58 



AIR-CELLS AIR- PLANTS. 



their specific gravities to different depths of water, 

 and the consequent pressures to which they are sub- 

 jected at those depths ; and the arguments in favour 

 of this theory are, that those fishes which have it not 

 are generally less discursive, both as to depth of 

 water and to range in distance, than those which 

 have it ; that when it is punctured the fishes remain 

 at the bottom, while so long as it is entire they can 

 come to the surface. These arguments are not, how- 

 ever, conclusive, or even very plausible. We know 

 of no animal which raises itselt by balloons, whether 

 in the air or in the water; and those fishes and 

 reptiles which have the power of inflating their inte- 

 guments, certainly never use that power to aid them 

 either in swimming or in flying. 



The fact is, that this mechanical theory refutes 

 itself, and never would have been adopted if the 

 parties adopting it had not been ignorant of those 

 very principles of mechanics of which they concluded 

 they were making a proper application. If the air- 

 bag were of such dimensions as that the bouyancy 

 produced by it could have any effect on the ascent of 

 the fish to the surface, it would destroy the fish's 

 command of itself in the water to a much greater ex- 

 tent. Besides, the air-bag has not been proved to be 

 muscular, or possessed of a contractile structure ; and 

 the reddish bodies which are sometimes attached to it 

 are organs of secretion. The air contained in the 

 bag is chiefly nitrogen, with a mere trace of oxygen in 

 fishes, such as the mullet, which live near the surface, 

 but with a little more oxygen in the fish which inhabit 

 at a greater depth. In no known case, however, has 

 the quantity of oxygen been found to be equal to one 

 tenth of the whole. This composition precludes the 

 possibility of supposing that the contents of this ves- 

 sel can be atmospheric air. As little are they the 

 remains of the air after having been breathed by the 

 animal ; for the chondropterygii with fixed gills, 

 which are the only fishes that receive the air into ca- 

 vities in the body, have no air-bladders. And yet, if it 

 were the residuum of air taken in from the water, they 

 are the ones in which we might expect to find it, as the 

 fishes with two gills pass the water in a current through 

 these. When there is any connexion of the air-bag 

 with any of the systems of the animal, it is always with 

 the alimentary system, the gullet, the stomach, or 

 both ; but there is often no perceptible communica- 

 tion even with these, and when there is not, the air- 

 bag is always furnished with glandular appendages 

 presumed to be for the purposes of secretion, though, 

 as in some cases it has both the connexions with the 

 gullet or- stomach and also the appendages, there is 

 doubt even upon that point of the subject. Indeed, 

 in those cases where the ducts of communication are 

 the most conspicuous, as with the gullet in the stur- 

 geon, and the stomach in the herring, these ducts are 

 air-tight, so that the bag can neither be inflated nor 

 emptied by pressure through them. That the air 

 contained in the bag is secreted cither by the glands 

 or the tunic itself is no doubt true, at least in those 

 cases where there is no communicating duct ; and as 

 when there is a communication, that is always with 

 the alimentary canal, the probability is that the air- 

 bag is, in some way or other, connected with the 

 digestive system, but in what way, the present state 

 of our knowledge does not enable us to decide. 

 Enough is known, however, to show that the use is 

 physiological, rather than mechanical to refute the 

 exfsting theory, but not to establish a better one. 



Before that can be done, there must be much careful 

 observation. 



The air-bags of fishes vary much in shape, in strength, 

 and in size, as compared with that of the species to 

 which they belong ; but till the use of the organ itself 

 is a little better known, no conclusion can be drawn 

 from these differences. 



.In the arts, the substance of this vesicle is of consi- 

 derable importance. When freed from fat and other 

 impurities, it is among the most pure animal gelatine 

 with which we are acquainted. Isinglass is the air- 

 bag of the sturgeon, freed from oily matter and dried ; 

 and the air-bags of cod are collected and salted in 

 large quantities at the cod-fisheries, and well-known 

 in commerce by the name of " cod-sounds." The air- 

 bags of all fishes may be used for the same purposes, 

 though some of them are much smaller than others 

 and have the gelatine less pure. Inferior ones are, 

 however, often sold for the genuine isinglass of the 

 sturgeon. 



AIR-CELLS, and tubes, in birds, are cavities in 

 the internal membranes, and also in the livers, feathers, 

 and other parts of these animals, some of which are of 

 use in respiration, and others render the parts in 

 which they are contained more strong and stiff, with 

 the same quantity and consistency of materials. As 

 birds have no motion of the chest in breathing, and no 

 diaphragm, the air-cells act as substitutes ; and as, in 

 some cases at least, the coats of the arteries are exposed 

 to the air in them, they answer some of the purposes of 

 lungs ; and the bird is enabled to continue long on the 

 wing without getting out of breath. In some species, 

 as for instance, in the gannet, there is a power of inflat- 

 ing the cellular membrane of the breast by means of 

 a communication with the air-cells ; but though the 

 quantity of air which the cells and tubes admit into 

 the body of the bird lessens its specific gravity as a 

 whole, it does not appear that they are used in altering 

 that specific gravity. 



AIR PLANTS are those vegetables which can 

 live for a considerable period of time, when sus- 

 pended in the air, without being attached to the 

 ground or any substance whence they can derive 

 nourishment. Many plants possess this property. 

 Thus several species of jicus, more especially the 

 ficus clastica or Indian-rubber tree, have been known 

 to live for a great number of years suspended in a 

 green-house, and nourished only by the air and the 

 moisture contained in that. Some of the aloe tribe 

 have also been kept growing in a similar manner for 

 a long time. 



The name of air plants has, however, been more 

 particularly applied to several members of the orchis 

 and pine-apple tribes, which are met with abundantly 

 in the damp shady woods of the tropical regions of 

 America and Asia. On account of their beauty and 

 the fragrance of their flowers, they are cultivated in 

 these countries for the purpose of adorning chambers 

 and balconies. 



The Tillandsia mneoides, with its wiry stems and 

 dense leaves, attaches itself to the trees in Louisiana, 

 forming an uninterrupted mass, suspended in the air, 

 which frequently extends over whole forests and 

 completely intercepts the sun's rays. 



Tillandsia xiphioidcs, Buenos Ayres, is another 

 species which grows on the bare trunks and 

 branches of trees, its airy .vegetation being fre- 

 quently suspended only by a thread. It is cultivated 

 in hot-houses in this country. 



