The internal ear, consisting of the labyrinth and the auditory nerve, is rarely the primary seat of disease, but is liable to be involved in affections of the middle ear on the one hand, and of the brain and its membranes on the other. It does, however, sometimes suffer directly from the effects of continuous loud noises (as in boilermakers) or from blows on the head. TheM lead to chronic inflammations of the structures. According to Ogston an inflammation of the labyrinth with effusion is not uncommon in gouty and rheumatic persons.

Inflammations of the middle ear by affecting the bone directly may lead to destruction, or in a less degree to thickenings and atrophies of the labyrinthine structures. Sometimes the base of the stapes, by pressing too firmly on the fenestra ovalis, causes increased pressure in the labyrinthine fluid and consequent atrophy of the nerve. The abnormal position of the stapes may be the result of inflammation or it may be produced by obstruction of the Eustachian tube.

Syphilis not infrequently attacks the labyrinth leading to thickenings and consequent injury to the nerve structures. It is said, also, that haemorrhage and serous exudation sometimes result from syphilis. Congenital syphilis may attack the labyrinth, so that deafness often coincides with keratitis and iritis.

Meningitis (cerebro-spinal and perhaps tubercular), by extending to the acoustic nerve or labyrinth, may lead in children to such damage to the structures as to result in total deafness. This is stated to be an important cause of deaf-mutism. Acute fevers, especially scarlet fever and typhoid fever, sometimes cause inflammation of the labyrinth. Mumps has occasionally a similar effect.

Diseases of the brain and its membranes, such as tumours, abscesses, and inflammations, often involve the auditory nerve within the skull. The auditory centre in the brain may also be involved by local diseases such as embolism and haemorrhage.

Affections involving the labyrinth and producing Alterations in the fluid contents of the semicircular canals are apt to produce subjective sensations of giddiness and sickness. This is due to the fact that these structures apparently exercise an important function in connection with the retention of the equilibrium of the body. The group of symptoms thus produced is included under the designation Meniere's disease. A minor degree of giddiness and sickness may be be produced by syringing the ear with cold water, especially when the membrane is perforated.

Any interference with the labyrinth leads to deafness, and this may be absolute. If this occurs in children the condition of Deaf-mutism results. Deaf-mutism will occur when deafness has been produced in the infant, from whatever cause, whether an affection of the middle or internal ear.