236 FUNCTIONS OF THE ORGAN OF CORTI. [BOOK in. 



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 analyse, and that 

 each terminal organ so affected by particular vibrations gives rise 

 to a sensory impulse and thus supplies 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 directly 

 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 musical sounds. 

 This last fact proves indubitably that the rods in question are not 

 absolutely essential for the recognition of tones, since it is in the 

 highest degree improbable that birds are able to recognize tones 

 in some manner absolutely different from that employed by 

 mammals. 



In the face of these difficulties it has been suggested that the 

 basilar membrane, which is present in birds as well as in mammals, 

 and which, being tense radially but loose longitudinally, i.e. along 

 the spiral of the cochlea, may be considered as consisting of a 

 number of parallel radial strings, each capable of independent 

 vibrations, is the sought-for organ of analysis ; for it may be 

 shewn mathematically that a membrane so stretched in one 

 direction only is capable of vibrating in such a manner. And 

 the radial dimensions of the basilar membrane increasing as 

 they do upwards from the bottom of the spiral to near the top 

 give a much greater range of difference than do the rods of Corti. 

 According to this view, when a composite vibration sweeps along 



