THE SENSES 845 



sympathetic vibration to a particular note, while the others 

 are unaffected ; just as when a note is sung before an open 

 piano it is taken up by the string which is attuned to the 

 same pitch and ignored by the rest. Let us suppose that a 

 given fibre of the auditory nerve ends in an organ which is 

 only set vibrating by waves impinging on it at the rate of 

 100 a second, and that the end-organ of another fibre is 

 only influenced by waves with a frequency of 200 a second. 

 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 sensa- 

 tion corresponding to a note with a pitch of 100 a second 

 will be perceived. Whenever the second fibre is excited, 

 the sensation will be 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 //, tc 

 740 /*; and in some insects, such as the locust, similai 

 hairs, also graduated in length, exist. 



To gain an anatomical basis for his theory, Helmholtz 

 supposed 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 remaining possibilities, 



