Download The external ear

‫حسين جاسم‬.‫د‬
The ear
L.1 Anatomy
The ear is an organ responsible for hearing sensation & balance
composed of 3- main parts:
1: the external ear: - consist of three parts: a. the auricle b. external
ear canal or meatus c. tympanic membrane(ear drum).
2: the middle ear: - it contain the auditory ossicles & this part
communicates with:
a. Eustachian tube
b. mastoid air cells.
3: the inner ear :- consist of 2- parts:
a. bonny labyrinth: localized in the temporal bone & consist of
the semicircular canal & the cochlea.
b. Membranous labyrinth: consist of :
1. Sac of the endolymph
2. Sac of the perilymph.
The external ear
A. The auricle:
It’s also called the (pinna) & is attached to the lateral surface
of the head, consisting of a yellow elastic cartilage covered by
perichondrium & skin .It has a medial & a lateral surfaces, the
lateral surface is bridged & the skin is closely adherent to this
surface, while the medial surface is smooth & the skin is loosely
attached to it.
The lobular part of the auricle is free of cartilage & consists of
skin & fibrous tissue.
B. The external auditory canal (EAC):
It extend from the auricle laterally to the tympanic
membrane
medially,its length is( 24 mm ) . The direction of the EAC is not
like an artificial tube, it’s a convoluted tube & this serves a
physiological function, so in its lateral (1/3)(8 mm) its directed
upwards &backwards, deep in the head in its medial (2/3)(16
mm) its directed downwards &forwards, this is important in ear
examination especially if we want to see the ear drum.
The EAC is composed of 2- parts:
1
1. Cartilaginous part: which represent the lateral 1/3 of the EAC
& is covered by skin which contains hair & sebaceous glands
& wax secreting glands , the skin here is thick & undergoes
exfoliation towards the surface & obliquely outwards,the wax
is a bactericidal material containing various amino acids,fatty
acids ,lysozymes & immunoglobulines.
2. Bonny part: its located deep in the petrous bone,representing
the medial 2/3 of the EAC & reaches the tympanic membrane
it’s also covered by skin which is thin,loose & easily
damaged,there are no hairs & no wax secreting glands.
The Middle ear
Its nearly a box shaped cavity (biconcave medially &
laterally) its also called the tympanic cavity or the middle ear cleft
which include the mastoid air cells & the Eustachian tube, its
situated in the temporal bone between the EAC & the inner ear
structures,the middle ear cavity is larger than the tympanic
membrane.
It has a superior, inferior, lateral, medial, anterior &
posterior walls with the following dimentions:length(15
mm),height(15 mm) & width(3.6 mm).
It’s divided into 3- parts:
a. Epitympanum: it lies above the tympanic membrane.
b. Mesotympanum: it lies opposite to the tympanic membrane.
c. Hypotympanum: it lies below the tympanic membrane.
The contents of the middle ear cleft are:
1. Chorda tympani nerve: which is a branch of the facial nerve that
supplies the parotid salivary gland & runs in the fallopian canal.
2. 2- small muscles:
a. Tensor tympani: attached to neck of the malleus.
c. stapedius: attached to the neck of the stapes.
3. The auditory ossicles: which are three bones connected to each
other:
a. Malleus: the handle of the malleus is firmly attached to middle
layer of the tympanic membrane (the fibrous layer) & its head
articulates with the body of the incus.
b. Incus: has 2-processes:
1- Short process: attached to the tip of the aditus.
2- Long process: attached to the head of the stapes.
2
c. Stapes: resembles stirrup & its footplate occupies the oval
window.
Walls of the middle ear:
1. The lateral wall: it’s formed mainly by the tympanic membrane
that separates the middle ear from the EAC.
2. The medial wall: it separate the middle ear (ME) from the inner
ear & is formed mainly by the bony cochlea & 2-openings the oval
& the round windows, these windows are not opened but is
covered by membrane & are important in conducting sound
waves to the inner ear. The oval window is above the promontory
(the bony cochlea) while the round window lies below the
promontory. The facial nerve run horizontally above the
promontory & is covered by a thin layer of bone.
3. The superior wall: it separates the (ME) from the middle cranial
fossa or cavity.
