478 



Comparative Animal Physiology 



In general, it seems that stimulation of the dendrites by sense cells may not 

 differ from stimulation of axons by dendrites of synapses. In both phenomena 

 the principal evidence against the electrical theory is the delay of the im- 

 pulse, and for this there seems to be an adequate explanation. 



The Range of Hearing. The normal human ear is sensitive to vibrations 

 of frequencies between 16 and 20,000 cycles (or 20 kilocycles— 20 kc.) per 

 second. At extremely high intensities the range has been reported even as 

 low as 1 cycle per second. Data showing the thresholds at various frequen- 

 cies as determined by numerous investigators are shown in Fig. 156. The 

 threshold is minimal at 1000 to 1500 cycles; at 100 cycles or at 15,000 cycles 

 it is 40 to 50 db higher on the pressure scale (in which 20 db denotes a ten- 

 fold increase in energy). 



\ 



FRCQUCNCY 



Fig. 156. The minimum audible pressure at the ear drum as a function of frequency, as 

 determined by various investigators. After Sivian and White," from Stevens and Davis." 



At the threshold of hearing the amplitude of vibration of the tympanic 

 membrane (calculations of Wilska,^'^ Fig 157) is less than 10'** cm. at 3000 

 cycles, and is lO'*^ cm. at 1000 cycles. This represents an extremely high 

 sensitivity to mechanical movement which is especially apparent when one 

 considers that the wave length of green light is about 5 X 10''^ cm. and the 

 diameter of a hydrogen molecule is only somewhat more than 10'** cm. 

 Since the amplitude of vibration of the basilar membrane is less than that 

 at the ear drum, it appears that the basilar membrane is sensitive to move- 

 ments of 10"^*^ cm., a distance equal to less than 1 per cent of the diameter 

 of a hydrogen molecule! 



The Mechanism of Frequency Discrimination. The human ear not only 

 is sensitive to the frequency range of 16 to 20,000 cycles, but it is able to dis- 

 tinguish between tones of different frequencies. At an intensity of 40 db 

 there are about 1500 recognizable differences in tone between 20 and 20,000 

 cycles; at 60 db the number is about 1800. 



It can be demonstrated in a variety of ways that the base of the cochlea, 

 i.e., the part next to the middle ear, is sensitive to vibration of high frequen- 



