HEARING OR AUDITION. 121 



ties in which it lies, only smaller. Between it and the walls of the 

 bony labyrinth lies the fluid called the perilymph, and within it is 

 contained the eyidolyniph, and some small calcareous particles called 

 otolithes or ear-stones. The use of the membranous labyrinth is 

 probably to afford a more extended surface for the expansion of the 

 auditory nerve. The otolithes, by being thrown into vibration, pro- 

 bably increase the impression upon the nerve. 



The sonorous waves reach the labyrinth from the membrana 

 tympani in three distinct ways. 1st. A portion of the vibrations 

 reflected from the walls of the tympanum reaches the external wall 

 of the vestibule immediately, this being at the same time the inter- 

 nal wall of the tympanum. 2d. Other vibrations are thrown directly 

 upon the fenestra rotunda, and in this way reach the cochlea. 3d. 

 A third set of vibrations travel along the chain of bones immediately 

 from the membrana tympani to the labyrinth. This is the most 

 powerful of any. The fluid of the labyrinth may also be thrown 

 into vibration, by undulations transmitted through the bones of the 

 head. 



A single impulse communicated in any of the above ways is suffi- 

 cient to excite the momentary sensation of sound, but a number of 

 them in rapid succession is essential to the production of a musical 

 tone. The acuteness or depth of the tone depends upon the rapidity 

 with which the impulses succeed each other. 



The acuteness of hearing varies very much in different individuals, 

 and its power may be very much increased by practice. A part of 

 this increase is due to the greater attention which its fainter indica- 

 *tions receive ; a part also to the increased use of the organ. A 

 want of musical ear is an encephalic defect, and not a deficiency of 

 the organ. 



The power of appreciating the direction of sounds is for the most 

 part acquired by habit. In some instances we are assisted by the 

 relative intensity of the sensations communicated to the two ears 

 respectively. The idea of distance is another acquired perception 

 depending principally upon the loudness ovfaintness of the sound. 



The sensation of sound often lasts longer than the exciting cause 

 of it. It is upon this circumstance that the continuity of a musical 

 tone depends ; a fresh impulse succeeding before the impression of 

 the first has disappeared. 



The subjective phenomena of hearing generally result from some 

 affection of the brain, or that part of it in which the auditory nerve 

 is implanted. Ringing sounds, or buzzing in the ears, are the most 

 common, and are indicative of either redundancy or deficiency of 

 blood in the brain. They may also be caused by some disturbance 

 of the local nutrition of the brain. 



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