THE SENSE OF HEARING. 161 



the r.apidity with which the nervous current is propagated, and the shortness of 

 the route through a nerve of hearing to the brain, aud theuee again through the 

 nerves of motion to the tensor muscle, that part of the waves of sound which 

 takes undiminished effect will, in most cases, be excessively small iii comparison 

 ■with that whoso intensity is broken by the obtunding apparatus. In the eye it 

 is no bettf^r. Here also the path of the reflex action, by which the protective 

 apparatus is brought into service, lies through the nerves of the sense itself: the 

 strong excitement of the optic nerve by dazzling rays of light is transferred in 

 the brain to tlie motive nerves of the .so-called iris, and determines this to a 

 contraction of the orifice by which the rays are admitted. So much concorning 

 this corrective apparatus to the ear. Respecting the operation and uses of the 

 small aud short muscle called the stapedius, which springs from the wall of the 

 cavity opposite to the stapes and is attached by its tendon to the neck of the 

 latter, there prevails much greater uncertainty ; we shall, therefore, spare our 

 readers a discussion of conjectures which are untenable, or, at least, destitute of 

 proof. Possibly this muscle also is destined to the purpose of lessening the 

 effect of intense air-waves. 



Before we direct our attention to the labyrinth and the apparatus of the nerves 

 of hearing, we should briefly consider some of the relations borne by the cavity 

 of the tympanum and the duct called the Eustachian tube, by which it is con- 

 nected with the cavity of the mouth. Although not the slightest doubt can exist 

 as to the immediate signification of these parts, and there are no grounds for 

 supposing this explanation of their existence inconclusive, yet much ingenuity 

 and fruitless pains have be^n employed in vindicating for them far otiier and the 

 most singular purposes. Obviously, the cavity and the tube must exist in the 

 ear as the air-space and air-hole in an -ordinary drum. The membrane of the 

 tympanum could not vibrate were it not surrounded on both sides by air; 

 the small bones of the ear would be immovable if, instead of being in air, they 

 were fixed in some mass of solids. The air in the cavity, since it would be 

 compressed hy the inward curvatures of the membrane, would oppose resistance 

 to the movements of the latter if it did not find in the tube a ready issue when 

 the membrane is driven inwards, and ready entrance when the membrane again 

 swings outwards. This is so simple and clear that all further explanation Avould 

 seem unnecessary. We shall, however, briefly notice some of the hypotheses 

 advanced on this subject, and hope to show, even to the uninitiated reader, how 

 untenable they are. It has been surmised, for instance, that the waves of sound 

 which the vibrating membrane of the tympanum must communicate as well to 

 the air contained in the cavity as to the ossicles cannot properly be supposed to 

 be lost, but must also be available for the use of the organ of hearing, just as if 

 it were a criminal prodigality of nature if every one of these waves which im- 

 part their effect with such constant and beneficent impartiality to all that can 

 vibrate did not reach the auditory nerves of man or beast. The ossicles of the 

 tympanum are so apposite, certain, and sensitive an apparatus for the propaga- 

 tion of these sound-producing undulations, that there needs no second route for 

 conducting them to the nerve; but were such an one really necessary, nature 

 could have chosen none more unsuitable than that through the air of the tym- 

 panic cavity and the Eustachian tube. For these self-propagating waves of the 

 air contained in the cavity, there is but one conceivable manner in which they 

 could reach the nerves; no one would think of asserting that they could be 

 transmitted to the osseous wall of the labyrinth, from this to the enclosed fluid, 

 and from this to the nerves; but the membrane which has been mentioned above 

 as closing iha fenestra rotunda, the opposed opening of the labyrinth, and which 

 is pressed on one side by the air of the cavity, on the other by the fluid of 

 the labyrinth, has been seized on as furnishing tlie rec[ui3ite mechanism. It has 

 been contended that the air- waves of the cavity must throw this membrane into 

 vibration, just as the waves in the external meatus of the ear do the tympanic 

 membrane, and that the vibrations of the former in the fluid of the labyrinth 



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