The mechanism of the ossicles of the ear and membrana tympani / by H. Helmholtz ; translated from the German, with the author's permission, by Albert H. Buck and Normand Smith.

  • Helmholtz, Hermann von, 1821-1894.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: The mechanism of the ossicles of the ear and membrana tympani / by H. Helmholtz ; translated from the German, with the author's permission, by Albert H. Buck and Normand Smith. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    tendon of the tensor tympani. As can be seen, the part a a, lies on the upper side of the mallco-incudal joint, while the line of junction of the two cogs c0c, lies lower down between the handle of the hammer and the long process of the anvil • the cog c, of the hammer lies on that side of the joint which is turned toward the membrana tympani; the cog of the anvil, on the median side. The handle of the hammer cannot therefore rotate inward without carrying the anvil with it • the limit of rotation outward is governed by the flexibility of the ligaments and cartilaginous covering of the articular surfaces. In the articular surfaces of both ossicles the point a0 ax lies at a greater distance from the axis than the border of the cog c„ ; in other words, that portion of the surfaces which is nearly plane is greater than the surfaces of the cogs, and consequently the entire surface of the joint is elliptical in shape, with the lunger axis running vertically. It should be stated, furthermore, that the apex of the cone-shaped articular surface (we have assumed the cone to be the type of this joint surface) does not come to a point, but is rounded off, as in a saddle. A conical surface, such as may be made by aid of Fig. 7 cannot be made to revolve upon its exact counterpart without loss of contact at some point; for when the two screw lines b0 ca and b, cx glide upon each other at the periphery of the joint, the central portion of the joint and the more level portion will separate each from the corresponding portion, leaving the two bones in contact only at the above-mentioned screw lines. Since the working space in such a motion is very small there- fore we may readily perceive that in a fresh specimen a com- pressible cartilage may quite fill it. This peculiar mechanism of the joint can be readily seen m dry specimens of the ossicles; one match may be fastened* with sealing-wax to the head of the hammer, just' above and in- the same direction with the processus Folianus, and another to the anvil, at the end of its processus brevis, and in the same direction with it; by using these matches as handles, and bringing the two articular surfaces together, the hammer may be rotated about its match as an axis, while the anvil is simply pressed lightly against it. If the hammer is rotated in the