THE SENSE OF HEARING. 257 



The endolymph occupies the interior of the utricle, saccule, and 

 membranous canals, and bathes the structures in the interior of the 

 membranous cochlea throughout its entire extent. 



The auditory nerve at the bottom of the internal auditory meatus 

 divides into 



1. A vestibular branch, which is distributed to the utricle and to the 

 semicircular canals. 



2. A cochlear branch, which passes into the central axis at its base 

 and ascends to its apex ; as it ascends, fibers are given off, which 

 pass between the plates of the osseous lamina, to be ultimately 

 connected with the organ of Corti. 



The function of the semicircular canals appears to be to assist 

 in maintaining the equilibrium of the body ; destruction of the 

 vertical canal is followed by an oscillation of the head upward and 

 downward; destruction of the horizontal canal is followed by oscil- 

 lations from left to right. When the canals are injured on both 

 sides, the animal loses the power of maintaining equilibrium upon 

 making muscular movements. 



Function of the Cochlea. It is regarded as possessing the power 

 of appreciating the quality of pitch and the shades of different 

 musical tones. The elements of the organ of Corti are analogous, in 

 some respects, to a musical instrument, and are supposed, by Helm- 

 holtz, to be tuned so as to vibrate in unison with the different tones 

 conveyed to the internal ear. 



Summary. The waves of sound are gathered together by the 

 pinna and external auditory meatus, and conveyed to the membrana 

 tympani. This membrane, made tense or lax by the action of the 

 tensor tympani and laxator tympani muscles, is enabled to receive 

 sound waves of either high or low pitch. The vibrations are con- 

 ducted across the middle ear by a chain of bones to the foramen 

 ovale, and by the column of air of the tympanum to the foramen 

 rotundum, which is closed by the second membrana tympani, the 

 pressure of the air in the tympanum being regulated by the Eu- 

 stachian tube. 



The internal ear finally receives the vibrations, which excite vi- 

 brations successively in the perilymph, the walls of the membranous 

 labyrinth, the endolymph, and, lastly, the terminal filaments of the 

 auditory nerve, by which they are conveyed to the brain. 



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