That the vestibule is the essential or fundamental part of the organ of hearing, is sufficiently proved by its constancy, other parts gradually disappearing as we descend the animal scale, and by its central position in the ears of the higher animals. The use of the otoconia or ear-powder is to strengthen the sonorous undulations, and to communi cate to the membranous vestibule and ampulla', and to their nerves, stronger impulses than the perilymph alone could impart. The action of otoliths or ear-stones, such as occur in osseous fishes, must be still more decided, and is well illustrated by the follow ing experiment of Camper: Fill a bladder with water, and place a pebble in it. The slightest impulse communicated to the bladder disturbs the pebble, which consequently produces a greater impression on the hand supporting the bladder than the water alone could do.
Nothing certain is known regarding the functions of the semicircular canals, but their constant existence and number* in the vertebrated animals indicate their impor tance; and in most cases of congenital deafness they are more or less defective. The fact of their position corresponding with the three dimensions of a cube—namely, its length, breadth, and height—has led to the opinion that they are concerned in convey ing a knowledge of the direction of sounds. This view is supported by prof. Wheat stone, who believes that we distinguish best the direction of those sounds which are sufficiently intense to affect the bones of the head, and that it is from the vibrations which are transmitted through these bones that our perception of direction is obtained. Thus, if the sound be transmitted in the plane of any one canal, the nervous matter in that canal will be more strongly acted on than in either of the other two; or if it be transmitted in a plane intermediate between the planes of this canal and the adjacent one, the relative intensity with which these two canals will be affected will depend upon, and indicate the direction of the intermediate plane.
The range of hearing, like that of vision, varies remarkably in different persons.
Some persons are insensible to sounds which others can readily hear. The ordinary range of human hearing comprised between the lowest notes of the organ and the high est known sound emitted by insects includes, according to Wollaston, more than nine octaves, the whole of which are distinctly perceptible by most ears. He relates, how ever, several cases in which the range, in reference to the perception of high notes, was much less. In one individual, the sense of hearing terminated at a note four octaves above the middle E of the pianoforte, the F above it being inaudible, although his hearing in other respects was as perfect as that of persons in general; another case was that of a lady who could never hear the chirping of the field-cricket; and in a third case the chirping of the common house-sparrow could not be heard. • See his memoir on sounds inaudible by certain ears, Phil. Trans., 1820.
The sensation of sound, like that of light, frequently lasts longer than the exciting cause. We have familiar proof of this fact iu the noise which remains in the ears after a long journey in a coach or railway; and it was clearly demonstrated by Savart, who found, in his experiments on toothed wheels, that the removal of one tooth did not pro duce any interruption of the sound.
For diseases of the E., see DEAFNESS.