in EXTERNAL CHARACTERS OF THE FROG 63 



It corresponds rather to the lower eyelid proper, plus a 

 nictitating membrane. The latter structure in most animals 

 in which it occurs is very distinct from both the other eye- 

 lids. In a bird, for instance, it appears as a thin membrane 

 which can be drawn over the eye from the inner angle of the 

 orbit. In the frog, however, it is situated just above the 

 lower lid, of which it appears to form a continuation. It is 

 thinner and more transparent than the lower lid and sepa- 

 rated from it by a shallow groove. 



Behind the eye is a nearly circular area covered by a 

 tense membrane, known as the tympanic membrane, which 

 forms the covering of the drum of the ear. Near the center 

 of this membrane may be seen a small prominence caused 

 by the end of the columella, or bone which connects at its 

 inner end with a small opening in the skull which commu- 

 nicates with the inner ear. When the tympanic membrane 

 is set in motion by the waves of sound which strike it, the 

 vibrations thus caused are communicated to the internal ear, 

 and thus give rise to the sensation of hearing, as will be 

 treated more in detail in a later chapter. On the inner side 

 of the tympanic membrane lies a cavity, the Eustachian tube, 

 which opens internally into the mouth. If a bristle be 

 passed through this membrane, it will be seen to emerge 

 through a rather large rounded opening near the angle of 

 the jaw. The external features of the auditory organ of the 

 frog differ markedly from those of man in that all traces of 

 an external ear are absent, and the tympanic membrane lies 

 exposed at the surface of the body instead of lying at the 

 inner end of a long passage. 



Above and behind the blunt tip of the snout lie the nos- 

 trils or external nares. These openings are guarded by 

 valves which open and close in connection with the move- 

 ments concerned in respiration. The tip of the upper jaw 



