1074 THE SENSES 



and that, directly or through the hair-cells, their mechanical vibra- 

 tions were translated into impulses in the auditory nerve-fibres. 

 But apart from the fact that their number is too small (about 3,000) 

 to allow us to assign one rod to each perceptible difference of pitch, 

 and their dimensions too similar to permit of the requisite range 

 of vibration frequency, it was pointed out that birds do not possess 

 pillars of Corti a fact which was decisive against the assumption 

 of Helmholtz, since nobody denies to singing-birds the power of 

 appreciating pitch. Helmholtz accordingly, choosing between the 

 remaining possibilities, gave up the pillars of Corti, and adopting 

 a suggestion of Hensen, substituted the radial fibres of the basilar 

 membrane as his hypothetical resonators. These are more ade- 

 quate to the task imposed on them, since their range of length is 

 far greater (41 fjL at the base to 495 fj, at the apex of the cochlea 

 Hensen); and the elaborate structure of Corti 's organ certainly 

 suggests that some one or other of its elements may be endowed 

 with such a function. Experimentally, too, it has been shown 

 that destruction of the apex of the cochlea causes loss of appreciation 

 of low notes, and destruction of the base loss of appreciation of high 

 notes, which agrees with Helmholtz's view. But while the theory 

 of peripheral analysis of pitch tends upon the whole to be strength- 

 ened as evidence gathers, it is possible that the analysis is accom- 

 plished in some other way than by sympathetic resonance. 



Ewald has developed a theory according to which each note causes 

 the basilar membrane to vibrate throughout its whple extent in 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 mem- 

 brane, 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 sensitive terminal appa- 

 ratus 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 support 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 

 complex tones. These can be observed with the microscope and photo- 

 graphed (Fig. 459). 



One of the best-known theories of central analysis may be con- 

 veniently labelled the 'telephone theory,' in accordance with the simile 

 used by Rutherford. He supposed 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 translated into impulses in the auditory 

 nerve, which are the physiological counterpart of the aerial waves 

 and tiie 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 



