Middle ear

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Middle ear
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Middle ear structures

• Middle ear cavity/tympanum
• Tympanic membrane
• Ossicles/Middle ear bones

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Middle ear cavity

• 2-4 mm horizontal, 13 mm vertical, approximately
  2 cm3 in volume
• Comprised of
• Tympanic cavity between outer and inner ear
• Epitympanic recess above the TM
• Mastoid recess contained in the mastoid region of
  the temporal bone

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Tympanic cavity

• Similar to a box with six surfaces
• Lateral wall or membraneous wall Formed by TM
  and squamous portion of TB.
• Medial wall or labyrinthine wall Promontory
  (outer wall of inner ear)
• (oval and round windows, prominence of facial
  nerve)
• Anterior wall or carotid wall (opening of ET and
  tendon of tensor tympani)
• Posterior wall or mastoid wall (tympanic aditus,
  fossa incudis, pyramidal prominence, facial nerve
  through tympanic sulcus)
• Inferior wall or jugular wall (tympanic plate of
  TB)
• Superior wall or tegmental wall (tegmen tympani,
  continuing posteriorly to tympanic atrium)

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Tympanic membrane

• Oval shaped lateral surface
• Thick periphery called annulus with the notch of
  Rivinus
• Located in a bony groove called tympanic sulcus
• 8-9 horizontal, 9-10 mm vertical axis, 0.1 mm
  thick.
• Held in place by fibers and cartilage
• Cone shaped, translucent
• 55 to 90 mm2 in area

http//www.ghorayeb.com/AuricleEACAnatomy2.html
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• Three layers
• Outer Epidermis of EAM
• Inner Mucosal lining of middle ear space
• Middle Collagen fibers, support for TM.
• Middle layer composed of two sets of fibers
• Radiates outward from center
• Concentric rings of fibers
• Two regions in tympanic membrane
• Superior region Pars flaccida (defined by
  anterior and posterior malleolar folds and
  malleolar prominence)
• Inferior region Pars tensa

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Tympanic membrane, contd.

• Umbo Region of maximum concavity
• Position changes as EAM lengthens after birth
• Important landmarks visible
• Tympanic membrane attached to ossicles.

http//hyperphysics.phy-astr.gsu.edu/hbase/sound/e
ar.html
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Ossicles

• Malleus Hammer
• Incus Anvil
• Stapes Stirrup http//www.ghorayeb.com/StapesPics
  .html
• Head of malleus and body of incus located in the
  epitympanic recess
• http//oto.wustl.edu/bbears/ossicle.htm

http//audilab.bmed.mcgill.ca/daren/3Dear/mid1.ht
ml
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Malleus

• 9 mm long, 23-37 mg weight
• Head with articular facet
• Neck
• Anterior and lateral processes
• Handle or Manubrium

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Incus

• About 23-32 mg weight
• Body
• Short crus/process in epitympanic recess, around
  5 mm
• Long crus/process with lenticular process, around
  7 mm
• (Plural of crus Crura)

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Stapes

• 2-5 to 3.8 mm tall, 2.1 to 4.3 mg weight.
  Footplate area around 3.2 mm2
• Head, with spine for attachment of stapedial
  tendon
• Neck
• Two bony crura, anterior and posterior
• Flat oval bone called footplate
• Medial surface of footplate fastened to wall of
  oval window by annular ligament.
• Unique connection allows for rocking, rather than
  piston-like, movement of stapes.

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Ossicular connections

• Malleus Manubrium of malleus connects to umbo of
  TM.
• Incus Head of malleus connects to body of incus
  (incudo-mallear joint)
• Stapes Inferior process of incus connects to
  form lenticular process, which connects to the
  stapes (incudo-stapedial joint)
• Footplate of the stapes connects to the oval
  window

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Ossicular connections, contd.

• Suspended in the middle ear cavity by axial
  ligaments and muscle tendons
• Middle ear muscles
• Tensor tympani and stapedius

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Tensor tympani

• Enters middle ear through anterior wall of
  tympanum.
• Connected to manubrium/neck of the malleus
• Activated by the trigeminal nerve
• Muscle runs parallel and superior to osseous
  foundation of the ET, separated by septum canalis
  musculotubarii)
• Pulls malleus in anterior and posterior direction

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Stapedius muscle

• Tendon enters the middle ear space through
  opening in the posterior wall of the tympanum
  (pyramidal eminence)
• Attached to the head of the stapes
• Activated by the facial nerve
• Pulls stapes in the posterior direction