296 Journal of Comparative Neurology and Psychology. 



reaction 62 % greater than the tactual, in the case of one frog, 

 and 103. 1 % greater in the case of the other. On the other hand, 

 when the sound preceded the touch by .45" the resulting audi- 

 tory-tactual reaction was 22.3 % and 4.8 %, less, respectively, 

 than the tactual. 



The figures on the left margin of the curves above the zero 

 point indicate reinforcement in per cent, of the tactual reaction, 

 those below the zero point, inhibition. Below the base line 

 the time intervals are given in tenths of a second. Each curve 

 is plotted, from the data of Table II, on the basis of seven hun- 

 dred reactions. It is to be noted that the curves of Figure 5, 

 for the two males, show considerably more reinforcement than 

 those for the females in Figure 6. Furthermore, the curves 

 for the females cross the transition line between reinforcement 

 and inhibition sooner and return to it more slowly than do those 

 for the males. In other words, inhibition begins with a shorter 

 interval between the stimuli and continues longer. 



These experiments prove conclusively that sounds, although 

 they do not call forth the reflex movement under consideration, 

 modify in important ways the action of other stimuli. It is 

 therefore certain that lack of auditory reaction is due to some 

 form of inhibition and not to insensitiveness. 



IV. Hearing of Frogs in Air and in Water. 



It has* been held by many investigators of the sense of 

 hearing that sounds in the air cannot be heard by animals under 

 water for the simple reason that the air waves cause only very 

 slight disturbances in the water. In view of this statement it is 

 of interest to test the ability of the frog to hear when the tym- 

 panum is exposed to air and when it is under water. 



Experiments were made with the apparatus represented in 

 Fig. 7. The reflex reaction method of testing the influence of 

 sounds was again employed, and pairs of reactions were record- 

 ed for frogs whose ears were either exposed to air waves or sub- 

 merged in the water of the aquarium. The level of the water, 

 with reference to the ear, was controlled by changing the vol- 

 ume in the aquarium, and the leg of the frog was kept from 



