The Auditory Ossicles

The Malleus

• Its superior part, the head, lies in the epitympanic recess.
• The head articulates with the incus.

• The neck, lies against the flaccid part of the tympanic membrane.
• The chorda tympani nerve crosses the medial surface of the neck of the malleus.

• The handle of the malleus (L. hammer) is embedded in the tympanic membrane and moves with it.
• The tendon of the tensor tympani muscle inserts into the handle.

The Incus

• Its large body lies in the epitympanic recess where it articulates with the head of the malleus.
• The long process of the incus (L. an anvil) articulates with the stapes.
• The short process is connected by a ligament to the posterior wall of the tympanic cavity.

The Stapes

• The base (footplate) of the stapes (L. a stirrup), the smallest ossicle, fits into the fenestra vestibuli or oval window on the medial wall of the tympanic cavity.

Functions of the Auditory Ossicles

• The auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane.

Walls of the Tympanic Cavity or Middle Ear

• This cavity is shaped like a narrow six-sided box that has convex medial and lateral walls.
• It has the shape of the biconcave lens in cross-section (like a red blood cell).

The Roof or Tegmental Wall

• This is formed by a thin plate of bone, called the tegmen tympani (L. tegmen, roof).
• It separates the tympanic cavity from the dura on the floor of middle cranial fossa.
• The tegmen tympani also covers the aditus ad antrum.

The Floor or Jugular Wall

• This wall is thicker than the roof.
• It separates the tympanic cavity from the superior bulb of the internal jugular vein. The internal jugular vein and the internal carotid artery diverge at the floor of the tympanic cavity.

• The tympanic nerve, a branch of the glossopharyngeal nerve (CN IX), passes through an aperture in the floor of the tympanic cavity and its branches form the tympanic plexus.

The Lateral or Membranous Wall

• This is formed almost entirely by the tympanic membrane.
• Superiorly it is formed by the lateral bony wall of the epitympanic recess.
• The handle of the malleus is incorporated in the tympanic membrane, and its head extends into the epitympanic recess.

The Medial or Labyrinthine Wall

• This separates the middle ear from the membranous labyrinth (semicircular ducts and cochlear duct) encased in the bony labyrinth.
• The medial wall of the tympanic cavity exhibits several important features.

• Centrally, opposite the tympanic membrane, there is a rounded promontory (L. eminence) formed by the first turn of the cochlea.
• The tympanic plexus of nerves, lying on the promontory, is formed by fibres of the facial and glossopharyngeal nerves.

• The medial wall of the tympanic cavity also has two small apertures or windows.

• The fenestra vestibuli (oval window) is closed by the base of the stapes, which is bound to its margins by an annular ligament.
• Through this window, vibrations of the stapes are transmitted to the perilymph window within the bony labyrinth of the inner ear.

• The fenestra cochleae (round window) is inferior to the fenestra vestibuli.
• This is closed by a second tympanic membrane.

The Posterior or Mastoid Wall

• This wall has several openings in it.
• In its superior part is the aditus ad antrum (mastoid antrum), which leads posteriorly from the epitympanic recess to the mastoid cells.

• Inferiorly is a pinpoint aperture on the apex of a tiny, hollow projection of bone, called the pyramidal eminence (pyramid).
• This eminence contains the stapedius muscle.
• Its aperture transmits the tendon of the stapedius, which enters the tympanic cavity and inserts into the stapes.

• Lateral to the pyramid, there is an aperture through which the chorda tympani nerve, a branch of the facial nerve (CN VII), enters the tympanic cavity.

The Anterior Wall or Carotid Wall

• This wall is a narrow as the medial and lateral walls converge anteriorly.
• There are two openings in the anterior wall.

• The superior opening communicates with a canal occupied by the tensor tympani muscle.
• Its tendon inserts into the handle of the malleus and keeps the tympanic membrane tense.

• Inferiorly, the tympanic cavity communicates with the nasopharynx through the auditory tube.

The skull, the skeleton of the head, is the most complex bony structure in the body because it:

1. Encloses the brain, which is irregular in shape;
2. Houses the organs of special senses for seeing, hearing, tasting, and smelling; and
3. Surrounds the openings in to the digestive and respiratory tracts.

• In the anatomical position, the skull is oriented so that the inferior margin of the orbit (eye socket) and the superior margin of the external acoustic meatus (auditory canal) are horizontal. This is called the orbitomenial plane (Frankfort plane).

• The term cranium (L. skull) is sometimes used when referring to the skull without the mandible (lower jaw), but the cranium is often used when referring to the part of the skull containing the brain.
• The superior part is the box-like structure called the calvaria (cranial vault, brain case); the remainder of the cranium, including the maxilla (upper jaw), orbits (eyeball sockets) and nasal cavities, forms the facial skeleton.
• The term skullcap (calotte) refers to the superior part of the calvaria, which is removed during autopsies and dissections. The inferior aspect of the cranium is called the cranial base.

Digastric Muscle

• Origin:
  • Anterior Belly: Digastric fossa of the mandible.
  • Posterior Belly: Mastoid notch of the temporal bone.
• Insertion: Intermediate tendon attached to the body of the hyoid bone.
• Nerve Supply:
  • Anterior Belly: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
  • Posterior Belly: Facial nerve (CN VII).
• Arterial Supply:
  • Anterior Belly: Branch of the submental artery.
  • Posterior Belly: Muscular branch of the posterior auricular artery and occipital artery.
• Action: Raises the hyoid bone and base of the tongue, steadies the hyoid bone, and opens the mouth by lowering the mandible.

• The palate has a rich blood supply from branches of the maxillary artery.