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the external acoustic meatus, the eustachian tube, the middle ear, the inner ear, the scala media, the important spaces, the physiology of ear.
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Surgical anatomy of the ear
Diksha CheetooRoll number 10
Table of content
• Anatomy of the External auditory meatus vs the eustachian tube
• The anatomy of the middle Ear and the tympanic membrane
• The anatomy of the inner ear especially the scala media
• The middle Ear Transformer mechanism• The important triangles and their significance
Anatomy of the middle Ear
1. The middle ear cleftMiddle ear + ET+ aditus+ antrum+ mastoid air
cells2. Middle Ear The hypotympanum
Lies below pars tensa
Epitympanum
Lies above pars tensa
Mesotympanum
Lies opposite the pars tensa
The middle ear can be linked to a six- sided box.
The roof
• The tegmen tympani
The floor
• A thin plate of bone which separates tympanic cavity from the jugular bulb
The medial wall• Promontory- bulge due
to coil of cochlea• Round window/fenestra
cochlea- lies post-inf to scala tympani
• Oval window-opens in scala vestibuli, foot plate of stapes is fixed to it
• Prominence of facial nerve canal-lies above oval window
• The processus cochleariformis- ant to oval window, hook like projection- for the tendon of tensor tympani
The lateral wall
• The tympanic membrane
• It is semitransparent• Pearly grey• Thickness is 0.1-
0.15mm• Placed at 55 degrees
from the floor• It separates the EAM
from ME
• Peripheral part of the TM is thickened to form a fibrocartilaginous ring attached to the tympanic sulcus
• Membrane-• 1. tense part is called pars
tensa • 2. flaccid part is called pars
flacida/shapnell’s membrane
• Outer TM- 3 layers• 1. outer cuticular• 2. middle fibrous of inner
circular and outer radiating fibres
• 3. inner mucosal• Blood supply-1. deep
auricular branch of maxillary artey 2.post auricular artery 3. tympanic branch of maxillary artery
• Veins- ext jug vein and transverse sinus
The posterior wall• Lies close to mastoid air
cells• Presents a bony
projection called as the pyramid/pyramidal recess- contains the stapedius muscle
• Aditus- opening through the attic, communicates with the antrum- lies above the pyramid
• Facial nerve runs behind the pyramid
• Facial recess/ suprapyramidal recess
• Collection of air cells lying lateral to facial nerve
• Medially- ext genu of facial nerve
• Lat-chorda tympani• Sup- fossa incudis• Anterolaterally- tympanic
membrane.
• The importance of this recess is that one can approach the ME without disturbing the post meatal wall
MCQ
• True about the facial recess is :
A. Bony landmark on the lateral wall of middle ear
B. Separates facial from vestibular nerve
C. Lateral boundary by fossa incudis
D. One can enter middle ear without removing posterior bony meatal wall
• May 2013
• Facial recess is bounded laterally by :
A. Facial nerveB. Chorda tympani nerveC. Short process of incus D. ponticulus
MCQ
• Facial recess is bounded by all except :
A. Facial nerveB. Chorda tympani
nerveC. Short process of
incusD. ponticulus
Anatomy of the EAM vs ET
EAM• LENGTH
24mmOuter 1/3rd is
cartilaginous and inner 1/3rd is bony
ETLength 36mmAlso called pharyngotympanic
tubeOsseus part is 12mm – from
ant tympanic wall, narrows to the end to attach to the cartilaginous part -24 mm
EAM• COURSE• Forms a mild s –shape• Curve is directed medially
upwards and forwards and then medially backwards downwards
ET• Descends at an angle of 45
degrees with sagital plane and 30 degrees with horizontal plane
• In infants the auditory tube is wide and short and is placed horizontally
EAM• RELATIONS1. Floor and ant part of the
meatus are longer than the roof and post part
2. ant, inf and post bony part is formed by the tympanic part of the temporal bone and roof is formed by the squamous part of the petrosal bone
3. The meatal recess is present in relation to the inf wall of the meatus
4. Ceruminous glands and hair are present mostly in sbc tissue of cartilaginous part.
ET
1. The muscles of ET are • Tensor palati• Levator palati• Tensor palati separates the ET
from the ottic ganglion, mandibular nerve , chorda tympani nerve and middle meningeal artery
EAM
• SUPPLY• Nerve supply• The ant and sup wall are
supplied through the auriculotemporal branch of mandibular nerve and inf wall is supplied by the auricular branch of vagus
ET
• Tympanic plexus and pharungeal branches of pterygopalatine ganglion.
Anatomy of the inner Ear
internal ear
Bony labyrinth Membranous labyrinth
bony labyrinth
Vestibule Semicircular canals
cochlea
Conical snail shell
At its base – 3 openings-oval window, round
window and cochlear
canaliculus
3 scc
Ant postlater
al
Forms central part of bony
labyrinth
Membranous labyrinth
utricle saccule Semicircular ducts
Scd open in it through 5
ampullary endsThe sensory part is
called maculaConcerned with
linear acc and dec
Connected to utricle through the utricosacular duct
3 in numberCorrespond to 3
bony canals. They open in utricle. The ampullated end of each duct
contain neuro epithelium called crista ampullaris
Cochear duct/ Scala Media
• Lies within the bony cochlea
• It is a blind coiled tube• Occupies the middle
part of the cochlear canal between the scala vestibuli and scala tympani
• It is
Floor- basilar membraneRoof – vestibular
membraneOuter wall- stria
vestibularis- contains vascular epithelium and secretes endolypm
The basilar membrane supports the organ of corti
The OOC is innervated by peripheral process of bipolar cells located in the spiral ganglion
• The ganglion is present within the spiral canal present within the modiolus at thebase of spiral lamina
• The central process of the ganglion form the cochlear nerve
• Post the scala media is connected to saccule by the ductus reuniens
• OOC consists of2 rows of cells
• External rod cells ( 4000)
• Internal rod cells (6000)• Supporting cells of
deiters are found between the outer hair cells
• Cells of hesner’s lie outside the deiter cells
Middle ear transformer mechanism/ impedance matching• Impedance matching is
the process in which the TM and auditory ossicles convert the sound energy to mechanical vibrations in the fluid of inner ear with min loss of energy by matching the impedance offered by fluid
• Hydaulic action of the TM
• Effective vibratory area of the TM is nearly 45- 55 mmsq as compared to footplate of stapes (3.2 sqmm). Hence it is 14 X greater.
• Hence when sound waves fall on the TM part of the sound is transmitted through the ossicles
• Lever action of the ossicles
• Handle of malleus is longer than the incus by 1.3 X so the pressure exerted at the oval window works out to be 14x 1.3= 18.3 times
Important triangles and their significance
• Macewan’s triangle• Sup- suprameatal recess• Antero-inf by the post
sup margin of the EAM• Post- tangent drawn
from zygomatic arch• Significance-1. Spine of henle lies in
the triangle2. Mastoid antrum lies
15mm deep
• Trautmann’s triangle• Post- sigmoid sinus• Ant- bony labyrinth• Sup-sup petrosal sinus• Significance• Inf into the post cranial
fossa can spread through this triangle and be can be approached by removing bone in between the triangle.
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