The tympanum is an irregular cavity in the temporal bone. At birth it is a recess in the outer wall of the petrosal, partially closed externally by the squamosal. When the various elements of the temporal bone coalesce, and the tympanic plate becomes fully developed, then the cavity is completely surrounded by bony walls, except where it communicates with the external auditory meatus.
The roof, or tegmen tympani, is a translucent plate of bone belonging to the petrosal; it separates the tympanum from the middle fossa of the skull. The floor is the plate of bone which forms the roof of the jugular fossa.
The inner wall is formed by the external surface of the petrosal bone, and presents the following points for study: in the angle between it and the roof is a horizontal ridge which extends backwards as far as the posterior wall, and then turns down- wards in the angle between the inner and posterior wall. This is the fallopian canal; it is occupied by the facial nerve (seventh). Near the roof, but below the fallopian canal, is the fenestra ovalis, which leads into the vestibule: this fenestra receives the base of the stapes. Below the fenestra ovalis is the promontory, which contains the commencement of the first turn of the cochlea. In the lower and posterior part of the promontory is the fenestra rotunda; this, in the recent state, is closed by the secondary membrane of the tympanum. In the macerated bone it leads into the spiral canal of the cochlea. The promontory is also furrowed by some delicate channels (sometimes canals) for the tympanic branch of the glosso-pharyngeal nerve, which enters the tympanum through the tympanic canaliculus. The posterior Avail of the tympanum is formed by the mastoid process. At the superior and internal angle of this wall an opening leads into the mastoid antrum. Immediately below this opening there is a small hollow cone, the posterior pyramid; its cavity is continuous with the descending limb of the fallopian canal. One or more bony spiculae often connect the apex of the pyramid with the promontory. The cavity of this cone is occupied by the stapedius and the tendon of the muscle emerges at the apex.
The roof and floor converge towards the anterior extremity of the tympanum, which is, in consequence, very narrow, and occupied by two canals: the lower for the Eustachian tube, the upper for the tensor tympani muscle. These grooves are sometimes described together as the canalis musculo-tubarius. In carefully prepared bones the upper canal is a small horizontal hollow cone (anterior pyramid), 12 mm. in length, which lodges the tensor tympani muscle ; the apex is just in front of the fenestra ovalis, and is perforated to permit the passage of the tendon of the muscle. As a rule the thin walls of the canal are damaged, and represented merely by a thin ridge of bone. The posterior portion of this ridge projects into the tympanum, and is known as the processus cochleariformis. The thin septum between the canal for the tensor tympani and the tube is pierced by a narrow canal which is traversed by the small deep petrosal nerve. The outer wall is occupied mainly by the external auditory meatus. This opening is closed in the recent state by the tympanic membrane. The rim of bone to which the membrane is attached is incomplete above; the defect is known as the notch of Rivinus. Anterior to this notch, in the angle between the squamosal and the tympanic plate, is the Glaserian fissure, and the small passage which transmits the chorda tympani nerve sometimes called the canal of Huguier.
The tympanic cavity may be divided into three parts. The part below the level of the superior margin of the external auditory meatus is the tympanum proper; the portion above this level is the attic of the tympanum; it receives the head of the malleus, the body of the incus, and leads posteriorly into a recess known as the mastoid antrum.