xlii • • AIR-BLADDER. 



they have usually two coats — an external, fibrouSi 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, inost 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 j 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 piscine 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 bloo'd 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 



* Qoekett, 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. 



