The ear is the organ of hearing and balancing. The organ of earing hearing and balance are divided into three parts:
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The external ear
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The middle ear
-
The inner ear
The external and middle ear are involved in hearing
only, whereas the inner ea functions in hearing and balance.
The external ear includes the auricle and external
auditory canal. The external ear terminates medially at the tympanic membrane
or eardrum.
The middle is an air-filled space within the petrous
portion of the temporal bone hat contains the auditory ossicles
The inner ear houses the sensory cells organ for both
hearing and balance. It consists of inner connecting fluid-filled tunnels
chambers within the petrous portion of the temporal bone.
External ear
The auricles or pinna is the fleshly part of the
external ear. On the outside of the head. It consists primarily of elastic
cartilage covered by skin, it shape helps collects sound waves and directs them
towards the external auditory canal. The eternal auditory canal is linked with
hairs and ceruminous gland which produce cerum a modified sebum commonly called
earwax. The hairs and cerum helps prevent foreign bodies from reaching the
delicate tympanic membrane.
The tympanic membrane is a thin, semi-transparent
nearly oval membrane hat separates the external ear from the middle ear. It
consists of three layers: a low simple cuboidal epithelium on the outer surface
and a thin stratified squarous epithelium on the outer surface with a layer of
connective tissue between. Sound waves reaching the tympanic membrane through
the external auditory canal causes it to vibrate. Rupture of the tympanic
membrane can be caused by foreign objects thrust into the ear, an infection of
the middle ear or sufficient differential pressure between middle ear and
outside ear as occurs when changing altitude n an airplane or diving into a
deep water. Rupture of the tympanic membrane may result in hearing impairment.
Middle ear
Medial to the tympanic membrane is the air-filled
cavity of the middle ear. Two covered openings, the round and oval window, on
the medial side of the inner ear. Two additional openings provide air passage
from the middle ear. One opens into the mastoid air cells into the mastoid
process of the temporal bone. The other passage way, the auditory tube, or
pharyngotympanic tube also called Eustachian tube opens into the pharynx and
equalizes air pressure between the outside air and he middle ear causing
unequal pressure between the middle ear and outside environment can distort he
tympanic membrane, dampens its vibration and makes hearing difficult.
Distortion of the tympanic membrane which occurs under
these conditions also stimulates pain fibres associated with it. Because of
this distortion, when a person changes altitude, sound seem muffled and eardrum
may become painful. Swallowing, yawning, chewing and holding the nose and mouth
shut gently forcing air out of the lungs can relieve distorting of the tympanic
membrane these actions open the auditory tube which allows air to pass through
the auditory tube and equalizes air pressure on each side of the eardrum.
The middle ear contains three auditory ossicles: the
malleus, the incus and the stapes which transmits vibration from the tympanic
membrane to the oval window. The handle of the malleus is attached to the inner
surface of the tympanic membrane and vibration of membranes causes the malleus
to vibrate as well.
The head of the malleus is attached by a very small
synovial joint to the incus, which in turn is attached by a small synovial
joint. To the stapes. The foot plate of the stapes fit into the oval window and
is held in place by a flexible annular ligament. Two skeletal muscles
originates from bone around the middle ear and insert into auditory ossicles.
The inner ear
The inner (internal ear) or labyrinth (meaning maize)
contains the organ of hearing and balance. It is described into two parts: the
bony labyrinth and the membranous labyrinth.
Bony labyrinth
This is the cavity within the temporal bone lined with
periosteum. It is larger than and encloses he membranous labyrinth of the same
shape that fits into it , like a tube, within a tube. Between the bony and
membranous labyrinth, there is a layer of watery fluid called perilymph, there
is a similar watery fluid, endolymph.
The bony labyrinth consists of:
-
Vestibule
-
Cochlea
-
Three semi-circular
canal.
The vestibule
This s he expanded part nearest the middle ear, he
oval and round window are located in its lateral wall. It contains two
membranous sacs: the utricle and baccule, which are important in balance.
The cochlea
This resembles a snail’s shell. It has a broad base
where it is continuous with the vestibule and a narrow apex and it spirals
around a central bony column.
The semi-circular canals
These are three
tubes arranged so that one is situated in each of the three planes of space.
Hey are continuous with the vestibules.
Membranous labyrinth
This is a network of delicate tubes, filled with
endolymph. It comprises:
-
The vestibule which
contains he utricle and saccule
-
The cochlea
-
The three semi-circular
canal
The cochlea
The cross-section of the cochlea contains three
compartments:
-
The scala vestibule
-
The scala medial or
cochlea duct
-
The scala tympanic
In cross section, the bony cochlea has two compartment
containing perilymph: the scala vestibule which originates at oval window and
the scala tympanic which ends at the round window. The two compartments are
continuous with each other. The cochlea duct is part of the membranous
labyrinth and is triangular in shape on the basilar membrane or base of the
triangle are supporting each cell and specialized cochlea hair cells containing
auditory receptors. The sensory organs that responds to vibration by initiating
nerve impulses that are perceived as hearing within the brain. The auditory
receptors are dendrites of efferent (sensory) nerves that combines forming the
cochlea (auditory) part of the vestibules cochlea nerves (8th cranial nerve)
which passes through a foramen in the temporal bone to reach the hearing area
in the temporal lobe of the cerebrum.
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