CHAP, in.] HEARING, SMELL, AND TASTE. 563 



since noise is at best but confused music, the cochlea must be a 

 means of appreciating noises as well as sounds. But this would 

 leave nothing whatever for the rest of the labyrinth to do in 

 respect to the appreciation of sound save so far as the difference in 

 structure between the hair-cells of Corti, with their short thick 

 rods, and the hair-bearing structures in the maculae and cristse 

 with their thin delicate hairs, may possibly indicate a difference of 

 function, the latter being more susceptible to the irregular vibra- 

 tions of noises. That the vestibule and semicircular canals are 

 however concerned in hearing is shewn by its being the only audi- 

 tory organ in the ichthyopsida, unless we suppose that in the 

 higher vertebrates its function has been wholly transferred to the 

 cochlea. That the semicircular canals may have duties apart from 

 hearing we shall shew later on. 



Concerning the function of the other parts of the internal ear 

 we know very little. The otoliths have been supposed to intensify 

 the vibrations of the endolymph ; but since apparently they are 

 lodged in a quantity of mucus it is probable that they really act as 

 dampers. A similar damping action has been suggested for the 

 membrane of Corti (membrana tectoria) overhanging the fibres and 

 hair-cells; and some writers have supposed that muscular fibres 

 present in the planum semilunare may by tightening the basilar 

 membrane serve as a sort of accommodation mechanism. 



It must however be borne in mind that even making the fullest 

 allowance for the assistance afforded us by the organ of Corti, the 

 appreciation of any sound is ultimately a mental act The analysis 

 of the vibrations by the fibres of Corti or the basilar membrane is 

 simply preliminary to a synthesis of the sensory impulses so gene- 

 rated into a complex sensation. We do not receive a distinct 

 series of specific auditory impulses resulting in a specific sensation 

 for every possible variation in the wave-length of sonorous vibra- 

 tions any more than we receive a distinct series of specific visual 

 impulses for every possible wave-length of luminous vibrations. 

 In each case we have probably a number of primary sensations, 

 from the various mingling of which, in different proportions, our 

 varied complex sensations arise; the difference between the eye 

 and the ear being that whereas in the former the number of 

 primary sensations appears to be limited to three or at least to six, 

 in the latter, thanks to the organ of Corti, the number is very 

 large ; what the exact number is we cannot at present tell. Our 

 appreciation of a sound is at bottom an appreciation of the com- 

 bined 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. 



Whatever be the explanation of the manner in which our 

 distinct auditory sensations arise, the range and precision of our 

 appreciation of musical sounds is very great. Vibrations with a 



362 



