Yerkes, The Sense of Hearing in Frogs. 



TABLE V. 



The Influence of Sounds on Frog with Eighth Nerves Cut. 



303 



Cutting of the eight Ji nerves j-enders the frog irresponsive to 

 sonitds which markedly influejice the tactual reactions of the 7ior- 

 nial animal. We may tJierefore conclude that the reactions zuith 

 zvhich ive have dealt in this investigation are due to stimulation of 

 certain sense organs of the ear, and that the use of the word hear- 

 ing in comiection zvith them is appropriate. 



VII. Summary and Conclusions. 



1. Observation of frogs in their natural habitat shows 

 that they are stimulated by sounds. The sense of hearing 

 apparently serves rather as a warning sense which modifies reac- 

 tions to other simultaneous or succeeding stimuli than as a con- 

 trol for definite auditory motor reactions. 



2. Experimental tests prove that sounds modify the frog's 

 reactions to visual and tactual stimuli. When the sound accom- 

 panies the visual or tactual stimulus it serves to reinforce the 

 visual or tactual reaction, but when given alone it never causes 

 a motor reaction. 



3. The sound of an electric bell occurring simultaneous- 

 ly with a tactual stimulus markedly increases (reinforces) the 

 leg reflex of green-, leopard- and bull-frogs. If the sound pre- 

 cedes the touch by i" it is without effect on the reaction ; 

 if the interval is not longer than .35" it usually causes reinforce- 

 ment, whereas for an interval of from .4" to .g" there is partial 

 inhibition of reaction. According to its temporal relation to 



