The Middle Ear (Tympanic cavity)

  • Is a cavity in the petrous temporal bone.
  • It is shaped like a narrow, six-sided box, with convex medial and lateral walls.
 

Boundaries
 
  • Roof (tegmental wall): A thin plate of bone, called tegmen tympani; which separates the tympanic cavity from the dura matter of the middle cranial fossa cavity.

  • Floor (jugular wall): Formed by a layer of bone, which separates the tympanic cavity from the superior bulb of the internal jugular vein

  • Lateral wall (membranous wall): The tympanic membrane.
    Medial wall (labyrinthine wall): Posterior (mastoid wall): Anterior (Carotid wall):
    Separates the tympanic cavity from the inner ear. This wall has important features, namely:

  • The promontory: Is formed by the first turn of the cochlea.
  • Tympanic plexus: Of nerves lying on the promontory is formed by fibres of the facial, and glossopharyngeal nerve.
  • Two openings: Fenestra vestibuli and fenestra cochleae.
Separates the cavity from the mastoid antrum air cells. It has the following openings:
  • Aditus to the mastoid antrum or aditus ad antrum- mastoid antrum or air cells.
  • Pyramidal opening: For tendon of stapedius
  • Posterior chorda tympani canaliculus: Transmits the chorda tympani nerve.
 Separates the cavity from the carotid canal, with its contained internal carotid artery. It has two openings:
  • Communicating with tensor tympani muscle canaliculae
  • Auditory tube (pharyngo-tympanic or Eustachian tube)
         
         
 
Contents  
 

Nerves:

  • Tympanic plexus: Formed by the fibres from facial and glossopharyngeal nerves.
  • The general sensory fibres supply the tympanic cavity and the tympanic membrane.
  • Chorda tympani: A branch of the facial nerve. Has two components:
    • Taste from the anterior 2/3 of the tongue
    • Parasympathetic fibres-SUBMANDIBULAR GANGLION-submandibular sublingual and small glands (via lingual nerve)
 

Muscles:

  • The stapedius, attached to the stapes and the tensor tympani attached to the handle of the malleus.

  • These muscles are innervated by the facial and Mandibular nerves respectively. They restrict movements of the ossicles, tense the tympanic membrane and dampen loud sounds.
  • The auditory ossicles are malleus, incus and stepes.

The tympanic membrane:

  • This is am thin, fibrous membrane, separating the external auditory meatus from the tympanic cavity.

  • Covered with thin skin externally, and mucous membrane internally, it slopes inferomedially.

  • Shows a concavity towards the meatus, with a central depression called the umbo.

  • It moves in response to air vibrations.
 

Blood supply
  • Deep auricular branch of the maxillary artery.

  • Stylomastoid branch of the posterior auricular artery.

  • Veins on the external surface – external jugular vein.

  • Veins on the internal surface- transverse sinus, sigmoid sinus.
 

Innervation
  • Internal surface: Tympanic plexus

  • External surface:

    • Auriculotemporal nerve (upper part)
    • Auricular branch of vagus (with facial and glossopharyngeal)
The Larynx and the Ear
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