1018 FUNCTIONS OF THE VESTIBULE. [Book hi. 



The changes in the endol3 r mph which give rise to the impulses 

 affecting equilibrium, namely a simple change in the amount of 

 pressure or a simple shifting of position, a simple flowing, are 

 so different from the changes, the rapid repeated vibrations pro- 

 duced by sound, that it seems permissible to conceive of the 

 cristse and maculse reacting differently towards the two agencies, 

 and so giving rise to impulses of different natures, one auditory 

 and the other not. The value of the hairs of the cristre and 

 ampullae as vibrating organs have probably been exaggerated ; 

 among other things the medium in which they move, the some- 

 what viscid endolymph, is unfavourable to vibrations ; and the 

 otoliths and otoconia, if they have any relations to vibrations, 

 probably serve as ' dampers.' Still the hairs probably do vibrate 

 as the endolymph vibrates ; and we may imagine that the changes 

 in the hair-cells and hence in the nerve fibres and hence in the 

 brain are different when the hairs are thrown into series of 

 vibrations from what they are when the hairs are gently pressed 

 or gently moved. Further we may reflect that in ourselves the 

 sensations gained by the cochlea are so dominant, that we may 

 be at the same time receiving sensations through the vestibule 

 without being aware that we are doing so ; these latter may, 

 further, be of a different nature from the former, and the vesti- 

 bular hearing of a fish may be something very different from 

 our, mainly, cochlear hearing. At any rate we may hesitate 

 to accept the view that no auditory impulses travel along the 

 vestibular nerve. But if we do thus hear by means so to speak 

 of a double organ, then the origin and nature and effects of 

 auditory impulses must be still more complex and difficult 

 than appears from the study of the cochlea alone, perplexing 

 as they even then seem. 



The difficulties attending an adequate conception of the 

 nature and origin of auditory impulses are further- increased by 

 the following observation. Two tuning-forks, not quite in uni- 

 son, produce 'beats' when they are sounding together; the beats 

 are due to the influence exerted by one set of waves on the other 

 set (§ 627). But it is stated that, if the two forks be listened to, 

 one with one ear and the other with the other, precautions being 

 taken so that the vibrations reaching the one auditory nerve by 

 the one ear, cannot, by conduction through the bones of the 

 head or otherwise, also gain access to the other auditory nerve, 

 the beats are still heard. This observation, unless there be 

 some hidden fallacy in it, seems to shew that the beats arise in 

 the brain itself, that the impulses travelling along the auditory 

 nerve so far resemble in their nature and character, the waves 

 of sound giving rise to them, that the two sets of impulses 

 along the two nerves meeting in the brain give rise to beats, 

 just as do the two sets of waves meeting in the air. If we fur- 

 ther couple with this conclusion the view referred to above 



