CHAP. XVJII.] THE SEMICIRCULAR CANALS. 99 



ledge of the direction of sounds, a view which is also advocated by 

 Wheatstone. The latter philosopher conceives that we distinguish 

 best the direction of those sounds which are sufficiently intense to 

 affect the bones of the head, and that it is from the vibrations which 

 are transmitted through these bones that our perception of the direc- 

 tion is obtained. The three canals being situate in planes at right 

 angles with each other are affected by the sounds transmitted through 

 the bones of the head with different degrees of intensity, according 

 to the direction in which the sound is transmitted : for instance, if 

 the sound be transmitted in the plane of any one canal, the nervous 

 matter in that canal will be more strongly acted on than that in 

 either of the other two; or if it be transmitted in the plane interme- 

 diate between the planes of this canal and the adjacent one, the 

 relative intensity with which those two canals will be affected will 

 depend upon the direction of the intermediate plane. The direction 

 suggested to the mind will correspond with the position of the canal 

 upon which the strongest impression has been made. 



There are remarkable differences in the range of the sense of hear- 

 ing in different individuals, analogous to the differences in the power 

 of vision with regard to colours. Some persons are insensible to 

 certain sounds, which are familiar to other ears, as some are unable 

 to see particular colours. The limits of audition in different indivi- 

 duals may partly depend on the condition of the auditory nerve, 

 and partly upon the size of the membrana tympani. It is probable 

 that animals with very large membranse tympani can hear much 

 graver sounds than man. The artificial tension of the membrana 

 tympani alluded to at a former page is capable of inducing insensi- 

 bility to sounds of a grave character, as Dr. Wollaston has shown. 

 The ordinary range of human hearing comprised between the lowest 

 notes of the organ, and the highest known cry of insects, includes, 

 according to Wollaston, more than nine octaves, the whole of which 

 are distinctly perceptible by most ears. Dr. Wollaston has, how- 

 ever, related some cases in which the range was much less, and 

 limited as regards the perception of high notes; in one individual, 

 the sense of hearing terminated at a note four octaves above the 

 middle E of the pianoforte; this note he appeared to hear rather 

 imperfectly, but the r above it was inaudible, although his hearing 

 in other respects was as perfect as that of ordinary ears; another 

 case was that of a lady who could never hear the chirping of the 

 field-cricket; and in a third case, the limit was such that the chirp- 

 ing of the common house-sparrow could not be heard. Dr. Wol- 

 laston supposes that inability to hear the piercing squeak of a bat 



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