900 OF SENSATION^ AND THE ORGANS OF THE SENSES. 



of the Membrana Tympani seems obviously to be the reception of sonorous un- 

 dulations from the air, in such a manner that it may be thrown by them into a 

 reciprocal vibration, which is communicated to the chain of bones. This mem- 

 brane, in its usual state, is scarcely on the stretch ; and this is found by experi- 

 ment to be, for a small membrane, the best condition for the propagation of 



Fig. 221. 



Membrana tympani from the outer (A) and from Ossicles of the left ear articulated, and seen from the 



the inner (B) sides : 1. Membrana tympani. 2. outside and below, m. Head of the malleus, below 

 Malleus. 3. Stapes. 4. Incus. which is the constriction, or neck. g. Processus gra- 



cilis, or long process, at the root of which is the shor t 

 process, h. Manubrium, or handle, sc. Short crus ; 

 and Ic, long crus of the incus. The body of this bone 

 is seen articulating with the malleus, and its long crus, 

 through the medium of the orbicular process, here 

 partly concealed, a, with the stapes, s. Base of the 

 stapes. Slagnified three diameters. From Arnold. 



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

 for an increased tension may be given to the membrana 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 found im- 

 mediately 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 dis- 

 tinctly 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 fundamental tone : and as no 

 proper reciprocation can take place in it, to any sound lower than its funda- 

 mental 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 relaxed condition, its funda- 

 mental 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 re- 

 ciprocates the sound, 1 at the same time rendering it more intense by multiplica- 



1 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 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. 



