ORGAN OF HEARING IN FISH. 293 



works of defence to the eye, namely, the eye- 

 brows and eyelids, which are met with among ter- 

 restrial animals. It is clear that these appendages 

 are rendered unnecessary by the nature of the 

 medium in which the creatures reside. The same 

 observation applies with even stronger force to the 

 organ of hearing : there is nothing on the exterior 

 of the head of the fish to indicate that it is pro- 

 vided with an ear. In the higher animals the me- 

 chanical apparatus of hearing consists of an exter- 

 nal and an internal portion ; in fishes the internal 

 portion alone exists, and is hardly inferior in per- 

 fection of form and structure to that of creatures 

 placed higher in the animal scale. The nerves 

 distributed to the organ of hearing are of large 

 size, and the vital apparatus or portion of brain, 

 from which the latter proceed, is also considerable. 

 There exists, however, this important difference 

 between the organ of hearing of terrestrial ani- 

 mals and fishes, namely, that the ear in the former 

 is organised for the reception of the more delicate 

 vibrations of the atmosphere, while in the latter, 

 it is adapted to the rude oscillations of a denser 

 element. We may make this difference apparent 

 by the following simple illustration. The im- 

 pulse occasioned to the air by the ticking of a 

 watch is so weak, as to be indistinctly heard when 

 the watch is brought close to the ear, but if we 



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