FUNCTIONS OF THE VESTIBULE. [BOOK in. 



visual impulses for every possible wave-length of luminous vibra- 

 tions. In each case we have probably a number of primary 

 sensations, from the various mingling of which, in different pro- 

 portions, our varied complex sensations arise ; but there is this 

 difference between the eye and the ear that whereas in the former 

 the number of primary sensations appears to be limited to three 

 or at least to six, in the latter the number is probably very large ; 

 what the exact number is has not at present been even suggested. 

 Our appreciation of a sound is at bottom an appreciation of the 

 combined effect produced by the relative intensities to which the 

 primary auditory sensations are, with the help of the organ of 

 Corti, excited by the sound. The appreciation and the subjective 

 analysis of sounds is ultimately a psychical process ; and though 

 there are individual differences in the structural finish and physical 

 capabilities of the auditory epithelium as of other parts of the ear, 

 the differences in the psychical or at least cerebral powers of in- 

 dividuals are far greater; and when we speak of a musical ear we 

 really mean a musical mind or a musical brain. 



852. If the organ of Corti, as appears from the above, affords 

 the means by which we appreciate tones, it is evident that by it 

 also we must be able to estimate loudness, for the quality of a 

 musical sound is dependent on the intensity, as well as on the 

 pitch, of the partial tones in relation to the fundamental tone 

 and to each other. And since noise is but confused music, and 

 music more or less orderly noise, the cochlea must be a means 

 of appreciating noises as well as sounds. But if this be so, what 

 functions are fulfilled by the vestibular division of the labyrinth ? 



We have seen ( 643) reason to think that the fibres of the 

 vestibular nerve convey afferent impulses which are not auditory 

 in nature in the sense of serving as the basis of sensations of sound, 

 but which affect in a special way the movements of the body. We 

 then discussed the matter exclusively in reference to the cristse of 

 the semicircular canals ; but the view has been extended to the 

 macula? of the utricle and saccule. It is contended that while 

 the former are affected by movements involving a rotation of the 

 head, the latter are affected by movements in which the head 

 is carried in a straight line, either vertical as when the body rises 

 or falls, or horizontal as when the body progresses forwards or 

 backwards. It is maintained that the utricle and saccule thus 

 share with the ampullae in providing impulses by which we become 

 aware of the direction and amount of the movements we execute. 

 Indeed it is urged that the true function of the otoliths and 

 otoconia has no connection with hearing, that these act neither as 

 adjuvants to nor as dampers of an auditory mechanism, but by 

 impinging on the so-called auditory hairs in the movements of the 

 head g ; ve rise to the impulses of which we are speaking ; and 

 arguments in favour of this view are drawn not only from the 

 arrangement of the otoliths in vertebrates, but also from the 



