Ill ON ANIMAL LIFE 89 



vibration of the air striking on the drum of 

 the ear, and the fewer are the vibrations in 

 a second, the deeper is tlie sound, which 

 becomes shriller and shriller as the waves of 

 sound become more rapid. In hiiman ears 

 the limits of hearing are reached when about 

 35,000 vibrations strike the drum of the ear 

 in a second. 



Whatever the explanation of the gift of 

 hearing in ourselves may be, different plans 

 seem to be adopted in the case of other 

 animals. In many Crustacea and Insects 

 there are flattened hairs each connected with 

 a nerve fibre, and so constituted as to vibrate 

 in response to particular notes. In others 

 the ear cavity contains certain minute solid 

 bodies, known as otoliths, which in the same 

 way play upon the nerve fibres. Sometimes 

 these are secreted by the walls of the cavity 

 itself, but certain Crustacea have acquired the 

 remarkable habit of selecting after each 

 moult suitable particles of sand, which they 

 pick up with their pincers and insert into 

 their ears. 



Many insects, besides the two large 



