260 PHYSIOLOGY CHAP. 



It is interesting at this point to consider the phenomena of 

 auditory fatigue. In order to demonstrate that the peripheral 

 and central neural apparatus of hearing can be fatigued, it is 

 necessary to let a very powerful sound act on the ear for some 

 time. The intensity of the sensation soon diminishes, and it may 

 disappear altogether. According to Sylvanus Thompson (1881) 

 there may also be the illusion that the source of sound is with- 

 drawn farther and farther from the ear ; in other words, owing 

 to auditory fatigue, sound is falsely located. Similar results 

 were obtained by Urbantschitsch (1881). He introduced two 

 rubber tubes into the ears, the free ends being brought close to 

 each other, so that the acuity of hearing in one or the other ear 

 might be tested rapidly by the note of a tuning-fork. After 

 convincing himself that both ears possessed the same degree of 

 auditory acuity, he let a loud tone from a large tuning-fork act 

 upon one ear for 10-15 seconds, and then damped the vibrations 

 by placing his finger on the prongs till the tone became almost 

 inaudible. As soon as the sensation ceased in the excited ear 

 he quickly brought the fork close to the tube in the other ear, 

 and observed that it was capable of perceiving the tone distinctly 

 for several seconds, demonstrating fatigue of the ear first excited. 

 This fatigue only lasted 2-5 seconds, after which time the fatigued 

 ear again acted like the non-stimulated ear. 



Another interesting experiment is reported by Tigerstedt. 

 When the vibrations of a tuning-fork are transmitted from a 

 distant room to both ears by a double telephone the tone perceived 

 is localised in the median plane of the head. But if the tone is 

 transmitted to one ear only, for a long enough time to fatigue it, 

 and the second telephone is then applied to the other ear, the 

 sound is no longer localised in the median plane, but on the side 

 of the non-fatigued ear. 



This very simple method not only demonstrates the pheno- 

 menon of auditory fatigue, but further brings out another interest- 

 ing fact, namely, that fatigue does not influence the auditory acute- 

 ness for different notes of the scale, but only for that individual 

 tone which produced it. When one ear is fatigued by one of the 

 telephones with a tone, e.g., of 360 double vibrations, and a tone 

 of 365 double vibrations is then immediately made to act on both 

 ears, no trace of fatigue can be discovered in the ear first stimu- 

 lated, because the tone of 365 vibrations is localised in the median 

 plane and not in the direction of the non- stimulated ear. 

 Undeniably, this limitation of fatigue to individual tones that 

 have affected the ear completely bears out Helmholtz' theory of 

 resonators ; but there is no proof that it cannot also be explained 

 by Ewald's theory of auditory images. 



Other phenomena in close relation with the function of the 

 peripheral and central apparatus of hearing are the entotic 





