194 HAND-BOOK OF PHYSIOLOGY. 



vibrations, or 24,000 impulses in a second; and this, probably, is not the 

 extreme limit in acuteness of sounds perceptible by the ear. For the op- 

 posite extreme, he has succeeded in rendering sounds audible which were 

 produced by only fourteen or eighteen half vibrations, or seven or eight 

 impulses in a second; and sounds still deeper might probably be heard, 

 if the individual impulses could be sufficiently prolonged. 



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 impression which 

 causes it; for a removal of a tooth from the wheel produced no interrup- 

 tion of the sound. The gradual cessation of the sensation of sound ren- 

 ders it difficult, however, to determine its exact duration beyond that of 

 the impression of the sonorous impulses. 



Direction of Sounds. The power of perceiving the direction of 

 sounds is not a faculty of the sense of hearing itself, but is an act of the 

 mind judging on experience previously acquired. From the modifications 

 which the sensation of sound undergoes according to the direction in 

 which the sound reaches us, the mind infers the position of the sounding 

 body. The only true guide for this inference is the more intense action 

 of the sound upon one than upon the other ear. But even here there is 

 room for much deception, by the influence of reflexion or resonance, and 

 by 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 

 originate 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 of Sounds. The distance of the source of sounds is not 

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

 sound 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 in- 

 tensity 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 same time pretend, 

 themselves, to hear the sounds as coming from thence. 



The effect of the action of sonorous undulations upon the nerve of 



