224 ORGAN OF HEARING IN FISH. 



In examining the structure of the organ of 

 vision we are struck by the absence of the out- 

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

 brows and eye-lids, which are met with among 

 terrestrial animals. It is clear that these append- 

 ages 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 provided with an ear. In the higher animals 

 the mechanical apparatus of hearing consists of an 

 external and an internal portion; in fishes the 

 internal portion alone exists, and is hardly inferior 

 in perfection 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 con- 

 siderable. There exists, however, this important 

 difference between the organ of hearing of ter- 

 restrial animals 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 ma} 7 make this differ- 

 ence apparent by the following simple illustration. 

 The impulse occasioned to the air by the ticking 

 of a watch is so weak, as to be indistinctly heard 



