692 HANDBOOK OF PHYSIOLOGY. 



the propagation of sound from a distance, without loss of intensity, 

 through curved conducting tubes filled with air. By means of such tubes, 

 or of solid conductors, which convey the sonorous vibrations from their 

 source to a distant resonant body, sounds may be made to appear to orig- 

 inate in a new situation. The direction of sound may also be judged 

 of by means of one ear only; the position of the ear and head being 

 varied, so that the sonorous undulations at one moment fall upon the 

 ear in a perpendicular direction, at another moment obliquely. But 

 when neither of these circumstances can guide us in distinguishing the 

 direction of sound, as when it falls equally upon both ears, its source 

 being, for example, either directly in front or behind us, it becomes 

 impossible to determine whence the sound comes. 



Distance. The distance of the source of sounds is not recognized by 

 the sense itself, but is inferred from their intensity. The sense itself is 

 always seated but in one place, namely, in our ear; but it is interpreted 

 as coming from an exterior soniferous body. When the intensity of the. 

 voice is modified in imitation of the effect of distance, it excites the 

 idea of its originating at a distance. Ventriloquists take advantage of 

 the difficulty with which the direction of sound is recognized, and also 

 the influence of the imagination over our judgment, when they direct 

 their voice in a certain direction, and at the sanje time pretend, them- 

 selves, to hear the sounds as coming from thence. 



Intensity. By removing one or several teeth from the toothed wheel 

 the fact has been demonstrated that in the case of the auditory nerve^ 

 as in that of the optic nerve, the sensation continues longer than the im- 

 pression which causes it; for a removal of a tooth from the wheel pro- 

 duced no interruption of the sound. The gradual cessation of the sen- 

 sation of sound renders it difficult, however, to determine its exact 

 duration beyond that of the impression of the sonorous impulses. * 



So we see that the effect of the action of sonorous undulations upon 

 the nerve of hearing, endures somewhat longer than the period during 

 which the undulations are passing through the ear. If, however, the 

 impressions of the same sound be very long continued, or constantly 

 repeated for a long time, then the sensation produced may continue for 

 a very long time, more than twelve or twenty-four hours even, after the 

 original cause of the sound has ceased. 



Binaural Sensations. Corresponding to the double vision of the- 

 same object with the two eyes, is the double hearing with the two ears; 

 and analogous to the double vision with one eye, dependent on unequal 

 refraction, is the double hearing of a single sound with one ear, owing 

 to the sound coming to the ear through media of unequal conducting; 

 power. The first kind of double hearing is very rare ; instances of it, 

 however, have been recorded. The second kind which depends on the 



