EXTERNAL EAR
Ear consists of three
parts, namely external ear, middle ear and internal ear.
External ear is formed by
two parts:
1. Auricle or pinna
2. External auditory
meatus.
AURICLE OR PINNA
Auricle or pinna of the
external ear consists of fibrocartilaginous
plate covered by connective tissue
and skin. This plate is
characteristically folded and ridged. Skin covering this plate is thin and
contains
many fine
hairs and sebaceous glands. On the posterior surface of auricles, many
sweat glands are present. In many animals, auricle can be
moved and turned to locate the source of sound or
the auricle can be folded to avoid unwanted sound.
But in man, extrinsic and intrinsic muscles of auricles
are rudimentary and the movement is not possible. The
depression of auricle, which forms the orifice of
external auditory meatus, is called concha.
EXTERNAL AUDITORY
MEATUS
External auditory meatus
starts from the concha and extends inside as a slightly curved canal, with a
length of about 55 mm.
Meatus consists of two
parts:
i. Outer
cartilaginous part
ii. Inner bony part.
i.
Outer
Cartilaginous Part
Cartilaginous part is the
initial part of external auditory meatus and is made up of cartilage. It is
covered
by thick skin, which
contains stiff hairs. These hairs prevent the entry of foreign particles. Large
sebaceous glands and ceruminous glands are also present in the skin
covering this portion. These
glands are coiled and
tubular in nature and open on the surface of the skin. Columnar epithelial
cells of the glands contain brown pigment granules and fat droplets. Secretions
of ceruminous glands, sebaceous glands and desquamated epithelial cells form
the earwax.
ii. Inner Bony Part
Inner part of the external
auditory meatus is also covered by skin, which adheres closely to periosteum.
Only
sebaceous glands are present here. Small fine hairs are present on the
superior wall of the canal. Skin covering
this portion is continuous with cuticular layer of tympanic membrane.
MIDDLE EAR
Middle ear
or tympanic cavity is a small, narrow, irregular, laterally compressed
chamber, situated within
the temporal bone. It is
also known as tympanum. It is separated
from external auditory meatus by tympanic membrane.
Middle ear consists of the
following structures:
1. Auditory ossicles
2. Auditory muscles
3. Eustachian tube.
Tympanic Membrane
Tympanic
membrane is a thin, semitransparent membrane, which separates the middle
ear from external
auditory
meatus. Periphery of the membrane is fixed totympanic sulcus in the
surrounding bony ring, by means of fibrocartilage .
Structure of Tympanic
Membrane
Tympanic membrane is
formed by three layers:
1. Lateral cutaneous layer,
which is the continuation of the skin of auditory meatu 2. Intermediate fibrous layer, which contains collagenous fibers
3. Medial mucus layer or
tympanic mucosa, which is composed of single layer of cuboidal epithelial
cells.
AUDITORY OSSICLES
Auditory ossicles are the
three miniature bones, which are arranged in the form of a chain, extending
across the middle ear from
the tympanic membrane to oval window.
Auditory ossicles are:
i. Malleus
ii. Incus
iii. Stapes.
i.
Malleus
Malleus is otherwise
called hammer. It has a
handle, head and neck. Hand is called
manubrium and it is
attached to tympanic
membrane. Neck extends from handle to the head. Head or capitulum articulates
with the body of incus.
ii.
Incus
Incus is also known as anvil. It looks like a premolar tooth. Incus has a body, one long process
and one
short process. Anterior surface
of the body articulates with the head of malleus. The short process is
attached to a ligament.
The long process runs parallel to handle of
malleus. Tip of the long process is like a knob called lenticular process and it articulates with the next
bone, stapes.s
iii. Stapes
Stapes is also called stirrup. It is the smallest bone in the body. It has a
head, neck, anterior crus, posterior crus and a footplate. Head articulates with incus. Footplate fits into oval window.
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