v THE SENSE OF HEAKING 235 



organ of Corti is provided with as many specifically differentiated 

 nerve -fibres as there are perceptible tones. This qualitative 

 difference in the fibres of the acoustic nerve, which is one of the 

 postulates of Helmholtz' theory, presents a certain analogy to 

 the difference in the sensations aroused by the tactile nerves in 

 different parts of the skin, in so far as these can be distinguished 

 by their local signs. 



A simpler hypothesis to explain the ability of the ear to 

 distinguish different tones is that which assumes that each tone 

 of any given number of vibrations, which agitates the fluid of 

 the labyrinth, simultaneously excites all the fibres of the auditory 

 nerve, and that the pitch of the note is distinguished in the brain 

 by the number of the waves carried to it by the nerve in the 

 time-unit. But when we remember that, on Bernstein's compu- 

 tation, the duration of the nerve-wave is usually at least 0'0006 

 sees., and that, on the other hand, the number of perceptible tones 

 may amount to 40'000 or 50'000 per sec., this assumption appears 

 extremely improbable little less so than the hypothesis that 

 we distinguish colours by the number of the nerve-waves trans- 

 mitted to the brain by the fibres of the optic nerve. 



To explain the mechanism by which single tones are capable 

 of separately exciting one or a few specifically different fibres 

 of the acoustic nerve, Helmholtz assumed that the organ of Corti 

 is a graduated system of resonators capable of vibrating to the 

 different tones of the scale. This theory was repeatedly advanced 

 before the time of Helmholtz. To cite one name, Cotugno, in an 

 anatomical treatise on the ear (Naples, 1760), compared the 

 cochlea to a lute, and held that the perception of the higher tones 

 depends on the lower spirals, andi that of the lower tones on the 

 apical convolution. Helmholtz, from similar anatomical con- 

 siderations, urged the same hypothesis in a most striking manner 

 when he compared the organ of Corti to a piano. 



Starting from the non-homogeneous structure of the basilar 

 membrane, and specially from the fact that it has a continuous 

 series of elastic fibres stretched in a radial direction, which 

 increase in length twelvefold as they ascend from base to apex 

 of the cochlea, and are connected by a membrane which is longi- 

 tudinally but little distensible and easily lacerated, Helmholtz 

 imagined that these radial fibres might represent a system of 

 strings, similar to those of a piano tuned and capable of. 

 vibrating with the different notes of the scale. The wonderful 

 faculty of the ear for analysing complex tones would depend on 

 the fact that each radial fibre of the basilar membrane can vibrate 

 to a given tone, so that when a complex vibration is transmitted 

 to the cochlea the partial tones of which the sound is composed 

 throw separate fibres into vibration, and these excite distinct 

 nerve filaments, which are specifically differentiated at the 



