CHAP, iv.] HEARING. 237 



the cochlea it throws into sympathetic vibrations those small portions 

 and those portions only of the basilar membrane, the vibrations of 

 which correspond to the single vibrations of which the composite 

 vibration is made up , and the vibrations in turn so affect the 

 overlying structures, that auditory impulses are generated in 

 particular groups of fibrils of the auditory nerve. These auditory 

 impulses reaching the brain give rise to a corresponding sensation 

 of a particular sound. 



But the dimensions of even the basilar membrane do not seem 

 wholly adequate for the purpose ; since the latest measurements 

 shew that in man its range is very limited. If we take the whole 

 width of the membrane, the range is from 21 mm. at the base 

 to '36 mm. at the top, though if we take the specially modified 

 part reaching ( 831) from the outer feet of the rods to the spiral 

 ligament, we get a wider range, namely, from -075 mm. at the base 

 to '126 mm. at the top. On the other hand the estimated number 

 of radial fibres of the membrane is very large, 24,000 ; and even if we 

 suppose that several fibres always vibrate together, this would 

 still leave some thousands of groups of strings, each group acting 

 as an analyser. 



In the present state of our knowledge the whole matter must 

 be left as uncertain. Even if the basilar membrane acts in some 

 such way as suggested, the other structures in the auditory epithe- 

 lium present problems as yet insoluble. The true function of the 

 rods of Corti and of the reticulate membrane of which these form a 

 part, of the cells of Deiters, of the inner hair-cells as distinguished 

 from the outer hair-cells, as well as the reason there are four rows 

 of the latter (whereby probably the effect of the vibrations of a 

 group of the basilar fibres is increased) and only one of the former, 

 all these are as yet merely questions which cannot be answered. 



851. Even admitting that, in some way or another, sets of 

 vibrations or, to use a more general term, sets of molecular 

 movements are started in the auditory epithelium, in more or 

 less complete correspondence with the sets of vibrations which 

 originate from the musical instrument or other sounding body, 

 and admitting further that each set of such molecular movements 

 in the auditory epithelium starts a particular nervous impulse in 

 a fibril or in a set of fibrils of the auditory nerve, we are very far 

 from having solved the problem of hearing. 



It must be borne in mind that making the fullest allowance 

 for the assistance afforded us by the organ of Corti, the apprecia- 

 tion of any sound is ultimately a psychical act. The analysis of 

 the vibrations by help of the basilar membrane or otherwise is 

 simply preliminary to a synthesis of the auditory impulses so 

 generated into a complex sensation. We do not receive a 

 distinct series of specific auditory impulses resulting in a specific 

 sensation for every possible variation in the wave-length of sonorous 

 vibrations any more than we receive a distinct series of specific 



