THE ORGANS OF SPECIAL SENSE. 65 



organ of Corti, a group of specially modified epithelial 

 cells in the cochlea of the membranous labyrinth, which 

 is very important in transmitting the impulses to the 

 brain. The nerve also breaks up into very small 

 branches and is distributed practically throughout the 

 wall of the labyrinth. 



The sensation of hearing is the result of impulses trans- 

 mitted to the auditory nerve and so conveyed to the 

 auditory center in the brain. It is caused by sound 

 waves which travel through the air from their point 

 of origin and enter the external ear. This collects and 

 selects the waves of sound and helps one to a certain 

 extent to determine the direction from which the sound 

 comes. As they pass through the external meatus the 

 sound waves are collected into a comparatively small 

 area for transmission to the middle ear, where, by means 

 of the drum, they set in vibration the chain of ossicles. 

 Through these the vibrations are in turn transmitted 

 to the oval window, being intensified in the process. 

 Here again they are taken up by the perilymph, from 

 which they pass through the wall of the membranous 

 labyrinth to the endolymph, affecting the epithelial 

 lining of the labyrinth in such a way that the impulses 

 are transmitted to the auditory nerve, more particu- 

 larly in the vestibule, from which the vibrations 

 enter the cochlea. They also affect the cells of the or- 

 gan of Corti in like manner as they pass from the peri- 

 lymph to the endolymph. The membrane that covers 

 the fenestra rotunda or round window relaxes and 

 expands as the vibrations strike it, thus serving to 

 eliminate the shock of impact. 



Musical sounds are caused by rhythmical or regu- 

 larly repeated vibrations, while irregular vibrations 

 give rise to noises. In musical sounds loudness is deter- 

 mined by the height or amplitude of the vibrations, 

 pitch by the length of the wave, and quality by the 

 number of so-called partial tones. A sensation of sound 

 cannot be produced by less than 30 vibrations a second 



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