P honor eception 



489 



Reptiles. It is in certain reptiles that structures characteristic of a true coch- 

 lea make their first appearance (Figs. 161, 166). In alligators the lagena is 

 attached to two sides of the surrounding cavity, therehy forming three ducts— 

 the scalae vestihuli, media, and tympani,— and the floor of the lagena forms 

 the basilar membrane. In all reptiles other than snakes there is a middle ear 

 which contains a bony structure similar to the columella of frogs, except that 

 it usually is composed of two bones. The tympanic membrane is usually de- 

 pressed below the body surface to form an external auditory meatus, and in 

 alligators there is even a skin fold which may serve as an outer ear. Snakes 

 have no middle ear, and the outer end of the columella is attached to the 

 quadrate bone of the skull. This arrangement makes snakes (even the 

 cobra!) insensitive to air-borne sounds but very sensitive to earth-borne vibra- 

 tions, such as those resulting from the footfalls of approaching animals. 

 There is considerable evidence that rattlesnakes cannot hear their own 

 ratthng.^"'' 



4ndolymph duct 



brain 



anterior canal 

 posterior canal 



lateral canal 



extra ■ columella 

 tympanic membrane 



Fig. 



lagena 

 stapes 



166. Diagrammatic presentation of the ear of a reptile, as 

 seen from the posterior. (Adams.') 



The microphonic effect and the auditory nerve responses of reptiles have 

 been studied by several investigators. Foa and Peroni^-' recorded impulses 

 from the auditory nerve of Thalassochelys. Wever and Bray"'-^ found 

 a microphonic effect between 120 and 1000 cycles for Chrysemys picta, 

 with a high response below 500, and Adrian- found that the ear of 

 Cistudo responded only to the narrow band of 80 to 130 cycles, with a maxi- 

 mum at 110 cycles, and the impulse frequency of the nerve was the same as 

 the stimulating frequency. A snake gave an irregular nerve response dis- 

 charge in response to earth-borne vibrations but none to air-borne sound. 

 Adrian, Craik, and Sturdy* obtained microphonic and nerve responses from 

 tortoises, box turtles, and alligators. The range of the microphonic effect 

 for the tortoise was 50 to 300 cycles with a maximal sensitivity of 50 to 200; 

 that for the box turtle was 50 to 2000 cycles, with a maximum between 200 

 and 800; the range for the alligator was 50 to 4000, with a maximum between 

 300 and 2500. It was found that the upper limit of the nerve responses varied 

 with temperature, and that a rise in temperature from 9° to 26° C. raised the 

 maximal frequency of the nerve response from 200 to 500 impulses per sec- 



