Interior of the ear. There is external to the head a wide-mouthed tube, or ear-trumpet (a), for catching and concentrating the waves of sound. It is movable in many beings, so that they can direct it to the place from which the sound comes. The sound concentrated at the bottom of the ear-tube falls upon a membrane stretched across the channel, like the parchment of an ordinary drum, over the space called the tympanum, or drum of the ear (b), and causes the membrane to vibrate. That its motion may be free, the air contained within the drum has free communication with the external air by the open passage (f), called the eustachian tube, leading to the back of the mouth. A degree of deafness ensues when this tube is obstructed, as in a cold; and a crack, or sudden noise, with immediate return of natural hearing, is, generally experienced when, in the effort of sneezing or otherwise, the obstruction is removed. The vibrations of the membrane of the drum are conveyed further inwards, through the cavity of the drum, by a chain of four bones (not here represented on account of their minuteness), reaching from the centre of the membrane to the oval door or window, leading into the labyrinth (e). The labyrinth, or complex inner compartment of the ear, over which the nerve of hearing is spread as a lining, is full of watery fluid; and, therefore, by the law of fluid pressure, when the force of the moving membrane of the drum, acting through the chain of bones, is made to compress the water, the pressure is felt instantly over the whole cavity. The labyrinth consists of the vestibule (e), the three semicircular canals (c), imbedded in the hard bone, and a winding cavity, called the cochlea (d), like that of a snail-shell, in which fibres, stretched across like harp-strings, constitute the lyra.