264 OF SENSATION, AND THE ORGANS OF THE SENSES. 



so as to distend it from within, or Ipy 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 membrana tympani is subjected, the more acute will be its fun- 

 damental 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 fundamental 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 fundamental note is very low, and it is capable of 

 repeating a much greater variety 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 under- 

 stand the influence of the tensor tympani muscle, in modifying the tension of 

 the membrane, and thus causing it to vibrate in reciprocation to sounds having 

 a great variety of fundamental 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 opera- 

 tion by a reflex action ; and it is by no means improbable that one of its func- 

 tions 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. 



357. 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 Miiller, it appears that it does not increase 

 the intensity 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 dulness 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 canal serves for the conduction of the speaker's voice to his 

 ears ; but this is certainly not the case in any considerable degree ; for, when 

 the Eustachian tubes are obstructed by disease, the patient hears his own 

 voice well, though other sounds are indistinct ; and it is easily shown, that its 

 transmission is chiefly accomplished in other ways. The common idea is, 

 that it serves the same purpose with the hole in an ordinary drum ; the effect 

 of which js generally supposed to be, the removal of the impediment to the 

 vibrations* of the membrane, that would be offered by the complete enclosure 

 of the air within. It does not appear, however, that any such impediment is 

 really offered ; and the effect of the hole in the drum seems rather to be the 



* 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 produced 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 intermediate lines of rest, forming a variety of curious 

 figures, which are known as the nodal lines. 



