OF HEARING.
THE sense of hearing comes next to that of sight, both as to its real importance, and the elaborate structure of its appropriate organ. We have also a tolerably accurate knowledge of the nature of sound, and of the process by which the ear receives the impression, although less so than we possess concerning the eye, and the action of light upon it ; we have likewise less correct ideas of the acquired perceptions of hearing.
The essential parts of the car are, a cavity in the tem poral bone, the membrana tympani stretched across this cavity, by which it is divided into two parts, called, re spectively, the meatus auditorius and the tympanum, a number of small bones or ossicles connected with the tympanum, the auditory nerve, and the Eustachian tube passing from the tympanum to the lances. On the internal surface of the tympanum, or drum as it is popularly styled, the termination of the nerve is spread out in a manner analogous to the expansion of the optic nerve at the back of the eye ; and this, like the retina, is supposed to con stitute the immediate scat of the impression. Sound is excited by the vibration or oscillation of the particles of certain bodies, named, from this circumstance, sonorous, and is capable of being transmitted from one body to ano ther until it arrives at the tympanum, and strikes upon the nerve. Sonorous bodies are of various kinds, but the medium by which sound is usually conveyed to the ear is the atmosphere; liquids and solids arc, however, better conductors than the air, both with respect to the strength of the vibration and the velocity with which it is conveyed. When a gun is fired at sea, if we apply the ear to the sur face of the water, we receive two successive impressions of sound, the first the one that is carried by the water, the other through the air. The same thing occurs if a sound be produced in the immediate vicinity of long metallic rods or tubes, the sound being conveyed by these bodies more quickly, and with greater intensity, than by the air which is contiguous to them. The different states of the atmosphere affect its power of transmitting sounds ; but we find that all sounds, whether loud or weak, are con veyed by the same medium with the same velocity. The vibi.ation of the air which constitutes sound, travels at the rate of about thirteen miles in a minute, very nearly a mile in five seconds.
Sound is capable of being reflected from the surface of bodies at a determinate angle, so as to be concentrated into a focus, although in a less precise manner than the rays of light. Upon this principle echoes depend ; and, as modified by the form of the surface, the incruace of sound is produced in speaking and hearing trumpets, domes, whispering galleries, &c. The external parts of the ear probably act in the same way by affording an elastic surface on which the undulations of the air impinge, and which, after a number of successive reflections, are collected in the tympanum. We have continual oppor tunities of observing the use of the external car among the inferior animals, where this part is large and is fur nished with muscles, which give it the power of being readily turned to the sounding body, and thus conveying the impression with more force to the seat of sensation.
The membrana tympani, from its size and position, would appear to be a part of considerable importance in the economy of the ear, yet it must be acknowledged that its use is but very imperfectly known. Boerhaave con ceived that we possess the power of contracting and relax ing this part, by means of the ossicles attached to it, ac cording as we are more or less attentive to sounds. This conjecture may be considered as, in some measure, verified by the observations of Sir E. Home upon the ear of the elephant, where, in consequence of the size of the parts, he was able to detect a muscular structure so connected with the membrana tympani, that, by the action of the muscle, the membrane would be contracted or relaxed. In consequence of the delicacy of the mechanism, lie was led to suppose that this is the part of the organ which is adapted to receive the impression of musical sounds; but the hypothesis was overthrown by a case which occur red to Sir A. Cooper, in which the membrane of one ear was entirely destroyed, and that of the other nearly so, and yet the person retained the complete power of perceiving musical sounds. It may appear not a little remarkable, that we should be ignorant of the uses of that part, both of the eye and the ear, which, from their structure and situation, might be supposed to be among the most im portant to the respective organs.