On Modes of Demonstrating the Action of the Membrana Tympani

Abstract

THE movements of the bones of the tympanum in connection with sound-waves were first observed and their excursions measured by Buck (“Archiv. of Ophthalmology and Otology,” vol. i., 1870), and more recently by Dr. Charles H. Burnett of Philadelphia, as recorded in the same Journal for 1872. The method followed in these researches was to expose the bones and membrane by chiselling away a portion of the wall of the tympanum, sprinkling on the chain of ossicles a little powdered amylum, so as to secure bright vibrating points, throwing light into the cavity by means of a condenser, and observing, with a microscope of low power, the excursions of the vibrating points when sound was conducted into the external ear. Various interesting deductions were drawn from these experiments as regards the amplitude of the excursions of vibrating points on different portions of the conducting mechanism, and as to the effect of variations of the fluid pressure within the labyrinth on the extent of movement. In the last number of the same journal an interesting paper appears from Dr. Clarence J. Blake of Boston on “the use of the Membrana Tympani as a Phonautograph and Logograph,” accompanied by a plate. Dr. Blake's method consists of exposing the membrane and chain of bones, and fixing a light style (made “by splitting long wheat straw, scraping the inner cortical substance away, and separating single fibres”) to the membrane. This style is caused to record its movements on a plate of smoked glass which is “carried smoothly and at a uniform speed in a direction at a right angle to the direction of the excursion of the style.”... “The membrana tympani being set in vibration, and the carriage [bearing the smoked glass] drawn by its weight, moving at right angles to the excursions of the style, a wave-line, corresponding to the character and pitch of the musical tone sounded into the ear, is traced on the smoked glass.”