HEARING. 439 



Aquatic reptiles have ears constructed nearly 

 on the same plan as those of fishes; thus the 

 Triton or Newt, has a vestibule containing only 

 one cretaceous body, and three semicircular 

 canals, unprotected by any surrounding bone. 

 In the Frog, however, we first perceive the ad- 

 dition of a distinct cavity, closed by a mem- 

 brane, which is on a level with the integuments, 

 on each side of the head. From this cavity, 

 which corresponds to that of the tympanum, 

 there proceeds an Eustachian tube ; and within 

 it, extending from the external membrane, 

 which must here be regarded as an ear-drum, 

 to the membrane of the vestibule, or fenestra 

 ovalis, is found a bone, shaped like a trumpet, 

 and termed the Columella. This bone is seen 

 at c in Fig. 404, attached by its base (b) to the 

 fenestra ovalis of the vestibule (v), which con- 



tains the cretaceous body (o). There is also a 

 small bone (i) attached in front to the columella. 

 In the Chelonia, the structure of the ear is 

 essentially the same as in the Frog, but tlie 



