ADJUSTMENT, DIRECT AND INDIRECT 



either a very subordinate part, or none at all. 

 As we have just been considering eyes and ears, 

 let us once more return to them, to show how 

 certain peculiarities in their structure must be 

 chiefly due to directly adaptive changes. Within 

 the human ear, firmly fastened in the temporal 

 bone, is a spirally coiled chamber, known as 

 the cochlea. Within this chamber there is a very 

 elastic membrane, and on it lie the so-called 

 fibres of Corti, which are a series of fibrous fila- 

 ments placed side by side, with great regularity, 

 so as to present somewhat the appearance of the 

 keyboard on a piano. It is now held by phy- 

 siologists that this row of fibres is really a key- 

 board, and that each fibre is set in vibration 

 only by a particular musical note, exactly as 

 an A-tuning-fork is set vibrating when A is 

 sounded near it, but not when any other note is 

 sounded.^ The auditory nerve, in passing into 

 the cochlea, branches into an immense number 

 of nerve filaments, each of which communicates 

 with ofie of the keys of this ear-piano. So that 

 when A is sounded on a musical instrument, the 

 A-key within the ear vibrates, and transmits its 

 vibrations to a special filament of the auditory 



^ [The hypothesis here stated has since been altered in cur- 

 rent literature in the sense of regarding, not the rods of Corti, 

 but the fibres of the basilar membrane as constituting the 

 "key-board" in question. Fiske would have modified this 

 passage accordingly.] 



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