FUNCTIONS OF THE OUTER EAR. 8oi 



whose number of vibrations is some multiple of the number of vibrations of the 

 same, as the octave. When other tones act on them, they exhibit only inconsider- 

 able sympathetic vibration. If a membrane be stretched over a funnel or cylinder, 

 and if a nodule of sealing wax attached to a silk thread be made just to touch the 

 centre of the membrane, then the sealing wax remains nearly at rest when tones or 

 sounds are made in the neighbourhood ; as soon, however, as the fundamental or 

 proper tone of this arrangement is sounded, the nodule is propelled by the strong 

 vibrations of the membrane. 



If we apply this to the tympanic membrane, then it also should exhibit very 

 great vibrations when its own fundamental note is sounded, but only slight 

 vibrations when other tones are produced. This, however, would produce great 

 inequality in the audible sounds. There is an arrangement of the membrane where- 

 by this is prevented. (1) Great resistance is offered to the vibrations of the 

 tympanic membrane, Owing to its union with the auditory ossicles. These act as a 

 damping apparatus, which provides, as in damped membranes generally, that the 

 tympanic membrane shall not exhibit excessive sympathetic vibrations for its own 

 fundamental note. But the damping also makes the sympathetic vibrations less 

 for all the other tones. In this way, alt vibrations of the tympanic membrane are 

 modified ; especially, however, is the excessive vibration diminished during the 

 sounding of its fundamental tone. The membrane is at the same time rendered 

 more capable of responding to the vibrations of different wave-lengths. The damp- 

 ing also p> revent8 after-vibrations. (2) Corresponding to the small mass of the 

 tympanic membrane, its sympathetic vibrations must also be small. Nevertheless, 

 these slight elongations are quite sufficient to convey the sonorous movements to 

 the most delicate end-organs of the auditory nerve ; in fact, there are arrangements 

 in the tympanum which still further diminish the vibrations of the 

 tympanic membrane. 



As v. Helmholtz has shown, the strong sympathetic vibrations of the tym- 

 panic membrane are not completely set aside by this damping arrangement. 

 The painful sensations produced by some tones are, perhaps, due to the sympa- 

 thetic vibration of the membrana tympani. According to Kessel, certain parts 

 of the membrane vibrate to certain tones; the shortest radial fibres at the 

 upper part of the anterior and upper segment vibrate with the highest tones, 

 the longest fibres at the posterior segment with the deepest tones. At the 

 upper part of the posterior segment noises are transmitted. 



According to Fick, the tympanic membrane, besides possessing the property 

 of taking up all vibrations with nearly equal intensity, has also the properties 

 of a resonance apparatus; i.e., it causes a summation of the energy of suc- 

 cessive vibrations. This is due to the funnel-shape of the membrane, and to 

 the radial, rigid insertion of the handle of the malleus. Fig. 583. 



Pathological. Thickenings or inequalities of the tympanic membrane inter- Toynbee's artifi- 

 fere with the acuteness of hearing, owing to the diminished capacity for vibra- cial membrana 

 tion thereby produced. Holes in and loss of its substance act similarly. In tympani. 

 extensive destruction, an artificial tympanum is placed in the external meatus, 

 and its vibrations, to a certain extent, replace those of the lost membrane (Toynbee). [Fig. 

 583 shows an artificial tympanic membrane.] 



410. AUDITORY OSSICLES AND THEIR MUSCLES. The auditory 

 ossicles have a double function. (1) By means of the ; ' chain" which they form, 

 they transfer the vibrations of the tympanic membrane to the perilymph of the 

 labyrinth. (2) They also afford points of attachment for the muscles of the middle 

 ear, which can alter the tension of the membrana tympani, and the pressure on the 

 lymph of the labyrinth. 



Mechanism. The form and position of the ossicles are given in figures 584 and 

 585. They form a jointed chain which connects the tympanic membrane, M, by 

 means of the malleus, h, incus, a, and stapes, S, with the perilymph of the labyrinth. 

 The mode of movement of the ossicles is of special importance. The handle of 

 the malleus is firmly united to the fibres of the tympanic membrane (fig. 585, n). 



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