External Ear structure and function

EXTERNAL EAR

Ear consists of three parts, namely external ear, middle ear and internal ear.

External ear is formed by two parts:

External Ear structure and  function

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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