Fig. 17.—Semidiagrammatic section through the right ear (Czermak). M, concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.
The Ear.—"The auditory organ in man consists of three portions, known respectively as the external ear, the middle ear or tympanum, and the internal ear or labyrinth; the latter contains the end-organs of the auditory nerve. The external ear consists of the expansion seen on the exterior of the head, called the concha, M, [Fig. 17], and a passage leading in from it, the external auditory meatus, G. This passage is closed at its inner end by the tympanic or drum membrane, T. It is lined by skin, through which numerous small glands, secreting the wax of the ear, open.
Fig. 18.—Mcp, Mc, Ml, and Mm stand for different parts of the malleus; Jc, Jb, Jl, Jpl, for different parts of the incus. S is the stapes.
"The Tympanum (P, [Fig. 17]) is an irregular cavity in the temporal bone, closed externally by the drum membrane. From its inner side the Eustachian tube (R) proceeds and opens into the pharynx. The inner wall of the tympanum is bony except for two small apertures, the oval and round foramens, o and r, which lead into the labyrinth. During life the round aperture is closed by the lining mucous membrane, and the oval by the stirrup-bones. The tympanic membrane T, stretched across the outer side of the tympanum, forms a shallow funnel with its concavity outwards. It is pressed by the external air on its exterior, and by air entering the tympanic cavity through the Eustachian tube on its inner side. If the tympanum were closed these pressures would not be always equal when barometric pressure varied, and the membrane would be bulged in or out according as the external or internal pressure on it were the greater. On the other hand, were the Eustachian tube always open the sounds of our own voices would be loud and disconcerting, so it is usually closed; but every time we swallow it is opened, and thus the air-pressure in the cavity is kept equal to that in the external auditory meatus. On making a balloon ascent or going rapidly down a deep mine, the sudden and great change of aërial pressure outside frequently causes painful tension of the drum-membrane, which may be greatly alleviated by frequent swallowing.
The Auditory Ossicles.—Three small bones lie in the tympanum forming a chain from the drum-membrane to the oval foramen. The external bone is the malleus or hammer; the middle one, the incus or anvil; and the internal one, the stapes or stirrup. They are represented in [Fig. 18.][19]
Accommodation is provided for in the ear as well as in the eye. One muscle an inch long, the tensor tympani, arises in the petrous portion of the temporal bone (running in a canal parallel to the Eustachian tube) and is inserted into the malleus below its head. When it contracts, it makes the membrane of the tympanum more tense. Another smaller muscle, the stapedius, goes to the head of the stirrup-bone. These muscles are by many persons felt distinctly contracting when certain notes are heard, and some can make them contract at will. In spite of this, uncertainty still reigns as to their exact use in hearing, though it is highly probable that they give to the membranes which they influence the degree of tension best suited to take up whatever rates of vibration may fall upon them at the time. In listening, the head and ears in lower animals, and the head alone in man, are turned so as best to receive the sound. This also is a part of the reaction called 'adaptation' of the organ (see the chapter on Attention).