AIR-BLADDER OF FISHES. 277 



in the higher organised species above-cited, with short and wide uir- 

 ducts, these may, likewise, convey air to the bladder. 



The contents of the air-bladder consist, in most fresh-water fishes, 

 of nitrogen, and a very small quantity of oxygen, with a trace of 

 carbonic acid gas : but in the air-bladders of sea-fishes, and espe- 

 cially of those which frequent great depths, oxygen predominates.* 



In the genera Aiichenipterus, Si/nodon, Malapterurus, and some 

 other Siluroids, the axis vertebra sends out on each side a slender 

 process, which expands at its end into a large round plate : this is 

 applied to the side of the air-bladder, and can be made to press upon 

 it, and expel the air through the duct by the action of a small muscle 

 arising from the skull. In some species of Gadus muscular fibres 

 extend from the vertebral column upon the air-bladder. The nerves 

 of the air-bladder are derived from the vagus after it has received 

 organic fibres from the sympathetic {fig- 58. t). 



Viewing the general modifications and relations of the air-bladder 

 throughout the class of fishes, we cannot but discern and admit, not- 

 withstanding some seeming capricious varieties, that its chief and most 

 general function is a mechanical one, serving to regulate the specific 

 gravity of the fish, to aid it in maintaining a particular level in its 

 element, and to rise or sink as occasion may serve. The general 

 law of its absence in the parasitic and suctorial Dermopteri, and 

 in all ground-fishes, as the Pleuronectidce and Ray-tribe, supports 

 the above conclusion. Borelli found that those fishes, whose air- 

 bladders were burst, sank to the bottom and were unable to rise. 

 Nor does the absence of the air-bladder in the surface-swimming 

 Sharks militate against this view of its physical function : for 

 though the air-bladder serves, it also enslaves. It opposes, for 

 example, those fishes that possess it in their endeavours to turn 

 on one side, and it demands a constant action of the balancing 

 fins to prevent that complete upsetting of the body which it oc- 

 casions from the weight of the superimposed vertebral column 

 and muscles when life and action are extinct. The Sharks re- 

 quire, by the position of their mouth and their common pursuit of 

 living prey, freedom in turning and great variety as well as great 

 power of locomotion : if they are not aided by a swim-bladder, 

 neither are their muscular exertions impeded by one ; whilst their 

 swimming organs acquire that degree of development and force 



* Humboldt found the gas in tlio air-bladder of the electric Gymiiotus to con- 

 sist of 96° of nitrogen and 4° of oxygen. Biot found 87° of oxygen in some of 

 the deep-sea Mediterranean fishes, the rest nitrogen, with a trace of carbonic acid. 

 No hydrogen has ever been detected in the air-bladders of fishes. 



f cxxix. cap. 23. 



T 3 



