SENSE OF HEARING USES'OF EUSTACHIAN TUBE. 787 



continuous in like manner with the mucous membrane lining the tympanic 

 cavity, and also composed of dermoid tissue and epithelium ; and a middle 

 layer, which, according to Mr. Toynbee, 1 may be separated into two distinct 

 lumiurc whose fibres run in contrary directions, those of the external layer 

 (which is the stronger of the two) radiating from the malleus towards the 

 peripheral ring to which they are attached, whilst those of the internal are 

 annular. The fibres of which these laminae are composed appear to be ten- 

 dinous; and the membrane as a whole is almost inextensible. Mr. Toynbee 

 points out the existence of a tubular ligament, inclosing the tendon of the 

 tensor tympani muscle ; and considers that the membrane is maintained by 

 this ligament in a state of moderate tension, the assistance of the muscle 

 being only required to augment this. The function of the Membrana Tym- 

 pani seems obviously to be the reception of sonorous undulations from the 

 air, in such a manner that it may be thrown by them into a reciprocal vibra- 

 tion, which is communicated to the chain of bones, and, as is shown in the 

 accompanying little diagram, through them to the membrane of the fenestra 

 ovalis. Helmholtz has shown that the curvature of the membrana tympani, 

 whilst it diminishes the amplitude of the vibrations, increases 

 their force. In its usual state, this membrane is scarcely on FIG. 279. 



the stretch ; and this is found by experiment to be, for a small 

 membrane, the best condition for the propagation of ordinary 

 undulations. This is easily rendered sensible in one's own 

 person ; for an increased tension may be given to the mem- 

 brana tympani, 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; and in either case, the hearing is immediately found 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 

 readily understood, by referring to the laws of Acoustics already mentioned. 2 

 The greater the tension to which the membrana 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 fundamental note proper to the tense 

 membrane, the more distinctly will it be heard. On the other hand, when 

 the membrane is in its naturally relaxed 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 vibra- 

 tion, which are separated by lines of rest ; and every one of these recipro- 

 cates the sound, 3 at the same time rendering it more intense by multiplica- 



1 Philosophical Transactions, 1851. 



2 See the Experiments of Schapringer (Wien. Akad. Sitz.-bpr., Bd. Ixii, 1870), who 

 possessed the faculty of voluntarily contracting his ten a or tympani. 



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

 cavity; for 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 different 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, which are 

 known as the nodal lines, forming a variety of curious figures. See also Bonders on 

 the sound colors of the vowels, in Bonders and Berlius, Archiv f. d. Holland. Bei- 

 trage zur Natur und Heilkunde, Bd. iii, p. 446. By means of a modification of Scott's 

 form of Konig's phonautograph, Bonders obtained very beautiful pictures or series 



