Chap, rv.] HEARING. 1021 



§ 633. In one important respect the parallel between hearing 

 and sight fails. When we see an object, the rays of light com- 

 ing from the object excite a particular part of the retinal expan- 

 sion ; and our appreciation of the position which that object 

 holds in space is based on our power of "localizing" retinal 

 changes. The terminal expansion of the auditory nerve how- 

 ever has no such definite relations to the positions in space of 

 objects from whence sounds are proceeding ; we have no evi- 

 dence that any particular part either of the organ of Corti or 

 of the maculae is alone or specially affected by sounds coming 

 from a particular quarter, and the evidence that sounds affect 

 the three cristse differently according to the direction of the 

 sound is at least doubtful. Hence we possess no "auditory 

 field" which can be directly compared with the "visual field;" 

 and our conclusions as to the direction in which the sounds 

 which reach our ears have travelled, our judgments as to the 

 position in space of bodies exciting auditory sensations are 

 formed in an indirect manner. 



The vast majority of the sounds which we hear reach the 

 auditory epithelium by way of the tympanic membrane and chain 

 of ossicles ; even the sounds which are conducted to the ear 

 through the bones and hard parts of the head pass to a large 

 extent by this way (§ 616) ; in normal hearing the auditory sen- 

 sations which are generated by vibrations transmitted directly 

 through the bony walls of the labyrinth to the perilymph are 

 probably insignificant. Now it is only in relation to these latter 

 that the disposition in space of the three semicircular canals 

 can possibly have any meaning ; the vibrations reaching the peri- 

 lymph by way of the tympanic membrane, whatever their orig- 

 inal direction, have all the same direction when they enter at 

 the fenestra ovalis, and fall in the same way upon the three 

 semicircular canals. We may therefore conclude that the posi- 

 tion in space of the three canals in question has nothing to do 

 with our ordinary judgments as to the direction of sounds. In 

 forming those judgments we are assisted mainly by two things. 



In the first place a peculiar character of the outwardness 

 which we attribute to our usual auditory sensations, that by 

 which we judge the sound to arise not only outside the in- 

 ternal ear but outside our whole body, seems, in some way, 

 largely dependent on the vibrations which cause the sensation 

 having travelled along the external auditory passage. If the 

 two passages be filled with fluid the hearer refers the sounds 

 which he hears, in spite of their starting at some distance off, 

 not to the external world outside himself, but to the inside of 

 his own head; the sounds appear to him to come, not it may be 

 remarked from the internal ear or any part of it, but from the 

 roof of the mouth, or the top of the skull or the back of the 

 head. So also if the ear-pieces of a binaural stethoscope be 



