128 Mr. R. Moou on the Structure of the Human Ear, and on 



pressed home, and the membrane of the fenestra rotunda bulging 

 out into the tympanal cavity, it is evident that when a rare- 

 fied wave becomes incident upon the membrana tympani, the 

 latter will move outwards, drawing the stapes from the labyrinth, 

 the fluid in the latter following the stapes by reason of the 

 pressure of the air in the cavity of the tympanum on the mem- 

 brane of the cochlear fenestra, which would thus be driven 

 inwards. 



I think that the foregoing remarks will have made evident 

 what are the true relative functions of the two apertures from 

 the tympanal cavity into the labyrinth. So long as disturbance 

 was supposed to be transmitted along the chain of bones exactly 

 in the same manner as if they had constituted a rigid bar, with- 

 out producing in any degree that opening or shutting of the 

 labyrinth which the whole scheme of the mechanism proves is 

 possible, and was intended to be produced, the supposition that 

 a like transmission took place through the air in the tympanal 

 cavity was a perfectly natural and proper one. But if it be ad- 

 mitted that the stapes is so fitted to the vestibular aperture as 

 to admit of being pushed inwards and outwards — if the action of 

 a rarefied wave on the membrana tympani is to pull it out- 

 wards, while the action of the muscles of the ear is to pull it in- 

 wards — and if, as we have seen, none of these capacities or ten- 

 dencies can be carried into effect unless the action of the cochlear 

 membrane be such as we have described it, it is clear that the 

 action of the two fenestras must be opposite to each other — the 

 one tending to move in as the other tends to move out, and 

 vice versa, the two thus combining to produce that one effect 

 (to wit, the agitation of the fluid in the labyrinth) which is essen- 

 tial to the perception of sound. 



But although I consider the explanation above offered suffi- 

 cient, so far as relates to the action of the perfect ear, it is 

 evident that when the membrana tympani is destroyed, or, 

 being perfect, the ossicular connexion between it and the laby- 

 rinth is broken, the above reasoning ceases to be applicable ; 

 and yet in these latter cases a very considerable amount of audi- 

 tory power is frequently retained. 



I think, from what has preceded, we are entitled to assume 

 that it is the function of the muscles of the car to restore the 

 auditory apparatus to its normal position of equilibrium*; 

 whence it will follow, even where the membrana tympani is 



* In confirmation of this view, I cite the following passages from 

 Cruveilhier. 



" La base de l'etrier, est une plaque mince . . . dont la configura- 

 tion est exactement adaptee a celle de la fenetre ovale, qu'elle remplit par* 

 faitement, et dont on ne la retire qu'avec un ttger effort ; en sorte que 



