Middle ear
THE MIDDLE EAR (TYMPANIC CAVITY)
- The middle ear cleft is lined by mucous membrane and filled with air
- Consists of the middle ear, eustachian tube, aditus ad antrum, mastoid antrum and mastoid air cells.
- The distance between tympanic membrane and medial wall of middle ear at the level of center is 2 mm.
- Middle ear is a 1 to 2 cm3 air filled cavity that houses ossicles, stapedius and tensor tympani muscles and chorda tympani nerve and tympanic plexus.
PART OF MIDDLE EAR:
A. Mesotympanum:
- Lies at the level of pars tensa.
- Narrowest part of middle ear (transverse dimension is 2mm)
B. Epitympanum (attic):
- This is the portion of middle ear that lies above the level of pars tensa and medial to Shrapnell’s membrane and the bony lateral attic wall.
- Widest part of tympanic cavity (transverse cavity (transverse imension is 6 mm).
- Communicates with mastoid air cell system, through a small opening called aditus.
- Prussak’s space communicates with mastoid air cell system, through a small opening called aditus.
C. Hypotympanum:
- This is the portion of middle ear that lies below the level of pars tensa.
- Its transverse dimension is 4mm.
D. Protympanum:
- The portion of middle ear around the eustachian tube opening is termed as protympanum.
- Presence of more goblet cells near the orifice of E. Tube
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CONTENTS |
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Mesotympanum |
Handle of malleus, long process of incus, stapes, oval window, round window |
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Epitympanum |
Head & neck of malleus, Body & short process of incus |
BOUNDARIES OF MIDDLE EAR
- Middle ear has six boundaries: roof, floor, and medial, lateral, anterior and posterior walls.
1. Roof or Tegmental wall (Paries tegmentalis):
- It is formed by tegmen tympani (a thin plate of bone), which extends posteriorly to form the roof of the aditus and antrum (tegmen antri).
- Tegmen tympani separates middle ear from the middle cranial fossa.
2. Floor or Jugular wall (Paries jugularis):
- Consist of thin plate of bone called the fundus tympani.
- It separates the tympanic cavity from the jugular bulb.
- It presents, near medial (labyrinthic) wall, a small aperture for the passage of the tympanic branch of the glossopharyngeal nerve.
3. Anterior or carotid wall (Paries carotica):
- The anterior wall, a thin plate of bone, which separates the middle ear cavity from internal carotid artery.
Has following features:
- Eustachian tube: It connects the middle ear with nasopharynx.
- Canal of tensor tympani muscle: It is situated in the roof of eustachian tube.
- Canal for chorda tympani nerve (Canal of Huguier)
- Attachment of anterior malleolar ligament.
- Canal for Lesser Petrosal Nerve.
- Canal for branch from Int. Carotid Plexus to tympanic plexus (Caroticotympanic Nr)
4. Posterior (mastoid wall):
- It lies close to the mastoid air cells and presents following structures:
- Pyramid: It is a bony projection through the summit of which appears the tendon of the stapedius muscle that is inserted to the neck of stapes.
- Aditus ad antrum: It is an opening through which mastoid antrum opens into the attic. It lies above the pyramid.
- Facial nerve: The vertical mastoid part of the fallopian canal for facial nerve runs in the posterior wall just behind the pyramid.
- Facial (suprapyramidal) recess: This recess is a depression in the posterior wall lateral to the pyramid. Its boundaries are following:
- Medial: Vertical part of CN VII.
- Lateral: Chorda tympani and tympanic annulus.
- Superior: Fossa incudis, in which lies short process of incus.
- Sinus (infrapyramidal) tympani: This deep recess lies medial to the pyramid.
5. Medial or inner labyrinthic wall (Paries labyrinthica):
- It is formed by the lateral wall of labyrinth.
It presents following structures:
- Promontory: It is a bony bulge which is due to the basal coil of cochlea. Tympanic plexus present over it.
- Oval window (fenestra vestibuli/ovalis): Kidney shaped opening. Situated above and behind promontory. The footplate of stapes is placed over this window guarded by anular ligament.
- Round window(fenestra cochleae/rotunda): lies below & behind F.Ovalis. It is covered by the secondary tympanic membrane.
- Horizontal tympanic part of fallopian canal for facial nerve: It lies above the oval window.
- The tympanic segment of facial nerve canal may be congenitally dehiscent and the exposed facial nerve becomes vulnerable to injuries or infection.
- Lateral semicircular canal: It lies above the fallopian canal, facial nerve.
- Processus cochleariformis: It is a hook-like projection, which lies anterior to the oval window.
- The tendon of tensor tympani takes a turn on this process and then is inserted on the neck of malleus.
- Processus cochleariformis is an important surgical landmark for the level of the genu of the facial nerve.
6. Lateral or outer or membranous wall (Paries membranacea):
- Formed mainly by the tympanic membrane
- Scutum: An upper part of epitympanum is formed by outer bony attic wall called scutum.
- The ring of bone is incomplete at its upper part, forming a notch (notch of Rivinus).
INTRATYMPANIC NERVES:
- Tympanic plexus :supplies to the medial surface of the tympanic membrane, tympanic cavity, mastoid air cells and the bony eustachian tube.
- Formed by following nerves:
1. Tympanic branch of glossopharyngeal (Jacobson’s Nerve) :
- It carries secretomotor fibers to the parotid gland.
2. Sympathetic fibers:
- Caroticotympanic nerves come from the sympathetic plexus, which is present round the internal carotid artery
3. Chorda tympani nerve:
- This branch of the facial nerve enters the middle ear through posterior canaliculus, runs on the medial surface of the tympanic membrane (lateral wall of middle ear)between the malleus and long process of incus, above the insertion of tensor tympani.
Exam Question
- The chorda tympani nerve is related to the lateral wall of middle ear.
- The auditory tube connects the nasopharynx with the anterior wall of middle ear
- Tympanic plexus present in Medial projection of middle ear cavity
- The distance between tympanic membrane and medial wall of middle ear at the level of center is 2 mm
- Mesotympanum Narrowest part of middle ear
- Sensory nerve supply of middle ear cavity is provided by Glossopharyngeal nerve
- Tegmen seperates middle ear from the middle cranial fossa containing temporal lobe of brain by roof of middle ear
- Floor of middle ear cavity is in relation with Bulb of the internal jugular vein
- Promontory seen in the middle ear is Basal turn of cochlea
- Lateral wall of middle ear is formed byTympanic membrane
- Scutum is present in Lateral wall of middle ear
- The middle ear cleft in the temporal bone includes Air cells,Mastoid Antrum,Aditus ad Antrum,Attic (Epitympanum),Eustachian Tube,mesotympanum,Hypotympanum
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