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Monday, October 11, 2010

Anatomy of Ear

Ear has three main parts
External ear
Middle ear
Inner ear

  • External ear:
          It is consist of auricles (pinna), external auditory meatus, and external auditory canal which ends at the the tympanic membrane (ear drum).
          Auricle is consist of an upper helix and anti helix. lower part is called  lobule. The upper part of lobule is called antitragus which lies opposite to the tragus. The opening to the external auditory meatus is called conchae. External auditory meatus is the bony opening into the external auditory canal. The external 1/3 of the external auditory canal is covered by the skin, while the internal 2/3 is covered by mucous membrane. This part of the external auditory canal  contains specialized ceruminous glands which produce cerumen (wax like substance). Cerumen helps to protect the middle ear from foreign particles .External auditory canal is to 2.5cm in length. it ends at the tympanic membrane. Tympanic membrane is 1cm in diameter.
          Point of attachment of the handle of the malleus produces a projection at the centre of the tmpanic membrane called as umbo. Part of the membrane which lies above the umbo is called as pars flaccida. The remaining lower part is called pars tensa. The margins of the tympanic membrane attaches to the groove inside the temporal bone called as annulus.

  • Middle ear
          Also called as tympanic cavity. It is an air filled cavity which lies in the petrous part of the temporal bone. Externally it is separated from the external ear by the tympanic membrane (ear drum). Within the cavity of middle ear lie the three most smallest bones  of the body namely malleus (hammer), incus (anvil), stapes (stirrup). The handle of the malleus is attached to the internal surface of the tympanic membrane at its centre. the head of the malleus attaches to the incus. Incus in turn attaches to the head of the stapes.         
        Footplate (base) of the stapes attaches to the oval window. Two small muscles namely tensor tympani muscle is attached to the tympanic membrane, while the second muscle stapedius muscle is attached to the stapes. These muscle on contraction control the frequency of sound , by altering the vibrations of the the structures to which they are attached (tympanic membrane and stapes). Nerve damage cause hypercoustica. In the anterior wall of the middle ear is an opening which leads into the superior part of the nasopharnyx through a canal called Eusthacian tube (auditory tube).  This tube helps in the maintanance and regulation of atmospheric pressure inside and outside the middle ear.

  • Internal ear
           It is consist of three parts.
Semicircular canals
          All these parts have a bony portion and a membranous portion. Bony part is lined with periosteum and contains a fluid called perilymph. Membranous part is lined with epithelium and contains fluid called endolymph.
          Vestibule is the mddle part of the internal ear. Oval window opens into the vestibule. Vestibule in turn contains two small cavities which are called the utricle and saccule (little sac). Utricle and saccule contains receptors for equilibrium. Utricle is connected posteriorly to three semicircular canals. These three canals are oriented at 90 to eachother. Inside the semicircular canal lie the semicircular ducts which has a confined swelling at each end called ampulla. Ampulla also contains receptors for equilibrium. The three semicircular canals are called anterior, posterior and lateral.

          Conchae is a snail shaped organ which has 2 1/2 (2S) turns. The oval window opens into the vestibule to cochlea through the scala vestibule.The bony part of the cochlea make turns around a central pillar called modiolus. The modiolus at its upper end diverge into Y shaped membranes called vestibular membrane and basilar membrane. On the one side of the modiolus and vestibular membrane lie the scala vestibuli. On the other side of modiolus and basilar membrane lies the scala tympani. Scala vestibuli and tympani communicates with each other only at the apex of cochlea which is helicotrema. Between the vestibular membrane and basilar membrane lies the membranous cochlea which is called scala media (cochlear duct). On the internal surface of basilar membrane lies coiled  and arranged  in coiled form cells called hair cells and supporting cells. Hair cells are further divided into inner and outer hair cells.  Inner hair cells are arranged in 1 layer, while outer hair cells are arranged in 3 layers. These cells are covered by a thin membrane called tectorial membrane.


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