COMPARATIVE PHYSIOLOGY OF HEARING. 191 



rvith the integuments. The membrane corresponds to the 

 ear-drum of the mammalia, the cavity within, containing 

 air. From this cavity there proceeds a eustachian tube ; 

 and from the external membrane to the vestibule there 

 extends a small bone, shaped like a trumpet, and called 

 the columella. 



591. These parts are represented by Fig. 118 : where c, 

 is the .columella of an elegant trumpet-shape, having its 

 base 6, attached to the fenestra 



ovalis of the vestibule, v, and ' ^ 



which contains the chalky 

 body, o. There is also a small 

 bone, i, appended to the end 

 or front of the columella, 

 where this is attached to the 

 external membrane, or ear- 

 drum. 



592. In the frog, therefore, the sonorous undulations of 

 the air, instead of passing through an aperture, as in man, 

 strike the tympanum on the outside of the head, the vibra- 

 tions of which, are commnicated immediately to the fenes- 

 tra ovalis of the vestibule, and through the auditory nerves 

 spread over it, directly to the brain. In other amphibia, 

 these parts are essentially the same. 



593. This mechanism is probably designed to enable the 

 animal to hear both in air and water. 



594. In the fishes there is no internal cavity containing 

 air, as is also the case with the lobster, the ear of which, 

 as we have seen, is so contrived as to place the fenestra 

 or window of the vestibule, which contains the nerves of 

 hearing on the outside, so that the sonorous undulations of 

 the water are communicated immediately to the auditory 

 nerves. 



595. In the frog there is a communication from the ex- 

 ternal tympanum to the vestibule by a solid body, the op- 

 eration of which, therefore, does not differ from that of 

 the lobster ; but in addition to this, there is a cavity 



What is said of the hearing of the mollusca ? What description can 

 you give of a frog's ears? By what mechanism is the frog fitted to hear 

 both in the air and in water. 



