Mr. J. Jago on the Functions of the Tympanum. SOl 



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 parictes 

 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 throuyh a 

 medium, — the membrane. As no substance can be applied over the 

 meatus, however it be done, which does not hinder our hearing of 

 external sounds just as much as it occasions resonance of parietal 

 ones, the membrane on this supposition must in some degree or 

 other be a positive detriment to the auditory function. Besides, were 

 hearing aided by resonance within the drum, a patent Eustachian 

 tube by allowing vibrations to disperse must impair hearing, which 

 I know not to be the case. Again, if we assume the membrane to 

 but slightly arrest the transition of sound from the outer to the 

 tympanic air, to be, in short, an unavoidable impediment to hearing, 

 fidfilling some non-acoustic purpose, the loss of it would not prove 

 at once, as it does, a serious detriment to hearing rather than some 

 benefit. I may append too, that were it but a trifling obstacle, the 

 group of sounds occurring within it, so described, should be auo-- 

 mented by resonance in the external meatus, on its outlet beino- 

 stopped ; yet I can detect nothing of the sort. Further, I squeezed 

 a plug of chewed brown paper, and one of dry paper, firmly into the 

 bottom of the meaUis of the healthy ear, against the membrane, co- 

 vered the membrane with a stratum of wax, and filled the meatus 

 with water ; but in not one of these experiments were the said group 

 of sounds rendered louder. So that it aj)pcars that the application to 

 the membrane of even a highly reflecting surface fails to intercept 

 and cause to return intra-tympanic sounds, which can only be be- 

 cause the membrane is difficult for such sound to pass through. But 

 if the membrane highly resists the transition of aerial vibrations, it 

 (the fenestra rotunda being the chief portal) is a seiious detriment 

 to hearing. Hence this fenestra cannot be of this acoustic conse- 

 quence. And we must have recourse to the only other theory which 

 suggests itself, which is — 



4. That the membrane and ossicles form the essential path for 

 sonorous vibrations, ivhich traverse it by the mode of condensation 

 and rarefaction ; that aerial ones impinyincj upon the outer surface 

 of the membrane easily impress themselves upon its substance, and pass 

 into the ossicles, tvhilst the inner surface presents a yreat obstacle 



