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site of the membrane are described, which, hardly audible in a normal 

 ear, are loud in an ear thus diseased. 



Thus a group of phenomena beckon to the inference, that this 

 deafness had so modified the acoustic properties of the drum, as both 

 to render all sonorous vibrations affecting the air within it by far 

 more audible than before, and all those entering the meatus audi- 

 torius externus as much less audible than before. What physical 

 cause can bring about these inverse effects ? 



1 . If the fenestra rotunda be the chief portal for sound, no change 

 at it could render one set of sounds more audible without doing so 

 for the other also. 



2. If sound be mainly conveyed to the labyrinth by undulatory 

 displacements of the membrana tympani, causing bodily oscillation 

 of the ossicles, the membrane could not be rendered more responsive 

 to aerial waves falling upon one side of it without becoming equally 

 so for those falling upon its other. 



3. If the fenestra rotunda chiefly afford passage to sound, and the 

 membrana tympani has acquired an abnormally high reflecting 

 power, repelling vibrations that would heretofore have escaped 

 through it from the drum back upon this fenestra, and those that 

 fall upon its outer surface back through the meatus, effects of an in- 

 verse kind do result. This hypothesis, therefore, cannot be rejected 

 without a careful consideration. 



Let us inquire, then, what influence the existence of a membrana 

 tympani would, under this supposition, exert on hearing. Sonorous 

 vibrations impressed upon the walls of the head, that is, of the ex- 

 ternal meatus, are heard more loudly when we anyhow cover this 

 canal so as to close it, as any cavity when closed resounds like an 

 open one of greater size (J. Miiller). In again testing this principle, 

 I have used various materials for closing the meatus, have plugged 

 the entrance, and laid the thing over it, and observe always that the 

 smallest orifice in the occluding body detracts from the resonance ; 

 which I know to occur in the confined air, and not in the parietes 

 of the canal, for my deafened ear was deaf to it. Such experiments, 

 however, do not evince that the membrane aids hearing by reso- 

 nance, but the contrary. Dealing with vibrations already existing 

 in the walls of the cavity insulating the air, they do not at all imi- 

 tate the case of vibrations passing into the tympanum through a 



