(From audio, to hear). The sense of hearing, also called aco'e. By this sense we perceive the elastic tremors of the air; and to facilitate the function, the organ of hearing is made up of hard bones, and of elastic cartilages and membranes. The elastic air only receives sonorous tremors, and transfers them, as we see water transfer any impulse given to it. The sound is increased in air that is condensed, and is lost in a vessel emptied of its air. The body, which produces sound, ought to tremble or vibrate in the smallest of its particles. From such a tremor the contiguous air is beat into waves, whereby the parts of the air that lie outermost are compressed, and fly back again as soon as their elasticity conquers the impulse. The air consequently flies again towards the sonorous body, where it is now more loose and rarefied, to be again compressed by the impulsive power; and in the same manner the anterior and outer portion of air surrounding that which is impelled, is by the action of the latter compressed and removed further from the trembling body, yet so as to return again in its proper time by the force of elasticity, driving its contents to the tremulous body for the exciting a new wave. These impulsions of the air are required to succeed each other with a certain velocity; and in order to render them audible, they must not be fewer than thirty in a second of time. As these sonorous waves are more frequent in a given time, so much sharper is the sound heard, and the more strongly does it affect us, till we come to the most acute of audible, sounds, which have 7520 tremors in a second. Acute sounds are in general yielded from bodies that are hard, brittle, and violently shook or struck; but grave sounds from bodies of a contrary nature. Sounds, whether acute or grave, are carried through the air with a celerity equal to about 1038 Paris feet in a second; but a contrary wind retards their progress about one-twelfth of their velocity. Sounds, as arising from elastic tremors, are reflected from hard bodies in angles, equal to those of their incidence; but the same sound conveyed to the open air, and dilating through an immense sphere, grows proportionably weaker; but if it pass through a tube in a cylindrical shape, it is increased; therefore, the sonorous waves of the elastic air being driven into the cartilaginous funnel of the ear, are repelled and collected together by alternate reflections from its elastic sides into the cavity of the concha, from whence it proceeds through the auditory passage, with a force so much stronger as the surface of the outer ear is larger than the section of the auditory passage, through which the same force is continued entire forward, and increased by new reflections, excited from the percussion of the elastic cartilages and hard bones, so as to mix imperceptibly with the primitive sound. At the bottom of the auditory passage is the membrane of the drum of the ear, called membrana tympani; sometimes by the barbarous terms myringa, myrinx. It is a thin, transparent, flattish pellicle, the edge of which is round, and strongly fixed in the circular groove which divides the bony meatus of the external ear from the tympanum or barrel: this membrane consists of several plates, one of which is dry, rattling, splendid, and pellucid. It is very tense, easily put into a tremulous motion; and upon it the sonorous waves or modulations of external air strike, move the small bones fixed to it, and it proves a means of sound being readily conveyed to the common sensorium. This membrane is stretched over a cavity in the os petrosum, called the tympanum or drum, which consists of several cavities. In the tympanum, which is of an irregular oblong figure, are the bones of hearing, lodged in its hollow part, between the pars petrosa and squamosa of the temporal bones. The cavities of the tympanum are, the opening of the mastoid cells; that of the Eustachian tube, the canalis semi petrosus, half bony canal, the fenestra ova/is, and rotunda. Within the tympanum are suspended the bones of hearing moveably; the first of which is the malleus, or hammer, whose handle is fixed to the membrane of the drum; and at one end to the second bone, called the incus, or anvil, which it resembles in shape, to which it conveys the tremors impressed upon the membrane: the incus rests on the os orbiculare, a small round bone, and this upon the stapes or stirrup; the stapes and the air of the tympanum press the auditory nerve, whence the sense of sound is conveyed to the common sensory. When, by the force of external sounds, the membrane of the drum is forced too much inward, it is probably supported by air which passes from the mouth through the Eustachian tube into the inner car. The importance of the cochlea of the ear, in order to the conveyance of the sound, is very considerable. For a more minute information, consult Mailer's Physiology, and the ingenious observations of Dr. Shebbeare on this subject, in his Theory and Practice of Physic, and the article Sonus.