COMPARATIVE PHYSIOLOGY OP HEARING. 195 



with the integuments. This membrane corresponds to 

 the ear-drum of the Mammalia, the cavity within, con- 

 taining air. From this cavity there proceeds an Eusta- 

 chian tube ; and from the external membrane to the 

 vestibule there extends a small bone, shaped like a trum- 

 pet, and called the columella. 



These parts are represented by Fig. 118 ; where c,is 

 the columella of an elegant trumpet shape, having its 

 base, b, 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. 



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 vibrations of which, are communicated immediately 

 to the fenestra 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. 



This mechanism is probably designed to enable the 

 animal to hear both in air and water. 



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 audi- 

 tory nerves. 



In the Frog there is a communication from the exter- 

 nal tympanum to the vestibule by a solid body, the ope- 

 ration 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 1 What description can 

 you give of a frog's ears 1 By what mechanism is the frog fitted to hear 

 both in the air and in water ? 



