720 TEXT-BOOK OF PHYSIOLOGY 



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 sensations of sounds which physiologically result from the stimula- 

 tion of the auditory apparatus are characterized by loudness, pitch and qual- 

 ity or timbre and are the result of the intensity or vigor, frequency, and form 

 of the atmospheric vibrations. 



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 n 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. 



The quality of a sound depends on the form of the vibration. It is this 

 Feature which gives rise to those differences in sensations which permit one to 

 distinguish one instrument from another when both are emitting the same 

 note. ^ The form of the sound-wave in any given instance is the resultant of a 

 combination of a fundamental vibration and certain secondary vibrations of 

 subdivisions of the vibrating body. These secondary vibrations give rise 

 to what is known as overtones. By their union with and modification of the 

 fundamental vibration there is produced a special form of vibration which 

 gives rise not to a simple but a composite sensation. It is for this reason that 

 the same note of the piano, the violin, and the human voice varies in quality 



The Function of the Pinna and External Auditory Canal. In those 

 animals possessing movable ears the pinna plays an important part in the 

 collection of sound-waves. In man the pinna plays but a subordinate part 

 in this process. Nevertheless an individual with defective hearing may have 

 the perception of sound increased by placing the pinna at an angle of oo 

 degrees to the side of the head or by placing the hand behind it. The external 

 auditory canal transmits the sonorous vibrations to the tympanic membrane. 

 From the pbliquity of this canal it has been supposed that the vibrations, 



r passing the concha, undergo a series of reflections on their way to the 

 tympanic membrane, which, owing to its inclination, would be struck by 

 them in a much more effective manner. 



The Function of the Tympanic Membrane. The function of the 

 tympanic membrane is the reception of the atmospheric vibrations which 



