. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. THE SENSE OF HEARING. 155 action may take effect, the reservoir is provided with a second opening h, which is closed with an ehistic membi'one, {membrana tympani secondaria.) In pro- portion as the stirrup-plate is pressed deeper into its opening, the yielding fluid drives the membrane 7i before it, bending and stretching it outwards. We shall see hereafter that, in nature, this reservoir of fluid consists of a sac-shaped space in which is
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. THE SENSE OF HEARING. 155 action may take effect, the reservoir is provided with a second opening h, which is closed with an ehistic membi'one, {membrana tympani secondaria.) In pro- portion as the stirrup-plate is pressed deeper into its opening, the yielding fluid drives the membrane 7i before it, bending and stretching it outwards. We shall see hereafter that, in nature, this reservoir of fluid consists of a sac-shaped space in which is the opening for the stirrup or stapes, and of a long spirally convolved canal; the other opening h lies at the extreme end of this canal, so that the movement of the yielding fluid, propagated from particle to particle, must tra- verse the whole canal before it arrives at the elastic membrane. This extensive circuit is of the utmost importance, because the greatest part of the excitable nerves have their extremities within this canal, by all of which extremities there- fore the liquid waves must pass before they reach the membrane. If the con- densed wave, whose effect we have been considering, is succeeded by a rarified wave, all the movements will of course be reversed. The tympanum recoils and oscillates outwards, thus turning the rod a b on the axis d so that the lower por- tion a c moves outwards with the tympanum while its upper portion c b moves inwards; the portion c a draws a f after it, and this in turn draws the stirrup g in a corresponding degree out of the opening of the reservoir L ; the fluid pur- sues the withdrawing stirrup-plate and the membrane h becomes level or may even be drawn inwards. If condensed and rarified air-waves now follow one another in rapid succession, as in the case of our vibrating string, the little lever a h turns with equal velocity on its axis and moves the foot plate of the stirrup in its opening up and down, causing in the reservoir ad
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