The Senses of Animals. 263 



And the parts are so arranged that the vibrations, in 

 passing from the oval to the round membrane/ must run 

 up one side and down the other side of the cochlear canal. 

 As they run down they set in vibration a delicate mem- 

 brane which is supported on beautiful arched rods (the 



c.c 



Fig. 27. Diagram of ear. 



t.m., tympanic membrane, to which is attached a chain of small bones stretching across 

 the cavity of the drum, the innermost of which, St., fits into the " oval window." The vibra- 

 tions are transmitted up one side and down the other side of the cochlear canal, c.c., and thus 

 reach the "round window," /.r. ; s.c. is one of the semicircular canals, the other two are 

 omitted ; e.t. is the Eustachian tube connecting the cavity of the drum with the mouth-cavity. 



organs of Corti). And this membrane contains a number 

 of special hair-cells, so called because they bear minute 

 hair-like structures. These are the special end-organs of 

 hearing. It has been suggested that the fibres of the 

 membrane on the arched rods, which are of different 

 lengths and may be stretched with differing degrees of 

 tension, respond to vibrations of different pitch. Thus the 

 hair-cells on that particular part of the membrane would 

 be stimulated, and the note might be appreciated in its 

 true position in the scale. 



We must now pass on to consider the sense of hearing 

 in animals. That the mammalia have this sense well 

 developed is a matter of familiar observation, and in some 

 of them, such as the horse and the deer, it is exceedingly 



