644 TEXT-BOOK OF PHYSIOLOGY. 



THE PHYSIOLOGY OF AUDITION. 



The general function of the ear is the reception and transmission 

 of atmospheric vibrations from the concha to the percipient elements 

 the hair cells of the organ of Corti. The vibratory excitation of 

 these end-organs thus caused, is the basis of auditory perceptions. 

 The atmospheric vibrations are collected by the pinna and concha, 

 conveyed by the auditory canal to the tympanic membrane, trans- 

 mitted by the chain of bones to the labyrinth to pass successively 

 through the perilymph, the membranous walls, the endolymph, to 

 the hair cells. The nerve impulses generated by these vibrations are 

 then transmitted by the cochlear nerve to the auditory centers of the 

 cerebrum, where the sensations of sound are evoked. In order to 

 appreciate the function of the individual structures concerned in this 

 general function there must be kept in mind a few of the character- 

 istics of atmospheric vibrations. 



Atmospheric Vibrations. The vibrations of the atmosphere 

 which are the objective causes of the sensations of sound are com- 

 municated to it by the vibrations of elastic bodies such as tuning- 

 forks, rods, strings, membranes, etc. These produce in the air a 

 to-and-fro movement of its particles, resulting in a succession of 

 alternate condensations and rarefactions which are propagated in 

 all directions. The impact of a rhythmic succession of such con- 

 densations on the ear gives rise to musical sounds; the impact of an 

 arrhythmic or irregular succession gives rise to noises. 



If a writing point attached to a tuning-fork in vibration be placed 

 in contact with a traveling recording surface, each vibration will be 

 recorded in the form of a wave. For this reason atmospheric vibra- 

 tions are generally spoken of as sound-waves. A line drawn hori- 

 zontally through such a curve indicates the position of rest of the 

 fork; the extent of the curve on each side of this line indicates the 

 excursion of the fork or the amplitude of its movement. 



The sounds which physiologically result from the impact and 

 transmission of the effects of sound-waves, possess intensity, pitch, and 

 quality or tone. 



The intensity or loudness of a sound depends on the amplitude 

 of the vibration which causes it. The greater the amplitude or 

 swing of the vibrating body, the greater is the energy with which it 

 strikes the ear. 



The pitch of a sound depends on the number of vibrations which 

 strike the ear in a unit of time a second. The greater the number, 

 the higher the pitch. Thus while the pitch of the sound caused by 

 the note C, on the first leger line below, of the music scale, corre- 

 sponds to 256 vibrations, the pitch of the sound caused by the note C 

 an octave above, corresponds to 512 vibrations. The lowest rate of 



