DEAFNESS may be complete or partial, may affect both ears or only one, may date from birth, be permanent or only temporary, and is but too often one of the distressing symptoms of advancing age. The causes of deafness are numerous. On glancing at the article AUDITORY NERVE, the reader will at once remark the extraordinary intricacy of the hearing-apparatus there described, and will easily conceive that although it be contained in a little nut of densest bone (the petrous portion of the temporal), still it is exposed to many deteriorating influences, and that very slight causes may disarrange the exquisite adjustment of its parts. 1. The auditory nerve may itself be unsusceptible to the stimulus of sound, from some diseased condition at its origin in the brain, or at its final snbdivision in the labyrinth. This is termed nervous deafness. 2. The struc tures which conduct the vibrations of sound to the labyrinth may be faulty, from accident or disease. 3. The passage leading to the tympanum or drum may be blocked up. 4. The cavity of the drum may have ceased to be resonant, owing to deposits from inflammatory attacks, to loss of its membrane, or air being excluded, from obstructions in the passage between it and the gullet (the Eustachian tube).
Nervous deafness may be caused by a sudden concussion, as from a " box on the car," or a general shock to the whole body, as in the case of the celebrated Dr. Ditto, who lost his hearing, when a boy, by a fall from the top of a house. The concussion from loud sounds suddenly taking the ear unawares, before its small muscles have time to prepare themselves for the shock. causes the deafness which follows the firing of cannon. Even a loud yell close to the ear has peen sufficient to destroy the hearing power on that side. As such an accident is generally accompanied by an increased flow of blood to the part injured, it may be relieved by the application of leeches, applied behind the auricle, and the ear should for some time be protected from loud sounds as carefully as possible. In some of these cases the nerve gradually recovers its sensibility,
but in many the deafness continues, and is accompanied by a distressing singing in the ears. Exposure to cold affects the auditory nerve; and gouty persons, or those who are suffering from the poisons of typhus fever, scarlatina, measles, mumps, etc., frequently become deaf. Some medicines, as quinine, produce nervous deafness; so do debility and mental excitement; but all these causes seem to act in one way—viz., to increase the flow of blood to the ear, and should be treated accordingly.
The solid conductors of sound to the auditory nerve may be injured or diseased, so that the vibrations are interrupted. One curious cause of deafness has been recently shown to exist by Mr. Joseph Toynbee of London—viz., an increasing stiffness in the little joint by which the stirrup-bone moves in the oval window of the vestibule; this stiffness prevents the base of the stirrup pressing inwards sufficiently to affect the con tents of the labyrinth, therefore it ceases to keep the auditory nerve en rapport with the membrane of the drum. This condition may be recognized during life by the patient losing the power of adaptino. his hearing to varying sounds. Two persons speaking at once prevents his hearing the voice of either; there is a constant buzzing in the ear, and he gets deafer and deafer day by day. This curious disease is frequently associated with gout and rheumatism, and in its earlier stages may be influenced by the same remedies as these; but if once established, it is incurable.
Sound reaches the auditory nerve through the vibrations of the bones of the head, but chiefly through the external opening in the auricle, the passage leading from which is shut at the depth of an inch and a quarter from the surface by the membrane of the drum stretching across it. Should this passage be blocked up, so that the sounds can no longer pass along it to impinge upon the membrane, either total or partial deafness must result.