SENSE OF HEARING. 431 



of detail, into the action of the different parts of this apparatus ; and it will be 

 better to commence with that of the Internal Ear, the accessory organs being 

 afterwards considered. 



563. The object of the Membrana Tympani is evidently to receive the 

 sonorous undulations from the air, in such a manner as to be thrown by them 

 into a recriprocal vibration, which is to be communicated to the chain of bones. 

 This membrane is, in its usual state, rather lax than tense; and this laxity is 

 found by experiment to be, for a small membrane, the best condition for the 

 propagation of ordinary sounds. This is easily rendered sensible in one's 

 own person ; for an increased tension may be given to the membrana tym- 

 pani, either by holding the breath and forcing air into the Eustachian tube, so 

 as to distend it from within, or by exhausting the cavity, so as to cause the 

 external air to make increased pressure upon it. In either case the hearing is 

 found immediately to become indistinct. It is observed, however, that grave 

 and acute sounds are not equally affected by this action ; for the experimenter 

 renders himself deaf to grave sounds, whilst acute sounds are heard even more 

 distinctly than before. This fact is easily understood by referring to the laws 

 of Acoustics already mentioned. The greater the tension to which the mem- 

 brana tympani is subjected, the more acute will be its fundamental tone; and 

 as no proper reciprocation can take place in it, to any sound lower than its 

 fundamental tone, its power of repeating perfectly the vibrations proper to the 

 deeper notes will diminish. The nearer a sound approaches to the funda- 

 mental note proper to the tense membrane, the more distinctly will it be heard. 

 On the other hand, when the membrane is in its natural lax condition, its fun- 

 damental note is very low, and it is capable of repeating a much greater vari- 

 ety of sounds ; for, when it receives undulations of a higher tone, than those 

 to which the whole membrane would reciprocate, it divides itself into distinct 

 segments of vibration, which are separated by lines of rest; and every one of 

 these reciprocates the sound;* at the same time rendering it more intense by 

 multiplication. These facts enable us to understand the influence of the ten- 

 sor tympani muscle, in modifying the tension of the membrane, and thus caus- 

 ing it to vibrate in reciprocation to sounds having a great variety of funda- 

 mental notes. Moreover, the fact that some persons are deaf to grave sounds, 

 whilst they readily hear the more acute, is thus accounted for. The tensor 

 tympani, like the iris, is probably excited to operation by a reflex action ; and 

 it is by no means improbable that one of its functions may be, to prevent the 

 internal ear from being too violently affected by loud sounds, by putting the 

 membrana tympani into such a state of tension, as not readily to reciprocate 

 them. 



564. The uses of the Tympanic cavity are very obvious. One of its pur- 

 poses is, to render the vibrations of the membrane quite free ; and the other, 

 to isolate the chain of bones, in such a manner as to prevent their vibrations 

 from being weakened, by diffusion through the surrounding solid parts. As to 

 the objects of the Eustachian tube, however, opinions have been much divided. 

 From the experiments of Muller it appears, that it does not increase the intens- 

 ity of sound, but that it prevents a certain degree of dulness, which would 

 attend it, if the cavity of the tympanum were completely closed ; of this dul- 

 ness we are conscious, when any tumefaction of the fauces causes an occlusion 

 of the extremity of the tube. It has been supposed that, among other uses, 



This is very easily proved by experiments on a membrane stretched over a resonant 

 cavity ; if light sand be strewed upon it, and a strong musical tone be protluced in its vicinity, 

 the membrane will immediately be set in vibration, not as a whole (unless its fundamental note 

 be in unison with that sounded), but in distinct segments, of which every one reciprocates the 

 sound ; from the vibrating parts, the sand will be violently thrown off; but it will settle on the in- 

 termediate lines of rest, forming a variety of curious figures, which are known as the rcorfa/lines. 



