MODE OF ACTION OF AUDITORY ORGANS. 81 



The fibres of Corti, according to Helmholtz, may 

 be distributed among the seven octaves which are in 

 general use, so that there will be 33 J fibres to every 

 semitone, and 400 to each octave. Weber has esti- 

 mated that a skilful ear can perceive a difference even 

 of the g^ of a tone, or nearly four thousand sounds, and 

 this would agree fairly well with the number of fibres. 



But why, it may be asked, should a given musical 

 sound act more on one of these " keys " than another ? 

 If several tuniDg-forks which, sound different notes 

 are placed on a table, and another in vibration be 

 brought near them, the one sounding the same note is 

 thrown into vibration, while the others are unaffected. 

 A second tuning-fork would affect its owa fellow, but 

 no other, and so on.* A very slight change in the 

 tuning-fork, such, for instance, as would be made by 

 fastening a piece of wax to one of the prongs, is 

 sufficient to destroy the sympathetic vibrations. The 

 sound of the human voice has been known to break a 

 bell-shaped glass by the agitation thus caused. The 

 difficulty is to hit the pitch with sufficient precision, 

 and retain the tone long enough. It is probable, 

 therefore, that each of Corti's arches is set for a 

 particular sound, and sensitive to it alone. This 

 suggestion derives additional probability from the 

 observations of Hensen (see p. 93) on the auditory 

 hairs of Crustacea. 



We thus obtain a glimpse, though but a glimpse, of 

 the manner in which the arches of Corti may possibly 

 act. There are many problems still to be solved, but 

 it is at least easy to see that so complex an organ may 

 be capable of conveying very complex sensations. 



* Helmholtz, " Sensations of Tone." 



G 



