The method of recording the respiration was the direct transference of the movement of the throat by means of a pivoted lever, one end of which rested against the throat, while the other served as a marker on a revolving drum carrying smoked paper. The frog was put into a small box, visual stimuli were, so far as possible, excluded and the lever was adjusted carefully; a record was then taken for at least half a minute to determine the normal rate of respiration in the absence of the stimulus whose effect it was the chief purpose of the experiment to discover. Then, as soon as everything was running smoothly, the auditory stimulus was given. The following records indicate the effects of a few stimuli upon the rate of breathing:
1. Stimulus, 100 V. tuning fork.
Number of respirations for 10 cm. before stimulus 18.0, 17.0; number of respirations for 10 cm. after stimulus 19.0, 17.3.
The records indicate very little change, and contradict one another. For the same stimulus the experiment was tried of taking the normal respiration record for a complete revolution of the drum, and then at once taking the record for the same length of time (about two minutes) with the tuning-fork vibrating close to the frog. The following result is typical and proves that the sound has little effect.
Number of respirations in a revolution before stimulus: First rev. 88; second rev. 88. Number of respirations in a revolution during stimulus: First rev. 87; second rev. 88.
Concerning the influence of tuning-fork stimuli more will be said later in a consideration of the effects of auditory stimuli upon reactions to visual stimuli.
2. The influence of falling water as an auditory stimulus. Water was allowed to fall about two feet in imitation, first, of a plunging frog, and second, of water falling over rocks. In representing the effect of the stimulus on the rate of respiration, I have given the distance on the drum covered by the ten complete respirations just preceding the stimulus and the ten following it.
| 10 Respirations. Before Stimulus. | 10 Respirations. After Stimulus. | |
|---|---|---|
| 1st Stim. | 13.0 cm | 11.8 cm. |
| 2d Stim. | 12.7 cm. | 12.7 cm. |
| With a smaller animal. | ||
| 1st Stim. | 5.4 cm. | 4.8 cm. |
| 2d Stim. | 4.9 cm. | 4.7 cm. |
| Average for 5 | 5.00 cm. | 4.86 cm. |
These records show a marked increase in the rate of respiration just after the auditory stimulus is given for the first time. The stimulus has less effect when repeated after an interval of one or two minutes, and if repeated several times it finally causes no noticeable change. On the whole, the sound of falling water seems to arouse the animals to fuller life. The stimulus appears to interest them, and it certainly accelerates respiration. This is precisely what one would expect from a sound which is of special significance in the life of the animal.
3. In case of a loud shrill whistle inhibition of respiration resulted. This probably means that the frogs were frightened by the sound. Falling water served rather to excite their natural-habitat associations, whereas, the whistle, being an uncommon and unassociated sound, caused fear. It is evident to the casual observer that the frog sometimes inhibits and sometimes increases its respiratory movements when frightened, so the result in this experiment is in no way surprising. I am by no means certain, however, that a longer series of observations on several individuals would give constant inhibitory results. My immediate purpose in the work was to get evidence of hearing; the respiratory changes were of secondary importance, although of such great interest that I have planned a more thorough special study of them for the future.