The sense of hearing depends on the transmission of sonorous vibrations from the air to the auditory nerve by means of the membrana tympani and the ossicles of the ear, and upon the perception of those vibrations by the brain.

The centre for hearing, according to Ferrier, is in the superior temporo-sphenoidal convolution (16, Fig. 68, p. 185). It is probable that subjective sounds not depending on disturbance of the auditory apparatus, such as the sounds of voices, etc, heard in delirium or mania, or as the prodromata of an epileptic fit in certain individuals, or during intoxication by cannabis indica, are due to irritation of these centres.

1 Compare Schroff, Pharmacologic, 4th ed. p. 535, and Wood, Materia Medica, 3rd ed. p. 236.

2 Lauder Brunton, On Digitalis, etc.

The sense of hearing may be dulled by any interference with the passage of the sound into the ear, as by wax in the auditory meatus, by disease of the auditory nerve or of the brain itself.

The hearing may be rendered more acute by the removal of any obstacle in the way of transmission of sound to the auditory nerve, or by drugs which increase the excitability of the auditory nerve or of the brain; thus the wax may be removed by simply syringing; thickness and catarrh of the Eustachian tube which interfere with vibrations in the middle ear may be lessened by the inhalation of camphor and ammonia, or by the application of a solution of ammonium chloride and sodium bi-carbonate to the posterior nares either by the spray or nasal douche. The excitability of the auditory nerve or of the brain is increased by strychnine, which renders the hearing more acute.

Subjective noises in the ear, such as humming, buzzing, or ringing, are often very troublesome. Bubbling noises may be due to mucus in the Eustachian tube. Buzzing or humming are probably generally caused by vascular congestion either of the external meatus, of the middle ear, or of the Eustachian tube. Where the bubbling noises are due to the presence of mucus they may be to a considerable extent removed by washing out the mucus with a solution of carbonate of sodium applied by a nasal douche. Noises in the ears due to hyperaemia may be lessened or removed by cholagogue purgatives and by hydro-bromic acid. Where chronic thickening of the membrane is present, relief is usually afforded by iodide of potassium or iodide of ammonium, both applied locally and taken internally. Subjective noises in the ears are caused by quinine in large doses, and also by salicylate of sodium. Both of these drugs have their effect upon the ear to a great extent neutralised by hydrobromic acid, and ergot1 is said to have a similar power to prevent or remove the unpleasant singing. It is uncertain whether the singing caused by quinine and salicylates is due to their action on the auditory apparatus, or the cerebral centres; but the fact that in larger doses they may cause delirium indicates that even the earlier symptom of buzzing in the ears may be due, in part at least, to their action on the cerebral centres.

1 Schilling, Aertzl. Intelligenzblatt, 1883.