4. The inferior wall: it separates the ME from the bulb of the
internal jugular vein.
5. The anterior wall: it separate the ME from the internal carotid
artery,in the lower part of the wall there is an opening for the
Eustachian tube (which connect the ME to the
nasopharynx).tensor tympani muscle enter this wall through a
canal lies above the Eustachian tube opening.
6. The posterior wall: it has the opening of the aditus, which connect
the ME to the mastoid antrum.
The tympanic membrane:
It separate the EAC from the ME, its oval in shape,
semitransparent, pearly gray in color like a finger nail, its
longitudinal diameter is larger than the transverse diameter , its
concave laterally towards the EAC .
Structurally its divided into unequal parts, the larger part is
called the pars tensa which consist of 3-layers:
1. Single epithelial layer (laterally).
2. Fibrous layer (middle).
3. Mucous layer (thin & located medially).
While the other part of the eardrum is called: pars flaccida, which
is smaller than the pars tensa & has a similar layers but lacking the
fibrous layer in the middle.
3
The tympanic membrane also has a bright surface which reflect
light located in the anteroinferior part of the tympanic membrane &
end up in a depression called: umbo this surface is triangular in
shape & called the cone of light.
Communications of the ME:
The ME communicates with the nasopharynx through the
Eustachian tube & with the mastoid antrum through the aditus.
Eustachian tube:
It’s a tube connecting the ME with the nasopharynx, it’s 36-mm
in length & is directed downward, medially & forwards. In children
the tube is shorter, wider & more horizontal than in adults.
Its composed of 2-parts ; the bony part(lateral 1/3) & the
cartilaginous part(medial 2/3).
The whole tube is lined by mucous membrane that contains
goblet cells, physiologically the tube is closed at rest, but is opened
during swallowing &yawing. The tensor palati muscle is attached to
its wall & when it contract it causes E.tube opening.
Mastoid air cells:
Its located in the mastoid process (that appears as a projection
behind the auricle) it gives origin to the sternomastoid muscle &
considered as part of the temporal bone.
The mastoid air cells is a communicating cells (not isolated) the
number & the size of the cells vary between individuals, sometimes
its completely absent & here the mastoid process is called scleroting
bone, the mastoid process is normally absent at birth but it appears
as a process at the age of 2- years.
The function of the air cells is as a reservoir of air in normal
atmospheric pressure , but during pressure changes it may be filled
by blood , fluid , pus.
The inner ear
It’s called: the labyrinth & is an important part of the ear &
most of the symptoms & diseases are related to it, its located in the
petrous part of the temporal bone, between the ME & the EAC.
4
It measures about 20-mm length & consists of 2-parts; the
membranous labyrinth located within the 2nd part the bony labyrinth,
which is a bony cavity, the membranous part consist of a delicate sacs
& canals filled with fluid called: endolymph, while the space between
the bony & membranous labyrinth is filled with fluid called:
perilymph.
The membranous labyrinth consist of 2-parts:
1. The cochlea: which is a snail like structure (2 & ½ turns) this part
contain the organ of Corti responsible for hearing sensation by its
neuroepithelia located on the basilar membrane, when the sound
waves reach the ear it causes vibration of the ossicles, which
transmit the vibration to the oval window & causes vibration of
the fluid in the cochlea leading to vibration of the tectorial
membrane & stimulation of the auditory cells (neuroepithelium)
which passes to the higher centers through the cochlear part of the
8th cranial nerve.
2. The vestibule: it consists of 3-semicircular canals (horizontal,
anterior vertical & posterior vertical) & 2-sacs (utricle & saccule) ,
the neuroepithelia in this part of the inner ear is responding to
any change in position or movement & hence is responsible for
balance & equilibrium .
The stimulation of this system causes vibration of the
fluid in the membranous vestibule & cause movement of the
cupola (gelatinous material) causes stimulation of the hair cells
which transmit the impulses to the vestibular part of the 8 th
cranial nerve.the balance & equilibrium is a function of 3-systems
the ear, the joints & the cerebellum (it need 2-systems healthy to
gain a normal balance. Loss of balance or equilibrium causes
vertigo while affection of the cochlea causes deafness.
5