AIR BLADDER AIR CELLS 217 and highlands abounding in asses and goats. To the south are the groups of Mt. Sunday, Eghellal, Anderas, and Baghzen. A desert plateau, with an average elevation of 2,000 feet, the home of the giraffe, wild ox, and ostrich, divides Air from Soodan. The inhabitants of Air are blacker and shorter than those of Az- kar, and, instead of the austere and regular northern features, have a rounder and more cheerful expression of countenance. The prin- cipal places are Agades and Tintellust. This is probably the most southern place in central Africa where the plough is used ; for all over Soodan the hoe is the sole implement for pre- paring tlra ground. The government of the country is presided over by the sultan of Aga- des, and his chief vassal is emir of Tintellust. The inhabitants are fanatical Mohammedans. If a man marries a woman of another village, he must go and live in her village, not she in his. The hereditary power does not descend to the son, but to the sister's son. The arms, in general, are the spear, the sword, and the dagger, and an immense shield of antelope hide ; some use bows and arrows. A few only have muskets, and those few keep them for show- rather than actual use. The valleys are but poorly cultivated, and every piece of clothing material has to be imported, the population being sustained in large part by the salt trade of Bilma. The tolls levied on this article, in re- turn for protection afforded, constitute almost the whole source of revenue to the sheiks of Tintellust, Loosoo, and others. The name Air first appears in the description of Leo Africa- nus, written in 1526. It was introduced by the Berber conquerors, as Asben is the aboriginal name still used by the black and mixed popu- lation. See Richardson's "Journal of a Mis- sion to Central Africa" (London, 1853), and Dr. Earth's " Travels in Central Africa " (Lon- don, 1857). AIR BLADDER, an organ in some kinds of fishes, commonly called by fishermen the " swim." Fishes endowed with great powers of locomotion, and accustomed to pass rapidly from the surface to the bottom of the ocean, and vice versa, are provided with an air blad- der or a swim, by which they can modify at will the specific gravity of their bodies in the water, as birds do in the atmosphere by ad- mitting air when they wish to rise, and by ex- pelling it as they descend. Not that fish draw air into their swims and expel it, as birds do in their quills, &c., but they have the power of generating gas to fill the swim like a balloon within the body when they wish to ascend in the water, and expelling it when they descend. Fishermen are well acquainted with the func- tions of the bladder in the cod and other spe- cies, which require to be brought fresh to mar- ket at a great distance from the place where they are caught ; they perforate the air bladder with a fine needle, allowing the air to escape, and thus rendering the fish unable to rise from the bottom of the well-boats where they live for a considerable time, while brought to mar- ket. Cod sounds are the salted air bladders of these fishes. The Iceland fishermen, and those of Newfoundland, prepare isinglass from cod sounds ; and the Russians make a superior kind of isinglass from the sounds or swims of the sturgeon. The swim is composed of a length- ened sac, sometimes simple, as in the common perch, or divided into several compartments by transverse ligature, as in the trout and salmon ; sometimes furnished with appendices, more or less numerous in different species. It is com- posed of a thick internal coat of fibrous texture, and a thin external coat, the whole being en- veloped in the covering of the intestines. The swim has in many species no external opening, and the air or gas with which it is distended is supposed to be secreted in such cases by a glan- dulous organ with which it is always provided. In fresh-water fishes the air bladder communi- cates sometimes with the oesophagus and some- times with the stomach, by means of a small duct or tube ; and in these instances no secret- ing gland is found. A very few species, among which is the common eel, have air bladders opening by an external duct, and also provided with secreting glands. Fishes deprived of their air bladders sink helpless to the bottom of the water, and there remain. All the differ- ent species of flat fish, such as skates, soles, tur- bots, brills, &c., which live only on the coasts and on sand banks at the bottom of the ocean, where they find their food, have no air blad- ders ; their bodies are heavier than water, and their mode of life does not require them to as- cend. Mackerel and other species, which find their food entirely on the surface, and remain there, have no air bladders; their bodies are comparatively light, and they need not sink low down in search of food. Some zoologists have supposed that the air bladder of fishes may be connected with the respiration, and it is now generally admitted to be a rudimentary lung. Much remains to be yet observed with regard to the relation of this organ to the general con- formation of fishes ; for it is sometimes found in one species, and entirely absent in another which belongs to the same genus. AIR CELLS, hollow spaces within the cellular tissue of the stems, leaves, and other parts of plants, containing air only, the sap and other matters being contained in different receptacles. They most frequently occur in water plants, and very conspicuously in the splendid Victo- ria regia of the lakes of South America, ena- bling its rosy leaves to float ; and in the FaZ- lisneria spiralis, of which the male specimens, immersed in the water, rise from the bottom to meet the long-stalked females which stand over the surface. Other receptacles of air are to be found in the cambium (the layer of ge- latinous cellular tissue between the wood and the bark) of trees. Here the longitudinal rows of cells become broader, and exhibit in the progress of growth small flat air bubbles be- tween the walls of the contiguous cells ; grad-
