582 HANDBOOK OF PHYSIOLOGY. 



Sensibility of the Auditory Nerve. Any elastic body, e. #., air, a 

 membrane, or a string performing a certain number of regular vibrations 

 in the second, gives rise to what is termed a musical sound or tone. We 

 must, however, distinguish between a musical sound and a mere noise; 

 the latter being due to irregular vibrations. 



Sounds. 



Qualities of Musical Sounds. Musical sounds are distinguished from 

 each other by three qualities. 1. Strength or intensity, which is due to 

 the amplitude or length of the vibrations. 2. Pitch, which depends 

 upon the number of vibrations in a second. 3. Quality, Color, or Timbre. 

 It is by this property that we distinguish the same note sounded on two 

 instruments, e. g., a piano and a flute. It has been proved by Helm- 

 holtz to depend on the number of secondary notes, termed harmonics? 

 which are present with the predominating or fundamental tone. 



It would appear that two impulses, which are equivalent to four sin- 

 gle or half vibrations, are sufficient to produce a definite note, audible as 

 such through the auditory nerve. The note produced by the shocks of 

 the teeth of a revolving wheel, at regular intervals upon a solid body, is 

 still heard when the teeth of the wheel are removed in succession, until 

 two only are left; the sound produced by the impulse of these two teeth, 

 has still the same definite value in the scale of music. 



The maximum and minimum of the intervals of successive impulses 

 still appreciable through the auditory nerve as determinate sounds, have 

 been determined by M. Savart. If their intensity is sufficiently great, 

 sounds are still audible which result from the succession of 48,000 half 

 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 

 opposite 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 

 interruption of the sound. The gradual cessation of the sensation of 

 sound renders it difficult, however, to determine its exact duration be- 

 yond that of the impression of the sonorous impulses. 



Direction. 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 



