480 



Comparative Animal Physiology 



(Fig. 159). (6) Integration of curves for data on the just-perceptible-dif- 

 ference tones as a function of frequency yields a curve which can be super- 

 imposed on a plot of the position of the sensitive region as detected by the 

 drilling experiments mentioned in (4) above as a function of frequency. 

 This should be expected if the minimal detectable difference in frequency 

 corresponds to the minimal detectable distance between two adjacent regions 

 of excitation, and if this distance is constant throughout the length of the 

 cochlea. Since the sense cells are about the same size throughout the length of 

 the cochlea the latter assumption seems reasonable. (7) The mass of vibrat- 

 ing material along the path of a sound wave from oval to round window is 

 much less if the wave passes through the basilar membrane near the base 



250 500 1000 2000 



FREQUENCY 



HUMAN 



GUlNtA-PlG 



Fig. 158. Upper: Curve of intensity of stimulation necessary to elicit a just-measurable 

 microphonic response from a cochlea which has sustained an injury. Lower: Map of the 

 cochlea of the guinea pig, indicating localization of pitch reception. This map is recon- 

 structed from data of 20 experiments similar to that shown above, together with histo- 

 logical examination of basilar membrane damage. The pitch localization map for man is 

 deduced from the guinea pig data. (After Stevens, Davis and Lurie*" from Fulton.") 



than if it passes near the apex. Since the natural frequency of a vibrating 

 system increases with reductions in mass, the natural frequency of the path 

 through the basal region is higher than that through a region near the apex. 

 In general, there seems to be no reason for doubting the place theory of the 

 hearing mechanism. 



Transmission in the Nervous System. If we assume that each cross row 

 of hair cells (e.g., each such row usually consisting of one inner and three 

 outer hair cells), because of the geometry and physical character of its sur- 

 roundings, is most sensitive to one frequency, then the next step in an ex- 

 planation of the hearing mechanism is to show how information concerning 

 stimulation of these cells is transmitted to the brain. Ihcre are about 3500 

 hair cells on the inner row and about 20,000 equally distributed among the 



