168 HUMAN PHYSIOLOGY. 



The stapedius muscle emerges from the cavity of a pyramid of bone 

 projecting from the posterior wall of the tympanum; the tendon passes 

 forward and is inserted into the neck of the stapes bone posteriorly near its 

 point of articulation with the incus. 



The laxator tympani muscle, so-called, is now generally regarded as liga- 

 mentous in nature, and not muscular. 



The Eustachian tube, by means of which a free communication is 

 established between the middle ear and pharynx, is partly bony and partly 

 cartilaginous in structure. It measures about an inch and a half in length ; 

 commencing at its opening into the naso-pharynx it passes upward and out- 

 ward to the spine of the sphenoid bone, at which point it becomes some- 

 what 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 mastoid cells. 



The Function of the Ear, as a whole, is the reception and transmis- 

 sion of aerial vibrations to the terminal organs concealed within the in- 

 ternal ear and which are connected with the auditory nerve fibres. 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 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 

 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 each other with a certain rapidity. Noises are caused by the 

 impact of a series of irregular waves. 



All sound waves possess intensity, pitch, and quality. The intensity, or 

 loudness, of a sound depends upon the amplitude of the vibration, or the 

 extent of its excursion. The//&v 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 per second ; the pitch of the same note an octave higher is 

 caused by 512 vibrations per second. If the vibrations are too few per 

 second they fail to be perceived as a continuous sound ; the minimum 



