ear anatomy

Surgical Anatomy of the Ear Lecture No. 1
The ear is divided to External,Middle and Inner ear.
The external ear:
It composed of auricle (pinna) and the external auditory meatus.
It’s function is to collect and transmit sound to the tympanic membrane
The auricle is composed of cartilage covered with perichondrium to which the skin are very closely adherent
The lateral surface has characteristic prominences and depressions which are different in every individuals even identical twins,the curved rim is helix,anterior and parallel to it is another prominence,antihelix.Superiorly this divided into two crura,between which is is the triangular fossa,above the two crura is the scaphoid fossa. In front of antihelix,and partly encircled by it,is the concha Below the crus of the hlix and overlapping the external auditory meatus is the tragus.Opposite to it at the inferior limit of antihelix is the antitragus,below the antitragus is soft area composed oaf fibrous and adipose tissue called lobule.
The external auditory meatus:
The outer third is cartilaginous, t he inner two are bony , the outer cartilagenous portion is
covered with skin that contain hair follicles,sebaceous glands and
cerminous glands which secrete wax While these structures are lost in the inner bony meatus where the
skin is thin and hair-free
Owing to the tight union of cartilage and skin any inflammatory process will be extremely painful
The EU canal extends from the concha of the auricle to the TM is approximately 2.4 cm ,the diameter of the canal varies greatly between individuals and between different races
In adult the cartilagenous portion runs inward and slightly downwards and forward Therefor the canal is straightened by gently moving the auricle upwards and backwards to counteract the direction of the cartilagenous portion.
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In the neonate,there is vitually no bony external meatus as the tympanic bone is not not yet developed
So that the auricle must be gently drawn downwards and backwards for the best view of the tympanic membrane
Tympanic membrane:
The tympanic membrane or ear drum is oval in shape and measures about 1cm indiameter and supported around its periphery by a fibrous thickening (the annulus).This fibrous annulus fits in turn into a slot in the tympanic bone.
Ther is small deficiency superiorly,called the notch of Rivinus.
Tne ear drum consists of three layers
The outer layer is epithelial layer continous with the skin.
The middle layer which is fibrous layer
The inner layer which is mucous layer continous with the lining with tympanic cavity
The tympanic membrane is divided into two parts:
Pars tensa and pars flaccida
Pars flaccida is the most superior part occupying the notch of Rivinus and its medial layer is comprised of irregular elastic fibers,hence the flaccidity,,it issmall sometime difficult to see,it called some time (attic),perforation in this area are potentially unsafe.
The middle ear
The middle ear is an air-containing cavity in petrous part of the temporal bone lined with mucous membrane ,it contain the auditory ossicles,it is narrow,oblique,slit like cavity whose long axis lies approximately parallel to the plane of tympanic membrane.
It divided into:
?Epitympanum? upper most portion or attic above the level of the mallear fold
?Mesotympanum? middle portion
?Hypotympanum? lower portion
The middle ear cleft or tympanic cavity is an air filled space situated within temporal bone ,it made up of:
1 mastoid air cell
2 middle ear cleft
3 tympanic membrane
4 Eustachian tube
Function of the middle ear
Transmit sounds,which reach the TM in the form of air pressure waves,to the inner ear where a liquid wave is set up.
The sound energy is transmitted across the middle ear by a chain of three bones malleus,incus and stapes called ossicles the ossicular chain together with the ear drum amplifies the sound energy
The middle ear has six portions:
Roof(superior),floor(inferior),anterior wal,posterior wall,medial wall,lateral wall
Floor
The Ossicles
The auditory ossicles are:
Malleus
Incus
Stapes
The malleus is largest ossicle,had a head,a neck,a handle or long process,an anterior process,and alateral process
The head is rounded and articulate posteriorly with the incus.The neck is cnstricted part below the head.
The handle pesses downward and backward and firmily attach to the medial structure of the tympanic membrane,it can be seen through the tympanic membrsne on otoscopic eqxamination.
The anterior process is a specula of bone connected to the anterior wall of the tympanic cavity by a ligament.
