THE SENSE OF HEARING. 



767 



When the sound-waves vibrate the cells of Corti they make the 

 terminal filaments of the cochlear nerve vibrate, because they are in 

 Nation with the cells of Corti. The analysis of sounds takes place 

 the brain. 



Binaural Audition. The hearing of a single sound with both 

 ears may be due to habit or to the connection in the nerve-centers 

 of the fibers connected with both ears. Undoubtedly binaural audi- 

 tion facilitates our knowledge of the direction of sound, since each 

 jar has its own axis and direction. 



Fig. 317. Schema of the Semi-circular Canals, the Posterior Part of 

 the Skull Removed. (HEDON, after EWALD.) 



In the plane 1 lies the anterior canal. In the plane 2 the external canal. 

 In the plane 3 the posterior canal. 



We combine binauricular audition, just as we judge of the relief 

 of objects in binocular vision (stereoscopic vision), to determine the 

 direction of sounds. The tympanic membrane may be looked upon 

 as an organ of pressure-sense by variations of air-pressure, even 

 rhen sound-sensations are not produced. 



A blind man has been able to state correctly that he has passed 

 a fence, and whether it be of solid board or of open picket. It may 

 be stated that the membrana tympani is the outward organ of pressure- 

 sense, by which we know more or less the position of objects inde- 

 mdent of the sensations of sight and hearing. The air is in endless 

 lovement, and its waves, striking against various objects, must be 

 impinged against the drumhead with an intensity dependent upon 

 their position and the physical properties of the bodies reflecting it. 



