Human Anatomy - Kotsan I. Y. 2009

Vestibulocochlear organ (organ of hearing and balance)
Middle ear

The middle ear (auris media) consists of the tympanic cavity, auditory ossicles, and the auditory (Eustachian) tube. The middle ear cavity communicates with the mastoid air Cells located within the thickness of the mastoid process.

The tympanic cavity (cavitas tympanica) is located within the Base of the pyramid of the Temporal bone. In shape, the cavity resembles a drum set on its edge and tilted with its upper margin toward the external acoustic meatus. The volume of the tympanic cavity is approximately 1 cm³. The internal surface of the tympanic cavity is lined with a mucous membrane that covers all its walls and continues directly backward into the mucosa of the mastoid air cells, and forward into the mucosa of the auditory tube.

Six walls are distinguished in the tympanic cavity: superior, inferior, anterior, posterior, medial, and lateral.

The superior, or tegmental, wall (paries tegmentalis) is formed by a thin plate of bone belonging to the corresponding region of the petrous part of the temporal bone, which separates the tympanic cavity from the cranial cavity.

The inferior, or jugular, wall (paries jugularis) faces the jugular fossa. Its surface is uneven and contains tympanic air cells and the opening of the tympanic canaliculus.

The anterior, or carotid, wall (paries caroticus) separates the tympanic cavity from the canal for the Internal Carotid Artery, and it features the tympanic opening of the auditory tube.

The posterior, or mastoid, wall (paries mastoideus) communicates with the mastoid antrum (antrum mastoideum) via the aditus to the antrum (aditus ad antrum). The mastoid antrum, in turn, communicates with numerous mastoid air cells (cellulae mastoideae).

The medial, or labyrinthine, wall (paries labyrinthicus) separates the tympanic cavity from the bony labyrinth of the Inner ear. It has a rather complex Structure. In the upper part of this wall lies the oval-shaped vestibular window (fenestra vestibuli), which is closed by the base of the stapes. Slightly above and posterior to the vestibular window lies the prominent facial canal wall (prominentia canalis facialis). Below the vestibular window is the promontory (promontorium), formed by the first turn of the cochlea. Below and posterior to the promontory lies the round-shaped cochlear window (fenestra cochleae), which is closed by the secondary tympanic membrane. The secondary tympanic membrane (membrana tympani secundaria) separates the tympanic cavity from the scala tympani and attaches along the margin of the window to the crest of the cochlear window (crista fenestrae cochleae). The cochleariform process (processus cochleariformis), located above the promontory, acts as a pulley over which the tendon of the tensor tympani Muscle hooks.

The lateral, or membranous, wall (paries membranaceus) is formed by the tympanic membrane and the surrounding PARTS OF THE temporal bone.

The tympanic cavity contains three mucous-covered auditory ossicles, as well as Ligaments and Muscles.

The auditory ossicles (ossicula auditoria) are very small in size. Articulating with one another, they form a more or less mobile chain that spans the tympanic cavity from the tympanic membrane to the labyrinth, serving to conduct sound vibrations from the tympanic membrane to the vestibular window, which opens into the inner ear.

The auditory ossicles include the malleus, incus, and stapes, which received their names due to their shapes.

The malleus (malleus) has a rounded HEAD (caput mallei) that transitions into a long handle of the malleus (manubrium mallei) featuring two processes: the lateral process (processus lateralis) and the anterior process (processus anterior).

The incus (incus) consists of a body (corpus incudis), which contains an articular facet for articulation with the head of the malleus, and two limbs: a short limb (crus breve) and a long limb (crus longum). The tip of the long limb bears a thickening called the lenticular process (processus lenticularis), through which the incus articulates with the stapes.

The stapes (stapes) features a small head (caput stapedis) bearing an articular surface for the lenticular process of the incus; two limbs — an anterior limb (crus anterius), which is straighter, and a posterior limb (crus posterius), which is more curved; and a base (basis stapedis), which connects the limbs and is inserted into the vestibular window.

