226 HUMAN PHYSIOLOGY. 



The function of the ear, as a whole, is the reception and transmission of 

 aerial vibrations to the terminal organs concealed within the internal ear, 

 and which are connected with the auditory nerve-fibers. The excitation of 

 these end organs caused by the impact of the vibration arouses in the auditory 

 nerve impulses which are then transmitted to the brain, where the hearing 

 process takes place. In order to appreciate the functions of the individual 

 parts of the ear, a few of the characteristics of sound waves must be kept in 

 mind. 



Sound Waves. All sounds are caused by vibrations in the atmosphere 

 which have been communicated to it by vibrating elastic bodies, such as 

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

 to-and-fro movement of its particles, resulting in a series of alternate conden- 

 sations and rarefactions, which are propagated in all directions. A complete 

 oscillation of a particle of air forward and backward constitutes a sound wave. 

 Musical sounds are caused by a succession of regular waves, which follow one 

 another with a certain rapidity. Noises are caused by the impact of a series 

 of irregular waves. 



All -sound waves possess intensity, pitch, and equality. The intensity, or 

 loudness, of a sound depends upon the amplitude of its vibrations or on the 

 extent of its excursion. The pitch depends upon the number of vibrations 

 which effect the auditory nerve in a second of time; the pitch of the note C, the 

 first below the leger line of the musical scale, is caused by 256 vibrations a 

 second; the pitch of the same note an octave higher is caused by 512 vibra- 

 tions a second. If the vibrations are too few a second, they fail to be per- 

 ceived as a continuous sound; the minimum number of vibrations capable of 

 producing a sound has been fixed at sixteen a second; the highest pitched 

 musical note capable of being heard has been shown to be due to 38,000 vibra- 

 tions a second. In the ascent of the musical scales there is, therefore, a grad- 

 ual increase in the number of vibrations and a gradual increase in the pitch 

 of the sounds. Between the two extreme limits lies the range of audibility, 

 which embraces eleven octaves, of which seven are employed in the musical 

 scale. 



The quality of sound depends upon a combination of the fundamental 

 vibration with certain secondary vibrations of subdivisions of the vibrating 

 body. These so-called over-tones vary in intensity and pitch, and by modify- 

 ing the form of the primary wave produce that which is termed the quality 

 of sound. 



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

 possessing movable ears the pinna plays an important part in the collection of 

 sound waves. In man, in whom the capability of moving the pinna has 



