Ossicular motion was measured visually with stroboscopic illumination. Tonal stimulation ranged from 30 to 10 000 Hz. Up to 130 dB SPL (sound-pressure level), the motion of the stapes is predominantly pistonlike, and its displacement amplitude is linearly related to sound pressure. At frequencies under 3000 Itz, the ossicles move as one rigid body; at higher frequencies, the stapes and incus displacements lag behind the malleus displacement, which suggests that the incudo-malleolar joint flexes. From measurements of stapes displacement at known sound pressures, we calculate a transfer characteristic for the middle ear with the tympanic cavities open. The effects of closing the tympanic cavities on the transfer characteristic were determined from measurements in which the electric response recorded from the round window was used as an indicator of middle-ear output. By combining these data, we obtain a transfer characteristic for the middle ear with the tympanic cavities intact. An attempt is made to compare middle-ear characteristics of cat and man. * A portion of this work was submitted by J. J. G., Jr., as a master's thesis to the Department of Electrical Engineering, MIT, June 1964. • During most of the period in which this work was done, J. J. G., Jr., was a National Science Foundation Cooperative Fellow. Since June 1966, he has been a Fannie and John Hertz Foundation Fellow. done on cats, we chose the cat as our experimental animal. Since we used barbiturate anesthesia, which apparently eliminated activity of the middle-ear muscles in cats, • our results are applicable only to conditions in which the middle-ear muscles are in a relaxed state. Measurements of ossicular motion in human cadavers made with stroboscopic illumination were reported almost a century ago by Mach and Kessel (1874). Measurements of a middle-ear transfer characteristic were reported by von B•k•sy 2 in 1942, based on measurements of round-window volume displacement from temporal bones of cadavers. More recently, techniques for measuring acoustic impedance at the drum membrane s have improved our knowledge of the middle ear in living humans but have also raised some doubts about the usefulness of measurements made on cadavers (Zwislocki and Feldman, 1963). The first measurements of middle-ear transfer characteristics in living animals • For recently published evidence, see Simmons (1959), Carmel and Starr (1963), Baust and Berlucchi (1964). •' See von B•k•sy (1960), pp. 429-437. 3 For a summary of the results of this work see Zwislocki (1962).

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Tones were delivered directly to the stapes in anesthetized cats after removal of the tympanic membrane, malleus, and incus. Measurements were made of the complex amplitudes of the sound pressure on the stapes PS, stapes velocity VS, and sound pressure in the vestibule PV. From these data, acoustic impedance of the stapes and cochlea ZSC delta equal to PS/US, and of the cochlea alone ZC delta equal PV/US were computed (US delta equal to volume velocity of the stapes = VS X area of the stapes footplate). Some measurements were made on modified preparations in which (1) holes were drilled into the vestibule and scala tympani, (2) the basal end of the basilar membrane was destroyed, (3) cochlear fluid was removed, or (4) static pressure was applied to the stapes. For frequencies between 0.5 and 5 kHz, ZSC approximately equal to ZC; this impedance is primarily resistive ([ZC] approximately equal to 1.2 X 10(6) dyn-s/cm5) and is determined by the basilar membrane and cochlear fluids. For frequencies below 0.3 kHz, [ZSC] greater than [ZC] and ZSC is primarily determined by the stiffness of the annular ligament; drying of the ligament or changes in the static pressure difference across the footplate can produce large changes in [ZSC]. For frequencies below 30 Hz, ZC is apparently controlled by the stiffness of the round-window membrane. All of the results can be represented by an network of eight lumped elements in which some of the elements can be associated with specific anatomical structures. Computations indicate that for the cat the sound pressure at the input to the cochlea at behavioral threshold is constant between 1 and 8 kHz, but increases as frequency is decreased below 1 kHz. Apparently, mechanisms within the chochlea (or more centrally) have an important influence on the frequency dependence of behavioral threshold at low frequencies.