The surgical diseases of the ear / by Prof. von Troltsch. The mechanism of the ossicles and the membrana tympani / by Prof. Helmholtz ; translated from the German by James Hinton.

  • Tröltsch, Anton Friedrich, Freiherr von, 1829-1890.
Date:
1874
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Licence: Public Domain Mark

Credit: The surgical diseases of the ear / by Prof. von Troltsch. The mechanism of the ossicles and the membrana tympani / by Prof. Helmholtz ; translated from the German by James Hinton. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    the motions and the method of attachment of the ossicles, and have found them confirmed.' § 7. The mechanics of the nemhrana ti/mpani. The membrana tympani is to be considered as a tense membrane, but it differs from those membranes that have hitherto been investigated and employed in acoustics in this respect, that it is curved. Its tension is conditioned by the handle of the maUeus, which draws it inwards, and which is itself maintained in position by its ligaments and by the elasticity of the tensor tympani. Were the radial fibres of the membrane alone in existence and not joined with transverse fibres they would extend in straight lines. In fact they do not do this, but assume a curved form, convex on the side of the meatus, whence it is to be inferred that they are drawn to each other by the ring fibres, and that these latter are also put on the stretch. In fact, in the quiescent membrane there is no other force that can maintain the radial fibres in their curved form besides the tension of the ring fibres. On the impact of sound vibrations, the pressure of the air at one time acts against the convex surface of the membrane, at another against the concave, according as it is alternately greater or less in the meatus than it is in the tympanum. In each case it acts in a direction perpendicular to the surface of the membrane—thus perpendicularly also to the arching of the radial fibres wliich it tends alternately to increase and diminish. As the curves which the radial fibres of the membrane form are pretty shallow, there arises^ through this arrangement, as will be explained directly, the same mechanical action as if the pressure of the air acted on the end of a very long lever arm, whilst the point of the malleus handle forms the extremity of a very short lever arm. A relatively considerable displacement of the surface of the membrane in the direction of the pressure of the air necessitates a proportionately small displacement of the malleus point and vice versa. ]?rom this it follows further, according to the known general laws of virtual velocity, that a comparatively small amount of air 1 Copies of this model are constructed by the assistants at the Physiological Institute of this place, the Messrs. Sittel,