THE SENSE OF HEARING 



241 



Ewald starts from Waller's theory that single tones may 

 imprint different pressure-patterns upon the organ of Corti, com- 

 parable with the images on the retina. Every tone that impinges 

 on the ear causes the basilar membrane to vibrate in its entire 

 length, since it subdivides into a series of stationary waves of 

 definite form, constituting as a whole what Ewald terms " acoustic 

 images," which produce perceptions of sound in the brain by 

 means of the fibres of the auditory nerve. On Ewald's theory a 

 different acoustic image, consisting of a series 

 of stationary waves, corresponds to each tone. 

 When several tones impress the organ of Corti 

 simultaneously there occurs a superposition, 

 but no change in the length of the respective 

 waves, so that each of the acoustic images can 

 be recognised. In this way peripheral analysis 

 of the tones becomes possible. When the 

 sonorous vibrations are a-periodic, so that the 

 waves impinging on the basilar membrane are not stationary but 

 mobile, noises instead of tones are perceived. 



Ewald gave his theory an experimental basis. If a rubber 

 membrane, 15 cm. long and 6 wide, is stretched over a frame and 



3mninium,wifchB?raic?B 



s ..-!' 



Fio. 96. To show how Ewald's auditory images can be observed by the microscope on the 

 vibrating membrane. 



the surface smeared with oil to make it shine, a series of stationary 

 waves (an acoustic image) appears on it when a tuning-fork is 

 made to vibrate in its neighbourhood. These images vary per- 

 ceptibly if the note of the tuning-fork is altered. 



In order to reproduce the peculiar characteristics of the 

 vibrating apparatus of the internal ear as exactly as possible 

 Ewald (1903) prepared a delicate membrane, 8-5 mm. long and 

 0-55 mm. wide (Fig. 95). On this membrane, which he takes as 

 the counterpart of the basilar membrane, different sonorous images, 

 corresponding to the notes of certain octaves, can be observed 

 with the microscope (see diagram of Fig. 96). The lowest tone 

 capable, according to Ewald, of producing a visible image on this 

 membrane is B of the small octave ( = 247-5 d.v.), and the highest 

 is 4 ( = 3520 d.v.}. It remains to be seen whether further technical 



VOL. IV K 



