Ears:
(auditory sense and equilibrium)
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External
or outer ear:
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The
outer ear is designed to collect and channel sound waves
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The
pinna is the portion of the outer ear not connected to the head, also referred
to as the auricle or trumpet
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The
tragus is the small knob of cartilage anterior to the opening of the ear
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The
antitragus is another knob of cartilage, located superiorly to the earlobe
and opposite the tragus
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The
concha is the more interior hollow of the auricle
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The
helix is the the superior and posterior free margin of the outer ear
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The
antihelix is the fold of cartilage between the helix and the concha
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The
triangular fossa is the depression at the superior end of the antihelix
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The
lobule is the earlobe
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From
the pinna, a channel leads through the skull to the middle ear, called
the external auditory canal or meatus. Inside the canal are hairs and modified
sebaceous glands, called ceruminous glands, which secrete cerumen, or earwax,
which traps irritants and is a powerful insect repellent
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At
the end of the external auditory meatus is a thin, semitransparent membrane,
the tympanic membrane, commonly referred to as the eardrum
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Middle
ear: (tympanic cavity)
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The
middle ear is enclosed by the temporal bone
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The
posterior-inferior wall of the cavity communicates with the air cells of
the surrounding bone through the tympanic atrium and leads to the Eustachian
(auditory) tube, which connects to the nasopharynx
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Attached
to the tympanic membrane is a small bone, the
malleus (hammer). It's head articulates with the incus (anvil), which,
in turn, articulates with the stapes (stirrup). All three together are
the auditory ossicles. Small ligaments attach these bones to the walls
of the surrounding cavity
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The
base of the stapes fits into a small opening called the fenestra vestibuli,
or oval window. Directly bellow the oval window is the fenestra cochlea,
or round window. This opening houses the second tympanic membrane
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The
tensor tympani muscle draws the malleus medially to help prevent damage
to the membrane from loud sounds. The stapedius muscle is the smallest
skeletal muscles, used to draw the stapes posteriorly for the same purpose
as the tensor tympani
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Internal
(inner) ear: (labyrinth)
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The
bony labyrinth is made up of the vestibule, cochlea, and semicircular canals.
The bony labyrinth is lined with periosteum and contains a fluid by the
name of perilymph. This fluid surrounds the membranous labyrinth, containing
the fluid endolymph
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The
vestibule is the roughly oval-shaped central portion of the inner ear.
The membranous labyrinth in the vestibule consists of two sacs: the utricle
and the saccule, connected to one another by a small duct
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Projecting
from the vestibule are three semicircular canals. One end of each swells
slightly into an ampulla. The membranous labyrinth in the semicircular
canals are the semicircular ducts. There are three semicircular canals:
superior, posterior and lateral
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The
cochlea is the spiraling, shell-shaped portion of the bony labyrinth, making
about 2 ¾ turns. The central bony portion of the cochlea is called
the mobiolus. A cross-section of the cochlea shows that the canal is divided
into three channels by a Y-shaped partition, the stem of which is mostly
made up of tissue of the membranous labyrinth. The two larger channels
are the scala tympani (slightly larger than the other) and the scala vestibuli.
The perilymph of the vestibule is continuous with that of the scala vestibuli.
The third channels is the scala media, or cochlear duct. It is separated
from the scala vestibuli by the vestibular membrane, and from the scala
tympani by the basilar membrane
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Resting
on the basilar membrane of the scala media is the spiral organ (organ of
Corti), the organ of hearing. The spiral organ is a series of epithelial
cells on the inner surface of the basilar membrane. It consists of a number
of supporting cells and hair cells, which are the receptors form auditory
sensation. The inner hairs are located in a single row running the length
of the cochlea. The outer hairs are arranged in several rows throughout
the cochlea. The hair cells have long, thread-like processes at their free
ends that extend into the endolymph. The basal ends are in contact with
the eighth cranial nerve. Over an in contact
with the hair cells of the spiral organ is the tectorial membrane
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Sound
waves are directed by the external ear to strike the tympanic membrane
and causing it to vibrate, and thereby vibrating the auditory ossicles.
The stapes moves the oval window in and out, causing waves in the perilymph
of the scala vestibuli. The vestibular membrane gives under the waves in
the scala vestibuli, and the basilar membrane gives with the pressure from
waves in the scala tympani. The wave moves through the cochlea and causes
the round window to bulge into the middle ear. When the basilar membrane
vibrates, the hair cells in the scala media move against the tectorial
membrane, generating nerve impulses which travel along the vestibulocochlear
nerve to the temporal lobe of the cerebrum
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There
are two types of equilibrium: static and dynamic. Static equilibrium is
concerned with the orientation of the body relative to the ground. Dynamic
equilibrium is concerned with the maintenance of posture, especially in
the head
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Static
equilibrium receptors are located in the saccule and utricle of the vestibule.
Each contain a small, flat plaque-like region called a macula, where the
receptors are located. They are innervated by the vestibular branch of
the eighth cranial nerve. Microscopically, the maculae are a great deal
like the spiral organ in structure. Each of the hair cells (I have used
the term several times. Please note that these are not the cells responsible
for hair growth) has a major process called a kinocilium and several smaller
stereocilia. Lying over the supporting and hair cells is a gelatin-like
otolithic membrane. This membrane slides over the hair cells and stimulates
them when the head is moved
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The
semicircular canals are concerned with dynamic equilibrium. In each ampulla
is a small elevation called a crista. Each crista is made up of hair (receptor)
cells and supporting cells, and covered by a jelly-like material known
as the cupola. Movement of the cupola stimulates the hair cells
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