1.62 



ON THE SENSES. 



would be as capable of creating waves for the excitation of the nerves as the 

 movements of the stapes iu the fenestra oralis. In itself this last position is not to 

 be controverted; but it is to be inquired, first, whether the membrane is really 

 throAvn into sensible vibration ; and secondly, whether its oscillations can take 

 effect upon the nerves simultaneously with those propagated through the ossicles 

 of the tympanum. We do not hesitate to answer both questions in the negative. 

 To be thrown into vibration by air- waves, a membrane must receive their impact, 

 as far as possible, in a perpendicular direction. But this membrane of the fenes- 

 tra rotunda lies as unfavorably as possible in regard to waves proceeding from 

 the tympanum, inasmuch as it is turned away from the tympanum and towards 

 the Eustachian tube, so that the waves can as little strike it directly as, in the 

 dark, one could strike a disk whose edge instead of its flat surface was presented. 



Now, that it is not inaccessible to the waves is certainly true, since its surface 

 fronts directly on the air chamber; but its position would much sooner suggest 

 the idea that it is, as far as possible, sheltered from the effect of these waves 

 than that it is designed for their reception and transmission. But even if we 

 admit that it is, iu a slight degree, movable by the waves of the tympanic 

 cavity, it is not difficult to show that these movements must be overborne and 

 annulled by the opposed movements simultaneously imparted to it by the fluid 

 waves proceeding from the stapes. When an external wave of condensation 

 drives the membrane of the tympanum inwards, the stapes is driven deeper into 

 the fenestra ovalis and the fluid of the labyrinth is propelled before it; but this 

 propulsion is only possible, as has been shown, through the simultaneous out- 

 ward curvature of the opposite membrane, that, namely, of the fenestra rotunda. 

 Now, the same inward movement of the tympanum generates simultaneously a 

 condensed wave in the air of the tympanic cavity, and if this wave strikes the 

 membrane of the fenestra rotunda it cannot do otherwise than drive this mem- 

 brane inwards, and therefore in a direction opposite to the former. But as the 

 waves of the fluid in the labyrinth will, no doubt, act with more force on the 

 membrane than the air- waves of the cavity, the former must prevail and the 

 membrane curve outwards in spite of the latter force. Were the preponderance 

 reversed, it is manifest that any movement of the fluid by the stapes would be 

 impossible, and the whole system of ossicles would be brought to a stand. 

 Hence the sheltered and most unfavorable position of our membrane as regards 

 the reception of the air-waves would seem to have been wisely ordered, and to 

 be an indispensable condition for the propagation of the sound through the small 

 bones of the ear to the labyrinth. 



Other hypotheses respecting the intent of the cavity of the tympanum and 

 the Eustachian tube seem to have as little foundation as the above. It has been 

 thought by some that the tube was established for the purpose of conveying 

 waves of sound from the throat to the organ of hearing. When convinced that 

 this notion was untenable and that the tube could be of no avail in conducting 

 such sound-waves as reach the pharyngeal cavity from the outer air, the same 

 class of theorists fell upon the somewhat naive idea that the tube iu question 

 must be designed for sounds proceeding from the organs of speech in its close 

 vicinity; that it serves, in a word, for hearing our own voice. Now there are 

 many persons, certainly, well pleased to hear themselves speak, who would be 

 duly thankful to nature for her civility in establishing quite a special and private 

 route for the passage- of their own voice besides the common one into Avhich the 

 bellow of any casual bullock may wander. But, in the first place, nature has 

 created our voice rather for the ears of others than our own, and, except that 

 we might learn to speak, cared probably little about its being audible to our- 

 selves ; iu the second place, it may be shown that, setting aside any possible 

 concurrence with the sound-waves of bellowing bullocks, the common channel 

 of sound through the outer air and the outer ear to the tympanum not only suf- 

 fices fully for the perception of our own voice, but, under ordinary circumstances, 

 is the only channel for its admission. The Eustachian tube is not by any 



