VIII.] THE COMMON FROG. 125 



In AmpJiiicma they are completely wanting, and in 

 Proteus and in the Ophiomorpha the minute eyeballs 

 are covered with the ordinary and unchanged skin of 

 the head. 



The ear of the frog's class presents us with the 

 incipient condition of that part as an organ destined 

 to respond to sonorous vibrations conveyed to it by 

 the atmosphere. 



In man the internal ear (enclosed in the densest 

 bone of the skull, named, from its density, "petrous") 

 is a very complex organ. The aperture, surrounded 

 by the folds of the external ear, leads by a canal 

 towards a cavity called the tympanic cavity, which 

 cavity is shut off from the exterior by the tympanic 

 membrane (or drum of the ear), which stretches across 

 the canal at a considerable distance from its external 

 aperture. On the inner side of the tympanic cavity 

 lie the convoluted tubes (richly supplied with nerves) 

 which constitute the real organ of hearing or internal 

 ear. 



Although the tympanic cavity is shut off from the 

 exterior by the tympanum, it nevertheless is not alto- 

 gether shut off from the exterior, since it communi- 

 cates with the back of the mouth by a long and 

 narrow canal termed the Eustachian tube. 



It is the existence of these Eustachian openings 

 into the ear from the mouth which causes people 

 when intently listening to keep their mouth slightly 

 open. 



In the frog there is no such external canal, but the 

 tympanum is plainly to be seen in the way already 

 described, on the side of the head, covered only by a 



