The Middle Ear

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External ear lymphatic drainage goes to the superficial parotid, mastoid, upper deep cervical, and superficial cervical nodes.

Fig 1.0 – Overview of the ear

Parts of the Middle Ear

The middle ear can be divided into two parts:

  • Tympanic cavity – medial to the tympanic membrane. The auditory ossicles are three tiny bones: the malleus, incus, and stapes. They send vibrations of sound into the middle ear.
  • Epitympanic recess – space above the tympanic cavity, near the mastoid air cells. The malleus and incus extend slightly into the epitympanic recess.

Parts of the Middle Ear

The middle ear can be divided into two parts:

  • Tympanic cavity – medial to the tympanic membrane. The auditory ossicles are three tiny bones: the malleus, incus, and stapes. They send vibrations of sound into the middle ear.
  • Epitympanic recess – space above the tympanic cavity, near the mastoid air cells. The malleus and incus extend slightly into the epitympanic recess.

Borders

The middle ear can be visualised as a rectangular box with a roof and floor, medial and lateral and anterior and posterior walls.

  • Roof – formed by a thin bone from the petrous part of the temporal bone. It separates the middle ear from the middle cranial fossa.
  • Floor – known as the jugular wall, it consists of a thin layer of bone which separates the middle ear from the internal jugular vein
  • Lateral wall – made up of the tympanic membrane and the lateral wall of the epitympanic recess.
  • Medial wall – formed by the lateral wall of the internal ear. It contains a prominent bulge produced by the facial nerve as it travels nearby.
  • Anterior wall – a thin bony plate with two openings; for the auditory tube and the tensor tympani muscle. It separates the middle ear from the internal carotid artery.
  • Posterior wall (mastoid wall) consists of a bony partition between the tympanic cavity and the mastoid air cells.

o          Superiorly, there is a hole in this partition, allowing the two areas to communicate. This hole is known as the aditus to the mastoid antrum

Fig 2 – The middle ear. The two main parts of the middle ear have been labelled.

Bones

The auditory ossicles – the malleus, incus, and stapes – are the bones of the middle ear. They form a chain that connects the tympanic membrane to the oval window of the internal ear.

Sound vibrations move the tympanic membrane, which causes movement or oscillation in the auditory ossicles. This movement aids in the transmission of sound waves from the external ear’s tympanic membrane to the interior ear’s oval window.

The malleus is the largest and most lateral of the ear bones, with the handle of the malleus adhering to the tympanic membrane. The malleus head is located in the epitympanic recess, where it articulates with the incus, the next auditory ossicle.

The incus bone is made up of a body and two limbs. The body articulates with the malleus, the short limb connects to the middle’s posterior wall, and the long limb connects to the final of the ossicles, the stapes.

The stapes are the human body’s tiniest bone. It connects the incus to the inner ear’s oval window. It has a stirrup-shaped head, two limbs, and a base. The incus articulates with the head, and the base connects to the oval window.

Fig 1.1 – Bones of the middle ear.

Mastoid Air Cells

Mastoid air cells are found behind the epitympanic recess. They are a collection of air-filled pockets in the temporal bone’s mastoid process. The air cells are housed in a hollow known as the mastoid antrum. The aditus to the mastoid antrum connects the mastoid antrum to the middle ear.

When the pressure in the tympanic cavity is too low, the mastoid air cells operate as an air “buffer mechanism,” releasing air into the cavity.

Fig 1.2 – Coronal section of temporal bone, showing the mastoid air cells in more detail

Muscles

The tensor tympani and stapedius muscles offer a protective function in the middle ear. In reaction to loud noise, they contract, limiting the vibrations of the malleus, incus, and stapes and lowering sound transmission to the inner ear. The acoustic reflex is the name given to this action.

The tensor tympani arises from the auditory tube and connects to the malleus handle, drawing it medially when contracted. The tensor tympani nerve, a branch of the mandibular nerve, innervates it. The facial nerve innervates the stapedius muscle, which joins to the stapes.

Auditory Tube

The auditory tube (eustachian tube) is a bony and cartilaginous tube that connects the middle ear to the nasopharynx. It works by bringing the pressure in the middle ear in line with the pressure in the external auditory meatus.

It runs anteriorly and medioinferiorly from the anterior wall of the middle ear to the lateral wall of the nasopharynx. It creates a channel for an upper respiratory infection to spread into the middle ear by connecting the two structures.

Because the tube is shorter and straighter in youngsters, middle ear infections are more common than in adults.