1014 FUNCTIONS OF THE ORGAN OF COBTI. [BOOK in. 



tympanic membrane, all the constituent factors are merged into 

 one complex sweep. We may conclude then that we possess 

 some means of analyzing the composite waves of sound which 

 sweep through the perilymph, and of sorting out their constitu- 

 ent vibrations. 



There is at hand a simple and easy physical method of ana- 

 lyzing composite sounds. If a person standing before an open 

 pianoforte, the loud pedal being held down, sings out any note, 

 it will be observed that a number of the strings of the pianoforte 

 will be thrown into vibration, and on examination it will be found 

 that those strings which are thus set going correspond in pitch 

 to the fundamental tone and to the several partial tones of the 

 note sung. The note sung reaches the strings as a complex 

 wave, but the strings are able to analyze the wave into its con- 

 stituent vibrations, each string taking up those vibrations and 

 those vibrations only which belong to the tone given forth by 

 itself when struck. If we suppose that each terminal fibril or 

 each group of fibrils of the auditory nerve is connected with a 

 terminal organ so far like a pianoforte-string that it will readily 

 vibrate in response to a series of vibrating impulses of a given 

 period and to none other, and that we possess a number of such 

 terminal organs sufficient for the analysis of all the sounds which 

 we can analyze, and that each terminal organ so affected by par- 

 ticular vibrations gives rise to a sensory impulse and thus sup- 

 plies the basis for a sensation of a distinct character if we 

 suppose these organs to exist, our appreciation of sounds is in 

 part explained. 



When the rods of Corti were first discovered, it was thought 

 that they were specially connected with the nerve fibres, and 

 served mechanically to stimulate the fibrils passing along their 

 limbs, by striking them after the fashion of minute hammers. 

 Since these rods, to whose striking resemblance to the keys of a 

 pianoforte we have already called attention, are arranged in a 

 long series the members of which vary regularly in the length 

 and in the span of "their arch, from the bottom to the top of the 

 spiral, it was supposed that each pair would vibrate in response 

 to a particular tone, and hence that the whole series served for 

 the analysis of sound. 



But this view proved untenable. Whatever purpose they 

 serve, the rods of Corti produce their effect, not by acting di- 

 rectly on nerve fibrils, but by contributing in some way or other 

 to the play of the hair-cells ; and, whatever be the way in which 

 they intervene, they do not vary in length and arrangement 

 along the spiral to such an extent as the above view demands. 

 Moreover, they are wholly absent from the rudimentary cochlea 

 of birds, though these creatures very clearly can appreciate mu- 

 sical sounds. This last fact proves indubitably that the rods in 

 question are not absolutely essential for the recognition of tones, 



