HEARING. 311 



as the ray and shark tribes, it is surrounded by solid bone, 

 and is not visible within the cranium. In these latter fishes, 

 we first meet with a rudiment of the meatus, in a passage 

 extending from the inner side of the vestibule, to the upper 

 and back part of the skull, where it is closed by a mem- 

 brane, which is covered by the skin. 



Aquatic reptiles have ears constructed nearly on the same 

 plan as those of fishes: thus, the Triton or Newt has a vesti- 

 bule containing only one cretaceous body, and three semi- 

 circular canals, unprotected by any surrounding bone. In 

 the Frog, however, we first perceive the addition of a dis- 

 tinct cavity, closed by a membrane, 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 



404 



by its base (B) to the fenestra ovalis of the vestibule (v,) 

 which contains 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 the tympanum and columella are of greater 

 length. In the saurian reptiles the cavity of the tympanum 

 is still more capacious, and the ear-drum very distinctly 

 marked, and these animals possess great delicacy of hearing. 

 The labyrinth of the Crocodile is enclosed in bone, and con- 

 tains three calcareous bodies: it presents also an appendage 