The lateral process project laterally and attached to the anterior and posterior malleolar fold
The incus posses a large body and two processes
The body is rounded and articulates anteriorly with the head of malleus
The long process descends behind and parallel to the handle of malleus.Its lower end bends medially and articulates with head of stapes.Its shadow on T.M can sometime be seen on otoscopic examination.
The short process projects backward and is attached to the posterior wall of tympanic membrane by a ligament
The stapes has a head,a neck,two limbs(crura) and a base(foot plat

The head is small and articulates with the long process of incus.
The neck is narrow and receives the insertion of stapedius muscle
The two limbs diverge from the neck and attached to the oval base or foot plate
The foot plate is 3mm x1.4mm and it lies in the oval window
The Eustachian tube
The auditory tube extends from the anterior wall of tumpanic cavity downwards,forwards,and medially to the nasopharynx.
Its posterior third is bony,its anterior two-third is cartilaginous.
It serves to equalize pressure of air in tympanic cavity and nasopharynx..
The inner ear:
Called also the labyrinth,it consists of bony capsule that is almost embedded in the petrous part of temporal bone ,it consists of:
Bony labyrinth,comprising a series of cavities within the bone
Membranous labyrinth,comprising of series of membranous sacs and ducts contained within the bony labriynth
Bony labryrinth
It consists of three parts :
1-The vestibule
2-The semicircular canals
3-The cochlea
They are lined by endosteum and contain a clear fluid called the perilymph
The vestibule is the central part of the bony labyrinth .In its lateral wall is the fenestra vestibule (oval window) which is closed by the base of stapes,and the fenestra cochleae (round window) which is closed by secondary tympanic membrane.
Logged within the vestibule are the saccule and utricle of the membranous labyrinth
There are three semicircular canals,superior,posterior and lateral---- -open in posterior part of the vestibule by five orifices
The superior and posterior are vertical while the lateral semicircular canal is set in horizontal position
The cochlea resembles a snail shell,it opens in the anterior part of the vestibule.It consists of a centeral pillar(the modiolus) around which a tube makes two and one half spiral turn.The cochlea is divided by a membrane into scala vestibule above and scala tympani below
Membranous labyrinth
It is logged within the bony labyrinth and filled with endolymph and surrounded by perilymph and consists of utricle and saccule which are logged in the bony vestibule,also contain three semicircular ducts which lie within the bony semicircular canals,also contain the duct of cochlea which lie within the bony cochlea
Physiology of hearing
Airborne sound consists of vibration of the atmosphere and the purpose of auditory apparatus is toconvert this vibrations in air to vibrations in the inner ear fluid,and then to nerve impulses to be transmitted along the auditory nerve to the higher centrese of hearing.
The auricle collect sound waves to some extent,then pass along the external auditory meatus to the tympanic membrane,the vibration of tympanic membrane are transmitted to the malleus,incus and stapes.
Then the sound transmitted to the oval window,causing the vibration to be set up in the endolymphatic and prilymphatic compartments of the inner ear,so the middle ear structure convert the sound from air to fluid
The stapes moves in a rocking rather than a piston motion and as the fluids cannot be compressed,these vibrations are transmitted to the round window membrane. This reciprocal action of the oval and round windows is essential.
In the normal ear the presence of tympanic membrane and air containing middle ear prevents the sound pressure waves from reaching the round window and opposing the out ward movement of the round window membrane,this protection of the round window is lost when there is large perforation of the tympanic membrane,and this is one factor which may produce deafness.
The tympanic membrane is at its most efficient when the air pressure in the external auditory meatus and the middle ear is equal,and this equalization is achieved by the Eustachian tube.
Then the vibration transmitted to the inner ear produce displacement of the basilar membrane and shearing movement between the hair cell and tectorial membrane of the organ of Corti which intiates nerve impulse in the fibers of auditory nerve
Physiology of balance
The balance of the body is maintained by coordination of information from three systems;
1.proprioception. i.e sensation from muscle,joints,tendons and ligament
2.the eye
3.the vestibular system
The vestibular system cosists of the semicircular canals,the utricle and the saccule.
The semicircular canals respond to angular (rotatory) acceleration while the utricle and saccule respond to linear acceleration.