SENSE OF HEARING. 891 



pressure of the atmosphere is greater than that within the tympanum 

 the membrane of the tympanum would be pressed inward. On the 

 other hand, if the pressure within the tympanum be greater than that of 

 the atmosphere the membrane would be pressed outward, and in both 

 cases the movements of the tympanic membrane would be restricted and 

 sound to such an extent interfered with. The equilibrium is maintained 



through the opening of the Eustachian tube in the act of swallowing, 



an act which is performed not only during eating, but also at frequent 

 intervals to carry away the secreted saliva. At other times the Eusta- 

 chian tube is closed, and by this means the conduction of sound-waves 

 downward into the pharynx or the conduction upward of sound-waves 

 from the voice is rendered impossible. 



The sound-waves are thus conducted from the tympanic membrane 

 to the chain of ossicles, and thence by the vibration of the stapes to the 

 oval window. The membrane of the fenestra ovalis is, as a consequence, 

 set into transverse vibration, and these vibrations are directly communi- 

 cated to the fluid of the labyrinth. The lymph, like other fluids, is incom- 

 pressible, and if, therefore, the membrane of the oval window be pressed 

 inward there must be a corresponding exit at some other point of the 

 apparatus. This counter-opening is found in the round window, and as 

 soon, therefore, as the stapes vibrates inward the membrane of the circu- 

 lar window vibrates outward, and the pressure upon the fluid of the 

 labyrinth is thus relieved while being set into vibrations corresponding 

 with those of the stapes. 



From the oval window the wave travels into the vestibule and from 

 there into the cochlea, and we must assume that it there throws the 

 membranous apparatus with its organ of Corti into vibration. The ves- 

 tibule is, however, divided by the two membranous sacs which it contains 

 into two portions, each containing fluid, the one in connection with the 

 oval window and the other with the round window ; so that, therefore, 

 we cannot imagine that the fluid in the vestibule is directly driven by 

 the vibrations of the stirrup-bone to the round window, for the scala 

 vestibuli of the cochlea, which is in connection with the oval window, 

 is shut off from the scala tympani by the membrane, and any wave, there- 

 fore, started by the vibrations of the stapes will pass rapidly up the 

 scala vestibuli, while it will also transfer its vibrations to the membran- 

 ous partition and thus throw the fluid of the scala tympani likewise into 

 vibration. 



In the cochlea the vibrations of the perilymph throw into vibration 

 the fibres of the basilar membrane and the organ of Corti, consisting of 

 the rods and the inner and outer hair-cells, whicli may be regarded as a 

 series of stretched strings, a portion of which may be thrown into sym- 

 pathetic vibration independently of the whole. 



