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

Date:
1873
Catalogue details

Licence: Public Domain Mark

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

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

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    to which it is attached, but its centre or navel is strongly drawn inward by the handle of the hammer, with which it is united; for this reason the membrane lias the shape of a funnel whose point or end corresponds to the tip of the handle of the hammer, and whose meridian lines are convexed toward the hollow of the funnel. In order to represent this form of the membrana tympani, a point of great importance in the mechanics of the conduction of sound, I took a cast with stearin of the upper wall of the external auditory canal and of the outer surface of the membrane, after having first removed the lower wall of the canal, without, however, disturbing any of the connections of the $ membrana tympani. Its outlines are \ represented inFig. 2, just as I copied them in the camera clara; a b is Fi„ 2 ' the upper wall of the external au- ditory canal, b c the vertical outline of the membrana tympani. From the figure it is very clear that the radii drawn on the surface of the membrana tympani are convexed outward toward the external auditory canal. At the same time, it can be seen that, as a result of this drawing in of the navel, the upper half of the membrane is made to lie in almost the same direction as the upper Avail of the canal, Avhile the lower half stands almost at a right angle with the axis of this canal. This last circumstance is of importance in the examination of the ear with the reflector, inasmuch as this perpendicular portion of the membrana tympani, which is situated, as a rule, just below the tip of the manubrium, reflects back through the external audi- tory passage the light that is thrown in upon it, and thus gi\ es rise to the triangular “ bright spot.” _ - The outer surface of the membrana tympani, which is cov- ered with an epithelial layer, the continuation of the horny epi- dermis of the skin of the external auditory passage, owes its pro- perty of reflecting light to the fat Avhich it contains. In a very fresh specimen of the ear, drops of water can be seen running oil from this fatty surface as from oiled paper. The convexity of the meridians of the membrana tympani is least at that meridian in which the handle of the hammer lies.