The auditory ossicles are interconnected by two true joints reinforced by tiny ligaments. The handle of the malleus is fused with the tympanic membrane. The head of the malleus articulates with the body of the incus via the incudomalleolar joint (articulatio incudomallearis). The incus, in turn, connects with the head of the stapes via its lenticular process through the incudostapedial joint (articulatio incudostapedialis). The base of the stapes articulates with the margin of the vestibular window via the tympanostapedial syndesmosis (syndesmosis tympanostapedialis), which is reinforced peripherally by the annular ligament of the stapes (lig. anulare stapediale). The space between the stapes limbs is filled by the delicate stapedial membrane (membrana stapedialis).

The Movements of the auditory ossicles and protection against excessive vibrations during loud sounds are regulated by two small Muscles Attached to the auditory ossicles: the tensor tympani muscle and the stapedius muscle.

The tensor tympani muscle (m. tensor tympani) is located in the semicanal of the musculotubal canal of the same name, and its thin, long tendon attaches to the upper part of the handle of the malleus. By pulling the handle of the malleus medially, this muscle tensions the tympanic membrane. Consequently, the entire ossicular chain shifts inward, pressing the stapes into the vestibular window. The tensor tympani muscle is innervated by the mandibular nerve.

The stapedius muscle (m. stapedius) originates within the pyramidal eminence and attaches to the posterior limb of the stapes near its head. Functionally, this muscle acts as an antagonist to the previous one, producing the reverse movement of the ossicles in the middle ear away from the vestibular window. The stapedius muscle is innervated by the stapedial nerve (a branch of the Facial Nerve).

Overall, the Functions of the middle ear muscles are diverse:

✵ maintaining the normal tone of the tympanic membrane and the auditory ossicle chain;

✵ protecting the inner ear from excessive sound stimuli;

✵ accommodating the sound-conducting apparatus to sounds of varying loudness and pitch.

The primary operating principle of the middle ear as a whole is sound conduction from the tympanic membrane to the oval vestibular window.

The auditory, or Eustachian, tube (tuba auditoria, auditiva), averaging 35 mm in length and 2 mm in width, connects the tympanic cavity with the nasal part of the Pharynx. It serves for the passage of air from the pharynx into the tympanic cavity to maintain atmospheric pressure inside it, which is essential for the proper functioning of the sound-conducting apparatus (the tympanic membrane and auditory ossicles).

The auditory tube consists of a short bony part (pars ossea tubae auditoriae) and a long cartilaginous part (pars cartilaginea tubae auditoriae). At the transition from the bony part to the cartilaginous part lies the narrowest point of the auditory tube—its isthmus (isthmus tubae auditoriae).

The upper bony part of the tube is housed in the corresponding semicanal of the musculotubal canal of the temporal bone and opens on the anterior wall of the tympanic cavity via the tympanic ostium of the auditory tube (ostium tympanicum tubae auditoriae).

The wall of the bony portion of the tube is pitted with small pneumatic cells (cellae pneumaticae).

The lower (cartilaginous) part, which accounts for two-thirds of the tube's length, has the appearance of a downward-facing trough formed by two cartilaginous plates (medial and lateral) and a membranous lamina connecting them. Inferiorly, the auditory tube terminates on the lateral wall of the nasopharynx at the pharyngeal ostium of the auditory tube (ostium pharyngeum tubae auditoriae). The cartilaginous part of the tube gives rise to the tensor veli palatini and levator veli palatini muscles. Upon their contraction, the Cartilage and membranous lamina of the tube are pulled apart, widening the tube's lumen so that air passes from the pharynx into the tympanic cavity.

The internal surface of the auditory tube is lined with a thin mucous membrane forming longitudinal folds and is covered by ciliated epithelium, the cilia of which beat toward the pharynx. The mucous membrane of the auditory tube is rich in mucous glands and lymphoid tissue, which forms an aggregation near the torus tubarius in the nasopharynx, known as the tubal tonsil.