254 THE SPECIAL SENSES 



Eustachian tube. The cavity of the middle ear is 

 connected with the pharynx by a passageway called the 

 Eustachian tube, through which it is filled with air 

 under the same pressure as the outside air. Strain of 

 the delicate tympanum by air pressure from the out- 

 side is thus avoided, and it is free to move with 

 the slightest air vibration. Sometimes, however, the 

 Eustachian tube is closed by the swelling of the mucous 

 membrane of the pharynx. The air supply of the middle 

 ear is thus cut off and the air which it already has is 

 gradually absorbed. As a result, the pressure of air 

 upon the inner surface of the tympanum becomes less 

 than that upon its outer surface ; the tympanum is conse- 

 quently pushed inward and loses some of its sensitiveness 

 to air vibrations. This is the reason why with a head cold 

 we may become temporarily more or less deaf. If the 

 Eustachian tubes are permanently closed for any reason, 

 chronic deafness results. 



Inner ear. The bony case in which the inner 

 ear lies is coiled like a snail-shell and is therefore called 

 the cochlea or bony labyrinth. The cochlea contains a 

 small amount of fluid, the perilymph, and floating in 

 this fluid, an inner case, shaped like itself but made 

 of membrane, the membranous labyrinth. The en- 

 trance to the membranous labyrinth for vibrations is 

 through the oval window against which, as we have 

 seen, the stirrup lies. The vibrations of the tympanum 

 are thus transmitted through the ear bones, through 

 the oval window, through the perilymph to the mem- 

 branous labyrinth. 



Receiving apparatus. The membranous labyrinth is 

 itself filled with a thin watery fluid, the endolymph. 

 Stretched across it lies the receptive membrane for the vi- 

 brations, the basilar membrane, to which the vibrations of 



