 

 latter substance augmenting in sea fishes in accordance with the depth 

 at which the fish is captured. It has formed a subject of considerable 

 discussion as to how this gas is generated, but, as in those classes in which 

 the air-bladder is a closed sac (Physoclisti), it is as well seen as in others 

 possessing a pneumatic tube (Physostomi), one cannot resist believing that 

 the gas must be eliminated from the blood-vessels lining the interior of the 

 organ. Probably the gland serves the special purpose of removing super- 

 fluous gas or any deleterious substance, while the pneumatic tube is not 

 employed to admit the ingress of air, but acts as safety-valve when the organ 

 is too tightly distended. 



The air-bladder is 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 (whatever its functions may be) exists throughout 

 every variety and condition of air-bladder in the pisciue tribes. The arteries 

 which supply the air-bladder in teleosteans are offshoots direct from the 

 abdominal aorta, cceliac artery, or last branchial vein ; the blood is returned 

 to the portal, hepatic, or great cardiac vein. In the highest class of fishes 

 we find this organ differently supplied, as it is not only the homologue but 

 likewise the analogue of the lung, thus in Lepidosiren* venous blood is 

 distributed to the organ and arterial conveyed away, the two efferent veins 

 having coalesced, pierce the large post-caval, then pass forwards and through 

 the sinus and auricle, and thus discharge the blood into the ventricle. 

 Consequently we find that in this organ there are two distinct modes of 



* Quekett, who injected a small portion of the air-bladder of this fish, found the arrangement 

 of the vessels was precisely similar to that existing in the lungs of reptiles. 



