AIR-BLADDER OF FISHES. 497 



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. 229, t. 



Viewing the general modifications and relations of the air- 

 bladder throughout the class of Fishes, we cannot but discern and 

 admit, notwithstanding 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 in rising or sinking as occa- 

 sion 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 1 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 occasions from the weight of the superimposed 

 vertebral column and muscles when life and action are extinct. 

 The Sharks require, by the position of their mouth and in their 

 common pursuit of living prey, freedom in turning and great 

 variety as well as 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 

 developement and force which suffices for all the evolutions they 

 are called upon to perforin. With regard to the accessory offices 

 of the air-bladder in relation to the sense of hearing, the chief of 

 these remarkable modifications by which it is brought into com- 

 munication with the acoustic labyrinth have been already described, 

 p. 344. In a few genera ( Trigla), the air-bladder and its duct 

 are subservient to the production of sounds. 



Under all its diversities of structure and function the homolooy 

 of the swim-bladder with the lungs is clearly traceable ; and 

 finally, in those orders of Fishes which lead more directly to the 

 Reptilia, as, for example, the salamandroid Ganoidei and Protopteri, 

 those further modifications are superinduced upon the air-bladder, 



1 cxxxi. cap. 23. 



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