AIR- LADDER, • peculiar organ with which the great majority of fishes are provided, and by which they arc enabled to adapt the specific gravity of their belies to the various pressures of the super incumbent water at different depths. It is composed of a lengthened sac, sometimes simple, as in the common perch, sometimes divided into two or more compartments, by a lateral or transverse ligature, as in the trout and &Almon, and, at other times, furnished with appendices, more or less numerous according to the particular species. In all cased, it is composed of a thick internal coat of a fibrous texture, and of a very thin external coat ; the whole being enveloped in the general covering of the intestines.
The modifications of this organ are infinitely varied in different genera and species of fishes. In the greater number of instances it has no external opening, mid the air with which it is found distended is believed to be produced by the secretion of a certain glandulous organ, with which it is in all these cases provided. This air has been examined, and found to consist of oxygen and nitrogen, but with less oxygen than common air. In fresh-water fishes, the air-bladder communicates sometimes with the cesophagus, and sometimes with the stomach, by means of a small tube ; and it is observable, that in the greater number of these instances, in which it has a direct external communication with the intestines, the secreting glands shave mentioned do not exist ; thus giving us strong reason to believe that its functions and uses are not uniformly the same in all the different classes of fishes. A very limited number of species, among othsrs the common eel, have air-bladders not only opening by an external duct, but likewise provided with secreting glands; and thus occupying an intermediate station between the two larger classes, at least aa far as the nature and functions of this organ are concerned.
In general, all fishes which enjoy great powers of locomotion, and have occasion to pass through various degrees of superincumbent pressure in their rapid transitions from the surface to the bottom of the ocean, are provided with this important organ ; and so indispensable is it in their economy, that those which,: for the sake of experiment, have been deprived of it, have sunk ,Iselpleal to the bottom, and there remained incapable of moving, or even of maintaining their equilibrium. But to fishes whose habits and organization confine them either to the surface of the water or to the bottom of the sea, and which, therefore, do not require to mess through different depths, or to encounter different degrees of pressure, the pomession of an air bladder is by no means so essentially requisite. Accordingly we find,
that all the different specie; of rays and Pleuronectes or flat-fish, such as skates, soles, turbots, brills, etc., which live only upon the coasts and sandbanks at the bottom of the ocean, as well as the mackerel and others which find their food entirely at the surface, have no air-bladder; and so email is the relation of this otherwise important organ to the general conformation of fishes, that we sometimes find it present in one species, and wanting altogether in another of the same gums. Although it does not appear that the air-bladder is connected w.th the function of respiration in fishes, it accepted the position, and has the same relations, as the lungs in reptiles. it is, in fact, the homologue of these organ& Fishermen are well acquainted with the ' nature and functions of the air-bladder, or, as they most commonly call it, the Swim. They are accustomed to perforate this vessel with a fine needle in cod and other species which require to be brought fresh to mtrket, sometimes from a very great distance. By this operation, the confined air is allowed to escape, and the fish constrained to remain quiet at the bottom of their well-boats, where they live for a very considerable period. Cod-sounds, which are brought in great quantities from Newfoundland, are nothing more than the salted air-bladders of these fishes. The Iceland fieheriuen, as well as those of America, prepare isinglass of a very excellent quality from cod sounds, though they are not acquainted with the method of clarifying it, which the Russians practise in preparing isinaleast from the sound of the sturgeon.
(Owen, Lectures on Comparative Anatomy, voL ii.) Alit-CELLS, in plants, are cavities in the helves or stems, or other parts containing air. In water-plants they have a very definite form, and are built up of little vesicles of cellular tissue, with as much regularity as the walla of a house; they no doubt enable the plant to float. They are well men ia,6the structure of the Victoria regia. In plants which do not float, tLe form of the air-cella is lets definite they often appear to be mere lacerations of a mass of cellular sub stance, and their object is unknown ; well-known instanced of their presence are the chambers in the pith of the walnut-tree, and the tubular cavities in the stein of the bamboo, and other Grasses.