798 HEARING [CH. LVI. 



There can be little doubt that the cochlea is the organ specially 

 concerned in hearing. It first appears among vertebrata in certain 

 fishes as a very rudimentary structure. If the cochlea is removed 

 from dogs, they become deaf. 



There are two classes of theories of hearing, in both of which the 

 basilar membrane of the cochlea plays the essential part. 



The one class comprises the many "sound-picture" theories 

 which have been advanced in very various forms by Eutherford, 

 Waller, Hurst, Ewald, and Meyer. The entire basilar membrane is 

 supposed to vibrate either as a telephone plate, or as an elastic mem- 

 brane, different tones or combinations of tones giving rise to different 

 patterns of vibrations which are communicated to the hair-cells and 

 thence by the auditory nerve-fibres to the brain, where (in Euther- 

 ford's theory) the analysis of these patterns is held to take place. 



The other is the resonance-theory of Helmholtz, in which the 

 pitch of a tone, or the analysis of a complex sound into its constituent 

 tones, is determined not in the brain but in the cochlea. It depends 

 on the principle of sympathetic vibration. As is well known, if a 

 tone is sung in front of a piano (best with the loud pedal held down), 

 the string of the piano which is attuned to that tone will immediately 

 respond; another tone will elicit response from another string. So 

 in the cochlea the appropriate fibre of the basilar membrane is thrown 

 into vibration when the tone to which it is attuned reaches it. The 

 fibre thus stimulated affects the hair-cells above it, whence the stimulus 

 is conducted to the brain. If two tones are sounded together, the two 

 appropriate fibres respond, and the analysis of the now more complex 

 stimulus is performed in the cochlea. The fibres of the basilar 

 membrane increase in length from the base towards the apex of the 

 cochlea. According to the resonance-theory, the upper part of the 

 organ would thus be affected by low tones, the lower part by high 

 tones. 



The first of these two classes of theory makes it difficult or 

 impossible for us to explain our ability to analyse complex chords 

 into their component tones. The full acceptance of the second is 

 difficult in the face of the small difference of length (at most 1 : 12) 

 between the shortest and the longest of the basilar fibres. On the 

 other hand, it gains support from the effects of experiment on, and 

 disease of, different portions of the cochlea. For instance, the deaf- 

 ness to high-pitched tones (seen in boiler-makers) is stated to be 

 associated with disease of the lower whorl of the cochlea. 



It may be that the fibres of the basilar membrane do not 

 vibrate as Helmholtz supposed, but that the hair-cells themselves 

 are each in some unknown way specially attuned to respond only to 

 one of the many tonal stimuli which may reach them (Myers). 



