I 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 the sound, which 
becomes shriller and shriller as the waves of 
sound become more rapid. In human 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 
