Mesotympanum

The mesotympanum (rare plural: mesotympana) forms the main compartment of the tympanic cavity and contains most of the important structures of the middle ear, including most of the ossicular chain. 

Gross anatomy

Location

The mesotympanum is found in the middle ear.

Boundaries

• superiorly it is arbitrarily separated from the epitympanum by an axial plane extending medially from the superior margin of the external acoustic canal towards the lateral wall of the inner ear.
• inferiorly it is arbitrarily separated from the hypotympanum by an axial plane extending between the inferior margin of the external acoustic canal and the cochlear promontory. 
• laterally lies the tympanic membrane.
• medially is the lateral wall of the inner ear.
• anteriorly lies the protympanum which primarily comprises the bony portion of the Eustachian tube
• posteriorly the wall (also known as the retrotympanum) is irregular with several small important structures, which include lateral to medial:
  • facial recess
  • pyramidal eminence
  • sinus tympani
  • round window niche

Contents

Bones

The middle ear ossicles consist of three small bones, the malleus, incus and stapes, which form a mobile chain across the mesotympanum from the tympanic membrane to the oval window. Only the head of the malleus and the body and short process of the incus lie outside of the mesotympanum, in the epitympanum.

Muscles

There are two muscles, one for the malleus and one for the stapes, which act to damp down over-vibration from low-pitched sound waves. These are, respectively:

• tensor tympani muscle
• stapedius muscle

Nerves

The chorda tympani leaves the facial nerve in the facial canal and enters the tympanic cavity through the posterior wall, lateral to the pyramid, lying just underneath the mucous membrane. It runs over the pars flaccida of the tympanic membrane, and the neck of the malleus. It leaves at the anterior margin of the tympanic notch.