THE SENSE OF HEARING. 683 



THE PHYSIOLOGY OF HEARING. 



The general function of the ear is the reception and transmission of at- 

 mospheric 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 vibra- 

 tions are collected by the pinna and concha, conveyed by the auditory canal 

 to the tympanic membrane, transmitted by the chain of bones to the laby- 

 rinth to pass successively through the perilymph, the membranous walls, and 

 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 gen- 

 eral function there must be kept in mind a few of the characteristics of 

 atmospheric vibrations. 



Atmospheric Vibrations. The vibrations of the atmosphere which are 

 the objective causes of the sensations of sound are communicated 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, re- 

 sulting in a succession of alternate condensations and rarefactions which are 

 propagated in all directions. The impact of a rhythmic succession of such 

 condensations 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 con- 

 tact with a traveling recording surface, each vibration will be recorded in the 

 form of a wave. For this reason atmospheric vibrations are generally 

 spoken of as sound-waves. A line drawn horizontally 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 transmis- 

 sion of the effects of sound-waves, possess intensity, pitch, and quality or 

 timbre. 



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 vibrat- 

 ing 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 the G clef, of the music scale, corresponds to 256 vibrations, 

 the pitch of the sound caused by the note C an octave above, corresponds to 

 512 vibrations. The lowest rate of vibration which can produce a distinct 

 sound varies in different individuals from 14 to 18; the highest rate varies 

 from 35,000 to 40,000 per second. Between these two extremes lies the range 

 of audibility, which embraces about u octaves. Vibrations less than 14 per 

 second cannot be perceived as a continuous sound; vibrations beyond 

 40,000 also fail to be so perceived. In the ascent of the music scale from the 

 lowest to the highest regions there is a gradual increase in the vibration 

 frequency. 



