Topography and walls of the cavity
The tympanic cavity resembles a miniature hexagonal prism. Each of its six walls has a specific name reflecting the adjacent anatomical structures:
- The superior (membranous) wall (paries membranaceus) is formed directly by the tympanic membrane.
- The medial (labyrinthine) wall (paries labyrinthicus) isolates the cavity from the structures of the inner ear.
- The superior (tegmental) wall (paries tegmentalis) forms the roof of the cavity and serves as the boundary with the middle cranial fossa. Removing this wall reveals the epitympanic recess (recessus epitympanicus).
- The inferior (jugular) wall (paries jugularis) is located directly above the superior bulb of the internal jugular vein.
- The anterior (carotid) wall (paries caroticus) adjoins the osseous canal of the internal carotid artery. Its upper part opens into the tympanic ostium of the auditory tube (ostium tympanicum tubae auditivae).
- The posterior (mastoid) wall (paries mastoideus) contains the aditus to the antrum (aditus ad antrum), through which the cavity communicates with the mastoid air cells.
Relief of the medial wall and contents
The main functional contents of the tympanic cavity are the system of auditory ossicles (ossicula auditus), connected by joints and ligaments. From lateral to medial, they are arranged in the following order: first is the malleus (malleus), followed by the incus (incus), and the chain is completed by the stapes (stapes). The head of the malleus and part of the incus are hidden within the epitympanic recess.
A complex relief stands out on the medial (labyrinthine) wall. In the very center, the promontory (promontorium) projects, formed by the basal turn of the cochlea. Directly opposite the promontory, the umbo of the tympanic membrane bulges into the cavity lumen. Superior and slightly posterior to the promontory lies the oval fenestra vestibuli (fenestra vestibuli), in which the base of the stapes is firmly fixed. Inferior to the promontory is the round fenestra cochleae (fenestra cochleae), hermetically sealed by the secondary tympanic membrane (membrana tympani secundaria).
Auditory tube and cavity connections
The tympanic cavity is not a closed space. Posteriorly, it continues into the mastoid air cell system (cellulae mastoideae), and anteriorly, it communicates with the nasopharynx via the auditory tube (tuba auditiva).
The auditory tube is a channel 3–5 cm long and up to 2 cm in diameter. Its posterolateral third runs within the bone: the upper wall here merges into the roof of the tympanic cavity, and the medial wall adjoins the internal carotid artery canal. The anterior two-thirds of the tube have a fibrocartilaginous structure. At the junction of the osseous and cartilaginous parts lies the narrowest point — the isthmus (isthmus tubae), beyond which the lumen of the canal widens toward the pharynx. The tensor veli palatini and levator veli palatini muscles (m. tensor and m. levator veli palatini) attach to the cartilaginous portion of the tube.
Blood supply and innervation
The walls of the cavity are supplied by an extensive arterial network consisting of five branches:
- Anterior tympanic artery (from the maxillary artery).
- Caroticotympanic branch (from the internal carotid artery).
- Superior tympanic artery (from the middle meningeal artery).
- Posterior tympanic artery (from the stylomastoid artery).
- Inferior tympanic artery (from the ascending pharyngeal artery).
Innervation of the mucous membrane is provided by branches of the tympanic plexus (plexus tympanicus). Lymph drainage from the tympanic cavity goes to the deep parotid lymph nodes, and from the auditory tube, to the retropharyngeal nodes.
Inner ear (brief overview)
Medial to the tympanic cavity, within the petrous part of the temporal bone, lies the hidden inner ear (auris interna). Its structural basis is the osseous labyrinth (labyrinthus osseus), in the center of which is the vestibule (vestibulum). Anterior and medial to the vestibule lies the cochlea (cochlea), and posterior and lateral are the three bony semicircular canals (anterior, posterior, and lateral), which open into the vestibule through five openings.
Inside the bony case is the connective tissue membranous labyrinth filled with endolymph. It mirrors the shape of the bony structures but is smaller in volume. The space between the labyrinths is filled with perilymph. This area is supplied by the labyrinthine artery, while nearby, in the internal acoustic meatus, run the facial and vestibulocochlear nerves (cranial nerves VII and VIII).
Briefly: answers to topical questions
Which muscles attach to the auditory ossicles in the tympanic cavity?
Two muscles attach to the auditory ossicles in the tympanic cavity, regulating the transmission of sound vibrations.
- The tensor tympani muscle (m. tensor tympani) attaches by its tendon to the handle of the malleus.
- The stapedius muscle (m. stapedius) attaches by its tendon to the neck of the stapes; upon contraction, it pulls the stapes posteriorly and decreases its mobility, protecting the inner ear from excessive sound vibrations.
What anatomical structures form the relief of the medial (labyrinthine) wall of the tympanic cavity?
The relief of the medial (labyrinthine) wall of the tympanic cavity is formed by the following anatomical structures:
- The promontory (promontorium) — formed by the basal turn of the cochlea.
- The vestibular window (fenestra vestibuli) — an oval window superior and posterior to the promontory, closed by the base of the stapes.
- The cochlear window (fenestra cochleae) — a round window inferior to the promontory.
- The fissure / recess (fossula antefenestram) — located near the anterior margin of the vestibular window.
- The pyramidal eminence (eminentia pyramidalis) — contains the stapedius muscle.
- The cochleariform process (processus cochleariformis) — a projection of the semicanal of the tensor tympani muscle.
- The osseous aperture — the exit site of the chorda tympani.
Which nerve passes in transit through the tympanic cavity between the handle of the malleus and the long process of the incus?
A branch of the nervus intermedius, the chorda tympani (chorda tympani), passes in transit through the tympanic cavity. It originates from the facial nerve canal, crosses the tympanic cavity, and exits at the external skull base via the petrotympanic fissure (fissura petrotympanica). This nerve provides taste sensation for the anterior two-thirds of the tongue and carries parasympathetic secretory fibers to the salivary glands.
The names of the six walls are easy to remember by the structures they border: superior — roof (tegmental), inferior — vein (jugular), anterior — artery (carotid), posterior — process (mastoid), lateral — membrane (membranous), medial — labyrinth (labyrinthine).
Class and exam questions
What forms the promontory on the medial wall of the tympanic cavity?
The promontory (promontorium) is formed by the bulge of the basal turn of the cochlea of the inner ear.
Which anatomical structure closes the cochlear window?
The round cochlear window is hermetically sealed by the secondary tympanic membrane (membrana tympani secundaria).
What spaces communicate with the tympanic cavity?
Anteriorly, it connects to the nasopharynx via the auditory tube, and posteriorly, to the mastoid air cells through the aditus to the antrum (aditus ad antrum).
What else to study on this topic
- Mechanism of sound vibration transmission through the chain of auditory ossicles
- Topography of the internal acoustic meatus and the nerves passing through it
- Structural features and functions of the auditory tube
- Circulation of endolymph and perilymph in the inner ear
- Formation and branches of the tympanic plexus
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