5. The variability of reaction times of the frog is great, and increases as the strength of the stimulus decreases.
6. When two kinds of stimuli (e.g., electrical and tactual) give reaction times of equal variability, I consider them directly comparable.
7. According to this criterion of comparability the reaction time to electric stimulation which is comparable with that to tactual is 172.1σ; and it is to be compared with 205.7σ. Both of these have a variability of approximately 34σ. On this basis one may say that the tactual reaction time is considerably longer than the electrical.
PART III. AUDITORY REACTIONS OF FROGS.
X. HEARING IN THE FROG.
A. Influences of Sounds in the Laboratory.
After determining the simple reaction time of the green frog to tactual and electrical stimulation, I attempted to do the same in case of auditory stimuli. In this I was unsuccessful because of failure to get the animal to give a motor response which could be recorded. The animal was placed in an experimenting box with a string attached to one hind leg as in the experiments described in Part II., and after it had become accustomed to the situation a sound was made. A wide range of sounds were tried, but to none except the croak of another frog was a motor reaction frequently given. Even a loud noise, such as the explosion of a large pistol cap, caused a visible motor reaction only in rare cases. In fifty trials with this stimulus I succeeded in getting three reactions, and since all of them measured between 230 and 240σ it is perhaps worth while to record the result as indicative of the auditory reaction time. As these were the only measurements obtained, I have no satisfactory basis for the comparison of auditory with other reaction times.
The remarkable inhibition of movement shown by the frog in the presence of strong auditory stimulation, at least what is for the human being a strong stimulus, led me to inquire concerning the limits and delicacy of the sense of hearing in frogs. In the vast quantity of literature on the structure and functions of the sense organs of the animal I have been able to find only a few casual remarks concerning hearing.
In approaching the problem of frog audition we may first examine the structure of the ear for the purpose of ascertaining what sounds are likely to affect the organ. There is no outer ear, but the membrana tympani, or ear drum, covered with skin, appears as a flat disc from 5 to 10 mm. in diameter on the side of the head just back of the eye and a little below it. In the middle ear there is but one bone, the columella, forming the connecting link between the tympanum and the internal ear. The inner ear, which contains the sense organs, consists of a membranous bag, the chief parts of which are the utriculus, the sacculus, the lagena, and the three semicircular canals. The cavity of this membranous labyrinth is filled with a fluid, the endolymph; and within the utriculus, sacculus and lagena are masses of inorganic matter called the otoliths. The auditory nerve terminates in eight sense organs, which contain hair cells. There is no cochlea as in the mammalian ear. The assumption commonly made is that vibrations in the water or air by direct contact cause the tympanic membrane to vibrate; this in turn causes a movement of the columella, which is transmitted to the perilymphatic fluid of the inner ear. The sensory hair cells are disturbed by the movements of the otoliths in the endolymph, and thus an impulse is originated in the auditory nerve which results in a sensation more or less resembling our auditory sensation. It is quite probable that the frog's sense of hearing is very different from ours, and that it is affected only by gross air vibrations. This conclusion the anatomy of the ear supports.
Although there does not seem to be a structural basis for a delicate sense of hearing, one must examine the physiological facts at hand before concluding that frogs do not possess a sense of hearing similar to our own. First, the fact that frogs make vocal sounds is evidence in favor of the hearing of such sounds at least, since it is difficult to explain the origin of the ability to make a sound except through its utility to the species. Granting, however, that a frog is able to hear the croaks or pain-screams of its own species, the range of the sense still remains very small, for although the race of frogs makes a great variety of sounds, any one species croaks within a narrow range.