252 HUMAN PHYSIOLOGY. 



bony and partly cartilaginous in structure. It measures about 1^/2 

 inches in length ; commencing at its opening into the nasopharynx, 

 it passes upward and outward to the spine of the sphenoid bone, at 

 which point it becomes somewhat contracted; the tube then dilates 

 as it passes backward into the middle-ear cavity ; it is lined by 

 mucous membrane, which is continued into the middle ear and mas- 

 toid cells. 



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

 mission 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 vibrations 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 atmo- 

 sphere 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 condensations 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 in- 

 tensity, 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 affect 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 vibrations a second. If the 

 vibrations are too few a second, they fail to be perceived as a con- 

 tinuous sound ; the minimum number of vibrations capable of pro- 

 ducing 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 vibrations a second. In the ascent of the musical 

 scales there is, therefore, a gradual increase in the number of vibra- 



