498 HEARING, VOICE AND SPEECH 



to the labyrinth to the best advantage, it is necessary that all extraneous 

 vibrations be excluded as far as possible. Moreover, the tympanic cavity 

 ought, if possible at all, to have no tone of its own, and finally no difference 

 of atmospheric pressure, at least no permanent difference, ought to obtain 

 between the tympanic cavity and the outside air. 



These requirements are sufficiently fulfilled, one can readily see, by the 

 structure of the tympanic cavity, this being at once rather small and very 

 irregular in shape, so that resonance to special tones is prevented. 



The pressure inside the tympanic cavity is regulated through the Eusta- 

 chian tube communicating with the throat. Normally this tube is rather 

 tightly closed, but it is often opened as, e. g., in swallowing. Since it is in 

 this way that the pressure inside and outside the tympanic cavity is equalized, 

 it is well for a person inclosed within a pneumatic cabinet, where the air 

 pressure is considerably increased, to swallow frequently. The tube is opened 

 also in strong inspiration and in phonation, although to a less extent than in 

 swallowing. 



The Eustachian tube is lined with a ciliated epithelium which probably 

 serves to drive the mucus, etc., toward the throat. 



3. EXCITATION OF THE AUDITORY NERVE 



The vibrations of the stapes are transmitted to the perilymph, and these 

 in turn set the endolymph in vibration. 



A. THE RESONATORS IN THE COCHLEA 



We have already remarked that the analysis of sound leads us to assume 

 that the different perceptible sounds have their appropriate resonators in the 

 ear. But it is possible also to imagine that the fibers of the auditory nerve 

 themselves are thrown By the endolymph into vibrations which agree exactly 

 with those of the conducting apparatus. Against this hypothesis, however, 

 several objections may be urged, chief of which is that we have nowhere else 

 in physiology any analogous production in a nerve itself of 40,000 or 50,000 

 molecular vibrations per second. Besides, there are some observations which 

 appear to speak directly in favor of the resonance theory. For example, 

 Bezold has found by means of an instrument which enabled him to vary the 

 number of vibrations per second from that of the lowest sound to that of the 

 highest, without any omissions, that for different individuals there are gaps 

 of greater or less size in the series of perceptible tones. Some show defects 

 both in the upper and the lower ends of the series, others only in the lower, 

 and still others only in the upper end. Gaps of varying extent occur also at 

 different places along the course of the scale. All of them can be explained 

 by supposing that the corresponding resonators are wanting. 



That the fibers of the auditory nerve are not set in vibration directly by 

 the vibrations of the endolymph is indicated by the following considerations 

 with reference to fatigue of the ear : 



If the vibrations of a tuning fork in a distant room be transmitted by 

 means of two telephones to the two ears, the tone will appear to be located 



