Anatomy of the ear

Ï.ÍÓíä ÌÇÓã

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. .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.
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.
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.
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 8th 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.