The nerves forming the tympanic plexus are as follows: The mucous membrane is supplied by the tympanic plexus, formed from the tympanic branch of the petrous ganglion of the glosso-pharyngeal nerve—from the branch of the superficial petrosal and branches of the sympathetic nerve. The otic ganglion receives fibres from the inferior maxillary nerve, from the auriculo-temporal nerve, and from the sympathetic plexus, and it is distributed to the tensor tympani and tensor palati muscles.
The mastoid cells lead directly from the tympanum. They consist of one large opening, the antrum, and the lower mastoid cells. These cells consist of a large number of varying-sized cavities, and are enclosed by a dense layer of bone. The mucous membrane lining these cells is a direct extension of the tympanic membrane, and liable to the same pathological conditions as that mucous membrane.
The Eustachian tube connects the cavity of the tympanum with that of the naso-pharynx, and is mainly intended for the introduction of air into the tympanic cavity. It has a length of 35 mm., partly bone (11 mm. in length), partly cartilaginous (24 mm. in length). The pharyngeal opening is 8 mm. high and 5 mm. wide; the tympanic orifice, 5 mm. high and 3 mm. wide (Schwartze). The mucous membrane lining this canal is a continuation of that of the naso-pharynx, and affords an easy way for the transmission of disease from the naso-pharynx to the middle ear. The Eustachian tube at rest is probably closed, although this is a matter still discussed; but it is essential for normal hearing that the air-pressure exerted on the tympanic membrane through the external auditory canal should be equalized by that exerted through the Eustachian tube. This necessitates the opening of the tube from time to time for free admission of air into the tympanic cavity. This is accomplished by the action of the musculus dilator tubæ, the tensor veli palatini, and the salpino-pharyngeus muscle. In the act of swallowing the tube opens; also, if the nostrils are closed and the act of swallowing is performed, air will be pumped out of the middle ear; on the contrary, if the nostrils are open air will be forced into the middle ear.
Diseases of the middle ear can involve the superficial layers of the middle-ear mucous membrane only, and may be of a catarrhal character. Hyperæmia and swelling of the epithelial cells, with increased mucous secretion, will be found. Later on, if the inflammation assumes a higher degree, a serous fluid will be profusely poured out, with lessening of the mucous secretion. When the deeper epithelial cells are involved, then pus-cells often appear, and a suppurative process becomes established, with frequent destruction of the soft tissues of the middle ear.
These different grades of inflammation are seldom found distinct, but run one into another. A case can start as a pure catarrhal inflammation; this, after attaining its acme, may end in recovery or degenerate into a chronic catarrh; or, on the contrary, it may advance into an acute purulent inflammation with a subsequent chronic stage.
CAUSES OF INFLAMMATION OF THE MIDDLE EAR.—Change of temperature, causing a sudden cooling of the body, is a frequent cause of this disease; for instance, exposure to wind from a partly-open window, a sudden rush of cold water into the external canal, as in surf-bathing, etc. Irritating foreign bodies in the external auditory canal may also cause this disease.
But inflammation of the middle ear occurs most frequently as a sequela of diseases affecting the general body. Among these may be mentioned, in order of their relative importance—
1. Scarlet Fever.—This disease is apt to cause the purulent form of middle-ear inflammation, and often of a very grave character. The ear complication can occur during the existence of the rash or immediately after its cessation (Thomas), and may run a rapid course, causing destruction of the tympanic membrane and middle-ear ossicles. Destruction of the facial nerve in its passage through its bony canal is not infrequent. Wendt has noticed in severe cases that the periosteum of the mastoid process, also that of the squamous and petrous portions, may participate in the purulent process, and end in subsequent caries of the bone. The severity of the ear complication will largely depend upon the condition of the naso-pharyngeal mucous membrane. Light attacks of scarlet fever with slight throat symptoms would most probably cause slight irritation of the middle-ear mucous membrane, while the anginose variety would cause most violent inflammatory sequelæ.
2. Measles is apt to cause the catarrhal variety of middle-ear inflammation rather than the purulent form. It occurs during and immediately after this eruption, and is a direct continuation of the naso-pharyngeal inflammation viâ the Eustachian tube. Hearing, as a rule, is diminished, due to the swollen mucous membrane of the Eustachian tube and middle ear, and also to fluid accumulations that often exist in the middle ear. Wendt3 draws attention to the fact that chronic affections of the auditory apparatus, such as formation of adhesions between the ossicles or between the tympanic membrane and wall of the tympanum, may arise while the soft parts are in a swollen condition, and often chronic catarrhal sequelæ may be traced to this cause.