THE SENSES 963 



Then, on the doctrine of ' specific energy ' (according to which 

 the sensation caused by stimulation of a nerve depends not on 

 the particular kind of stimulus but on the anatomical connection 

 of the nerve with certain nerve centres), in whatever way the 

 first fibre is excited, a sensation corresponding to a note with a 

 pitch of 100 a second will be perceived. Whenever the second 

 fibre is excited, the sensation wilHbe that of a note of 200 a 

 second, or the octave of the first. If both fibres are excited at 

 the same time the two notes will be heard together. Now, 

 Hensen actually observed that in the auditory organs of some 

 crustaceans, the hair-like processes of certain epithelial cells 

 can be set swinging by waves of sound, and, further, that they 

 do not all vibrate to the same note unless the sound is very 

 loud. In the lobster there are between four and five hundred of 

 these hairs, varying in length from 14 yu, to 740 & ; and in some 

 insects, such as the locust, similar hairs, also graduated in length, 

 exist. 



To gain an anatomical basis for his theory, Helmholtz sup- 

 posed first of all that the pillars of Corti were the vibrating 

 structures, and that, directly or through the hair-cells, their 

 mechanical vibrations 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 remain- 

 ing possibilities, gave up the pillars of Corti, and adopting a sug- 

 gestion of Hensen, substituted the radial fibres of the basilar 

 membrane as his hypothetical resonators. These are more 

 adequate to the task imposed on them, since their range of 

 length is far greater (41 /j, at the base to 495 //, 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 Helm- 

 holtz' s view. But while the theory of peripheral analysis of 

 pitch tends upon the whole to be strengthened as evidence 

 gathers, it is possible that the analysis is accomplished 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 whole extent in 



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