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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    The shape of the last-named articular surface is usually de- scribed as resembling a saddle; it must be remarked, however, that, unlike the saddle, not only the convex sides come together to form a ridge that is almost sharp, but also the concave; and the union of the two forms a continuous and almost flat surface on either side of the ridge. In order to gain a clear idea of the mechanism of this joint, it is better, I believe, to make use of a different comparison than that of the surface of a saddle. Ir is, in fact, like the joint used in certain watch-keys, where the handle cannot be turned in one direction without carrying the steel shell with it, while in the opposite direction it meets with onlv slight resistance. As in the watch-key, so here the joint between hammer and anvil admits of a slight rotation about an axis drawn transversely through the head of the hammer toward the end of the short process of the anvil; a pair of cogs oppose the rotation of the manubrium inward, but it can be driven outward without carrying the anvil with it. If such a joint had to be constructed of metal, we should make use of screw-surfaces. A hollow cylinder, cut as A is represented in Fig. 6, and upon which the piece B (marked in dotted lines) fits, would represent the normal shape of such a joint. It is clear that A and B, revolving in the direction of their respective arrows, must necessarily strike against each other with their cogs a and 5/ hence, in this direction their rotation is limited. In the opposite direction, however, their rotation is free, and is accompanied by a slowly increasing sepa- ration of the two cylinders. The mechanic, in making such a joint, usually employs a hollow cylinder, because in the neighbor- hood of the axis, were the cylinder solid, the screw surface would incline upward at a pretty steep angle, like the inner border of a winding staircase, and hence would be difficult to execute. The articular ends of the bones, which are covered with a layer of elastic yielding cartilage, filling out all the irregularities of 3 i ; Fig o.