Phonoreception 



479 



cies, that the region toward the apex is sensitive to vibration of low frequen- 

 cies, and that sensitivity to intermediate frequencies is distributed throughout 

 the spiral. Maps of the cochleas of man and guinea pig are shown in Fig. 

 158. Men and guinea pigs are sensitive to vibrations of about the same 

 frequency range, but the guinea pig has one additional turn to the spiral. 



The allocation of response to particular frequencies to definite positions 

 along the cochlea is usually referred to as the "place theory" of hearing. 



Evidence for the place theory consists of the following: (1) Long exposure 

 to loud tones of a given frequency destroys the organ of Corti at the corre- 

 sponding place on the map in Figure 158. (2) Injections into the cochlea 

 at various points along the spiral of drugs which are destructive to the organ 

 of Corti will decrease the cochlear microphonics produced by the correspond- 



2 

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200 500 1000 



FREQUENCY 



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Fig. 157. Curve showing the calculated relation between the amplitude of vibration 

 of air molecules in a sound wave at threshold pressure. The circles show the amplitude 

 of vibration of the ear drum. (After Wilska,"' from Stevens and Davis.'") 



ing frequencies to which the particular areas are sensitive. (3) In human 

 high tone deafness there is a degeneration either of the organ of Corti near 

 the base of the cochlea or of the nerve fibers supplying this region. (4) Coch- 

 lear microphonics are recorded with greater voltage near the apex in response 

 to low tones and near the base in response to high tones. The region where 

 each frequency produces the highest voltage may also be mapped by moving 

 the recording electrode along the outside of the spiral. Furthermore, de- 

 struction or disturbance of certain regions of the organ of Corti by drilling 

 into the cochlea produces a rise in threshold for the cochlear microphonics 

 at the corresponding frequencies. (5) It has been demonstrated" that single 

 fibers of the auditory nerve are stimulated most easily by waves of a given 

 frequency, less easily by adjacent frequencies, and very much less easily 

 (threshold 30 db or higher) by frequencies differing by an octave or more 



