SECTION III 

 AUDITORY SENSATIONS 



HYPOTHESIS OF HEARING 



HELMHOLTZ' HYPOTHESIS was that the organ of Corti and the 

 basilar membrane form together a series of automatically recording reso- 

 nators. In the same way that each of the strings of a piano can be set into 

 vibration by the sounding of a note which corresponds with it in pitch, so 

 also can the different fibres of the basilar membrane vibrate to a certain 

 note, and so cause stimulation of the hair-cells which are attached. to it. 



Four objections have been made to this hypothesis. (1) That the fibres 

 of the basilar membrane are so short that they could not respond to the low 

 notes which the ear is able to hear. The answer to this criticism is that not 

 only the length but also the tension and weight of a cord determine its vibra- 

 tion rate. In the case of the basilar membrane the tensions in the fibres are 

 probably minute, while the weights of the arches of Corti and the hair-cells 

 must make the period of vibration so much the longer. 



(2) That the separate fibres of the basilar membrane are bound together 

 so that vibration of the separate fibres would be impossible. This objection 

 Helmholtz met by calculating that a uniform membrane, in which the tension 

 was greater from side to side than longitudinally, would be able to respond in 

 the manner required. 



(3) That the difference in length of the fibres is not sufficiently great 

 for the short ones to vibrate to notes of 4000 vibrations per second, while 

 the long ones vibrate to 40 vibrations per second only. This objection also 

 fails when we reflect that not only length but also tension and weight deter- 

 mine the period of vibration of a stretched cord. However accurately we 

 can determine length and weight by histological examination, the method 

 tells us nothing concerning tension. This objection therefore must faiL 



(4) That if the cochlea depends for its action on the resonance of the basilar 

 fibres, we should expect a musical note to seem to go on sounding after the 

 note has actually ceased. Since on the other hand we know from our own 

 experience that words such as ' utter,' in which there is an interval of silence 

 between the two "ts," are quite different from 'udder,' in which there is no 

 interval of silence, it follows that the fibres of the basilar membrane have not 

 been in vibration after the sound ceased, and therefore probably resonance of 

 the basilar membrane is imaginary. If on the other hand we suppose the 

 fibres to be highly damped so that they come to rest at once when the note 

 ceases, how comes it that they can so readily be set in motion so that in 

 only three or four vibrations a note is distinctly heard. The answer is, I 



6U 



