EARS

• The Ear


  External ear, middle ear, internal ear
• External ear


  • Auricle collects air vibrations
  • External auditory meatus conducts sound waves from auricle to tympanic membrane
• External auditory meatus


  • Outer third is elastic cartilage
  • Inner two thirds is bone
  • Lined by skin
  • Provided with hairs, sebaceous and ceruminous glands that secrete wax to prevent entrance of foreign bodies
• Sensory nerve supply of external auditory meatus

  • Auriculotemporal nerve and auricular branch of vagus nerve
• Lymph drainage of external auditory meatus

  • Superficial parotid, mastoid, and superficial cervical lymph nodes
• Middle ear (tympanic cavity)

  Air-containing cavity in petrous part of temporal bone, lined with mucous membrane, contains auditory ossicles that transmit vibrations of tympanic membrane to perilymph of internal ear
• Roof of middle ear

  • Tegmen tympani, part of petrous temporal bone, separates epitympanic recess from dura mater of middle cranial fossa
• Fracture of tegmen tympani

  May cause leaking of cerebrospinal fluid into tympanic cavity
• Floor of middle ear

  • Thin plate of bone, partly related to fibrous tissue, separates tympanic cavity from superior bulb of internal jugular vein
  • Tympanic nerve (branch of CN IX) pierces floor to enter tympanic cavity
• Anterior wall of middle ear

  • Has two openings in upper part: smaller one for tensor tympani muscle, larger one leading into auditory tube
  • Thin bony septum separating the canals is prolonged backwards on medial wall, forming a shelflike projection
• Posterior (mastoid) wall of middle ear

  • Has large, irregular opening (aditus ad antrum) in upper part, entrance to mastoid antrum and mastoid air cells
  • Has a small, hollow, conical projection (pyramid/pyramidal eminence) below aditus, housing stapedius muscle
• Lateral wall of middle ear


  • Largely formed by tympanic membrane
• Medial wall of middle ear

  • Formed by lateral wall of inner ear
  • Has rounded projection (promontory) from underlying first turn of cochlea
  • Has oval-shaped fenestra vestibuli closed by base of stapes
  • Has round fenestra cochleae closed by secondary tympanic membrane
  • Has bony shelf (processus cochleariformis) supporting tensor tympani muscle
  • Has prominence of facial nerve canal as rounded ridge
• Tympanic membrane (eardrum)


  Thin, fibrous membrane that separates outer ear from middle ear, vibrates when sound waves reach it
• Tympanic membrane


  • Concave laterally with a small depression (umbo) at the tip of malleus handle
  • Circular, about 1 cm in diameter, with thickened circumference slotted into a groove in bone
  • Has a notch superiorly with anterior and posterior malleolar folds to lateral process of malleus
  • Has a slack triangular area (pars flaccida) and a tense remainder (pars tensa)
  • Extremely sensitive to pain, innervated by auriculotemporal nerve and auricular branch of vagus
• Auditory ossicles


  Malleus, incus, and stapes, transmit vibrations from tympanic membrane to perilymph of internal ear
• Malleus


  • Largest ossicle, has head, neck, long handle attached to tympanic membrane, anterior process, and lateral process
• Incus


  • Has a large body and two processes: long process descends behind and parallel to malleus handle, short process attached to posterior wall
• Stapes


  • Has a head, neck, two limbs, and a base attached to margin of fenestra vestibuli by annular ligament
• Muscles of the ossicles

  Tensor tympani and stapedius muscles, dampen vibrations of tympanic membrane and stapes
• Tensor tympani muscle


  • Origin: wall of auditory tube and own canal

  Insertion: handle of malleus

  Nerve supply: mandibular division of trigeminal nerve

  Action: dampens vibrations of tympanic membrane
• Stapedius muscle


  • Origin: pyramid (bony projection) on posterior wall of middle ear

  Insertion: neck of stapes

  Nerve supply: facial nerve

  Action: dampens vibrations of stapes
• Movements of auditory ossicles

  Tympanic membrane moves medially, head of malleus and incus move laterally, long process of incus and stapes move laterally

  Base of stapes pushed medially in fenestra vestibuli causes motion in perilymph in scala vestibuli, compression wave passes down scala tympani causing lateral bulging of secondary tympanic membrane
• Leverage and pressure increase in ossicular chain

  • Leverage increase rate of 1.3 to 1, area of tympanic membrane about 17 times greater than base of stapes, total pressure increase of 22 to 1
• Auditory tube


  Also called eustachian or pharyngotympanic tube, connects anterior wall of tympanic cavity to nasopharynx, bony posterior third and cartilaginous anterior two-thirds, serves to equalize air pressure on both sides of tympanic membrane
• Functions of auditory tube

  • Keeping air pressure equal on both sides of eardrum, draining secretions from middle ear, protecting middle ear from pathogens, balancing pressure in middle ear, draining fluid from middle ear
• Mastoid antrum


  Cavity behind middle ear in petrous part of temporal bone, communicates with middle ear through aditus
• Relations of mastoid antrum

  • Anterior wall related to middle ear and contains aditus

  Lateral wall 1.5 cm thick, forms floor of suprameatal triangle

  Inferior wall perforated with holes communicating with mastoid air cells

  Posterior wall separates antrum from other structures
• Middle ear


  Commonly felt as your ears popping
• Mastoid Antrum


  • Lies behind the middle ear in the petrous part of the temporal bone
  • Communicates with the middle ear through the aditus
• Mastoid Antrum


  • Anterior wall is related to the middle ear and contains the aditus
  • Lateral wall is 1.5 cm thick and forms the floor of the suprameatal triangle
  • Inferior wall is perforated with holes through which the antrum communicates with the mastoid air cells
  • Posterior wall separates the antrum from the sigmoid venous sinus and the cerebellum
  • Superior wall is the thin plate of bone, the tegmen tympani, which is related to the meninges of middle cranial fossa and the temporal lobe of the brain
• Mastoid Air Cells


  • Series of communicating cavities within the mastoid process that are continuous above with the antrum and the middle ear
  • Lined with the mucous membrane
• Tympanic Nerve


  • Arises from the glossopharyngeal nerve, passes through the floor of the middle ear and onto the promontory
  • Splits into branches which form the tympanic plexus
  • Supplies the lining of the middle ear and gives off the lesser petrosal nerve
• Bony Labyrinth


  • Consists of the vestibule, semicircular canals, and cochlea
  • Contains the fenestra vestibuli and fenestra cochleae
• Membranous Labyrinth


  Consists of the utricle, saccule, semicircular ducts, and duct of the cochlea
• Vestibule


  The central part of the bony labyrinth, lies posterior to the cochlea and anterior to the semicircular canals
• Semicircular Canals


  • Superior, posterior, and lateral canals open into the posterior part of the vestibule
  • Each canal has a swelling at one end called the ampulla
• Cochlea


  Resembles a snail shell, consists of a central pillar (modiolus) around which a hollow bony tube makes two and one half spiral turns
• Spiral Lamina


  A spiral ledge that winds around the modiolus and projects into the interior of the cochlear canal, partially dividing it
• Basilar Membrane


  Stretches from the free edge of the spiral lamina to the outer bony wall, dividing the cochlear canal into the scala vestibuli above and the scala tympani below