Yerkes, The Sense of Hearing m Frogs. 291 



suspended leg, which varies in amount with the state of the 

 animal and the strength of the stimulus. A sound, on the 

 contrary, never causes the slightest movement, and that no 

 matter how loud or high it be. This makes it possible to study 

 the influence of sounds on tactual reactions, under different 

 temporal relations of the stimuli, for even when the sound pre- 

 cedes the touch there is no reaction until after tactual stimula- 

 tion. 



The results now to be presented were given by the green 

 frog, but test experiments indicate that both the leopard and 

 the bull- frog are influenced in similar manner by sounds. 



The experimental procedure was as follows. After an ani- 

 mal had been placed in the proper position and had ceased to 

 struggle to escape, reactions to stimuli were taken in pairs regu- 

 larly at half minute intervals, first a reaction to the tactual stim- 

 ulus alone, then a reaction to the same intensity of touch when 

 accompanied or preceded by an auditory stimulus. 



The series consisted of 50 pairs of reactions taken without 

 pause. So far as the frog is concerned there seems to be noth- 

 ing undesirable in long series, for there is no indication of 

 fatigue, and so long as the animal is kept moist and in a com- 

 fortable position it does not often struggle to escape. The 

 advantage, for the purposes of this investigation, of taking the 

 reactions in pairs is obvious. It permits us to compare directly 

 the reactions of each pair, and to note at once whether the 

 auditory stimulus has reinforced or inhibited the tactual reaction. 



During a series the intensity of the tactual stimulus was 

 changed as conditions demanded, but for any one pair of reac- 

 tions it was always the same. It not unfrequently happened 

 that an intensity which at first caused only slight movement of 

 the leg, later in the series uniformly brought about a maximum 

 contraction, or the reverse might be true, and since the maxi- 



Ftg. 4. Auditory-tactual apparatus. (Drawn by Dr. Wm. E. Hocking.) 

 P, pendulum; p, contact point of /"; b, attachment for electro-magnet, a; 

 m, key for electric bell circuit ; B, electric bell ; n, key for magnet circuit of 

 touch apparatus ; K, hand-key for release of pendulum and temporary closing of 

 electric bell circuit ; k^, k^, P, keys in circuits ; e,/, g, magnetic release for touch 

 apparatus ; /, pivoted lever bearing rubber cone, T, and weights, w. 



