230 SPECIAL SENSES. 



pitched sounds become more intense ; and, in cases of volun- 

 tary tension, the limit of perception of high tones may be 

 greatly increased. The membrana tympani obeys the laws 

 of consonance and vibrates strongly under the influence of 

 sounds in unison or in harmony with its fundamental tone, 

 returning, in this way, not only the pitch, but the quality of 

 tones and combinations of tones in harmony. Destruction 

 of the membrane does not necessarily of itself destroy hear- 

 ing, or even the appreciation of tones, for the impressions 

 may be conducted to the cochlea by the chain of ossicles. 



The arrangement of the ossicles and muscles of the middle 

 ear is such that contraction of the tensor tympani renders the 

 articulations firm, tightens the little ligaments, and presses 

 the stapes against the liquid of the labyrinth, so that the 

 chain resembles, in its action, a solid and continuous bony 

 rod. By this arrangement, the sonorous vibrations are con- 

 ducted to the labyrinth with very little loss of intensity. 



The cavity of the tympanum is filled with air, communi- 

 cates with the mastoid cells, and with the pharynx by means 

 of the Eustachian tube ; and, by this means, the pressure of 

 air in its interior is regulated. The labyrinth, consisting of 

 the vestibule, semicircular canals, and cochlea, is filled with 

 liquid, and the different cavities communicate with each 

 other. The vibrations, repeated by the membrana tympani, 

 are conveyed by the chain of bones to the liquid of the 

 labyrinth, and by it to the terminal filaments of the auditory 

 nerves. 



The vestibule and semicircular canals seem to possess 

 much less importance in the appreciation of sound than the 

 cochlea. In the cochlea, throughout the entire extent of the 

 spiral canal, is the organ of Corti, presenting, among other 

 structures, about 8,700 rods, varying in length, called the 

 rods of Corti. But little is known of the anatomical rela- 

 tions between the auditory nerves and the organ of Corti ; 

 still, it is thought, as a matter of pure theory, that the rods 

 of Corti are tuned in unison with different tones, that they 



