THE GASES OF THE AIR-BLADDER.—The gaseous contents of the air-bladder consist of oxygen and nitrogen, but the relative proportions of the two gases differ in different Fishes, and even in the same Fish, under different conditions. Normally the proportion of oxygen is considerably less in freshwater than in marine Fishes, and amongst the latter the proportion of oxygen is often enormously greater, amounting in some cases to 87 per cent., in deep-sea species as compared with their shallow water congeners. A trace of carbon dioxide is also usually present. The gases are derived from the blood as the latter circulates through the capillaries in the walls of the bladder, and it is highly probable that the "red glands" take an important part in the process; at all events, experimental research has shown that the "secretion" or diffusion of gas into the air-bladder, as well as the absorption of gas from the bladder into the blood, take place most rapidly in those Fishes in which "red glands" or "red bodies" are present.[354] [Illustration: FIG. 185.—Vertical section through a "red gland" (diagrammatic). _c_, Capillary blood-vessels; _g_, tubular glands. (From Vincent and Barnes.)] THE FUNCTIONS OF THE AIR-BLADDER.—Probably no single organ in any group of Vertebrata is associated with the performance of a greater variety of functions than the air-bladder of Fishes. Originally evolved, it may be, as a glandular caecum in certain {310}Sharks, the air-bladder in the Dipnoi, and some of the more generalised Teleostomi (e.g. _Amia_ and _Lepidosteus_), and perhaps also in a few of the more specialised members of the latter group (_e.g._ certain Teleosts), is to a greater or less extent an accessory respiratory organ. In not a few Teleosts it is an organ for sound-production, and in others again it is sometimes regarded as having an important relation to the sense of hearing. But omitting such subordinate functions which, as it were, have been grafted on to the air-bladder, there can be no doubt that in the great majority of Fishes its primary use is to act as a hydrostatic organ or "float." From this point of view experimental investigations[355] seem to justify the following conclusions:—