964 



A MANUAL OF PHYSIOLOGY 



such a way that stationary waves are produced in it, like the Chladni's 

 figures seen on a metal plate strewed with sand when it is set into 

 vibration. The pattern of the movement, the ' sound-picture,' will 

 be different for each tone, since the interval between the waves will 

 be different. The hair-cells and auditory fibres of particular parts 

 of the organ of Corti will therefore be stimulated by the pressure of 

 the membrane, or escape stimulation, according to the position of 

 the stationary waves with reference to them for each note. In this 

 way each sound-picture will be printed, so to speak, upon the sensi- 

 tive terminal apparatus of the auditory nerve, as a letter is printed 

 upon a piece of paper by a type. The corresponding excitation 

 pattern i.e., the particular distribution of cochlear fibres stimulated 

 is supposed to be associated in consciousness with the appreciation 

 of the pitch of the particular note. Ewald has endeavoured to sup- 

 port his theory by showing that fine membranes of the dimensions 

 of the basilar membrane do yield very distinct sound-pictures for 

 different simple tones as well as for com- 

 plex tones. These can be observed with 

 the microscope and photographed (Fig. 425) . 

 The best-known theory of central analysis 

 may be conveniently labelled the ' telephone 

 theory/ in accordance with the simile used 

 by Rutherford, to whom we owe it in its 

 present form. He supposes that the organ 

 of Corti (or at any rate the hair-cells) is 

 set into vibration as a whole by all audible 

 sounds, and that its vibrations are trans- 

 lated into impulses in the auditory nerve, 

 which are the physiological counterpart of 

 the aerial waves and the waves of increased 

 and diminished pressure in the liquids of 

 the labyrinth to which they give rise. 

 Thus, a sound of 100 vibrations a second 

 would start 100 impulses a second in the 

 auditory nerve ; a loud sound would set up 

 impulses more intense than a feeble sound ; 

 and a complex wave, which is the resultant 

 of several sounds of different vibration-fre- 

 quency, would also in some way or other 



stamp the impress of its form on the auditory excitation- wave ; just 

 as in a telephone every wave in the air causes a swing of the vibrating 

 plate, and thus sets up a current of corresponding intensity and dura- 

 tion in the wires. This theory evidently abandons the doctrine of 

 specific energy for the particular case of the analysis of pitch, for it 

 assumes that differences of auditory sensation are related to differ- 

 ences in the nature of the impulses travelling up the auditory 

 nerve, and not merely to differences in the anatomical connections 

 (peripheral and central) of the auditory nerve-fibres. It is un- 

 satisfactory because it takes no account of the remarkable and 

 suggestive structure of the telephone plate i.e., of the organ of 

 Corti and gives no hint of how the analysis is accomplished in the 

 central organ. 



The range of hearing is very great. The highest audible tone 

 corresponds to 30,000 to 40,000 vibrations a second, the lowest to 

 about 30. Between these limits as many as 6,000 variations of 

 pitch can be perceived. 



FIG. 425. PHOTOGRAPH OF 

 A SOUND-PICTURE 



(EWALD). 



