386 ELEMENTARY PHYSIOLOGY less. 



periods, such aa constitutes an ordinary musical sound, are 

 transmitted by tlie tympanum to the cochlea, these as 

 they sweep along the canalis cochlearis throw into sym- 

 pathetic movement those parts, and those parts only, of 

 the basilar membrane with their overlying epithelium, 

 whose periods of vibration correspond to tlieir own 

 vibrations, and thus excite certain nerve filaments, and 

 these only It is this excitement of a group of nerve 

 filaments, some more intensely than others, which, reaching 

 the brain, give rise to the sensation which we associate 

 with the particular musical sound. 



It must be distinctly understood that the picture which 

 has just been drawn only illustrates one way in which the 

 cochlear mechanism miijlit translate waves in the perilymph 

 into nervous impulses. There is no suggestion that this 

 indicates what actually takes place or even that it is 

 nearer the truth than any of several other suggestions 

 which have been made. It is possible for instance that 

 each wave in the perilymph causes the whole basilar 

 membrane to vibrate like the plate of a telephone or 

 itself to fall into waves which traverse its whole length, 

 in either case we can conceive that the hair cells could 

 appreciate such a condition. 



Little as we know of the cochlear mechanism, we know 

 still less about the nature of the auditory sensorium or 

 central end-organ of the auditory nerve ; but it may be 

 conceived that each filament of the cochlear nerve is 

 connected with a particular portion of the nervous matter 

 of the central end-organ, in such a way that the 

 molecular movements of one of these particular portions 

 of nervous matter, brought about by a molecular disturb- 

 ance reaching it through its appropriate filament, 

 produces a psychical effect of one kind only, more or less 

 intense it may be, but still always of one kind. If this 

 be so, each cochlear fibre or filament may be considered 

 as being pi'ovided with two end-organs : one, peripheral, 

 in the organ of Corti, capable of being set in motion by 



