Anterior Chamber
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This is the front portion of the eye containing
aqueous fluid. It is bounded in front by the cornea and in the
back by the iris and lens. An inner mucus film, produced by tiny
cells, adheres to the anterior globe surface as well. |
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| Aqueous Humor
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This transparent liquid is found in both the anterior
and posterior chambers of the eye. It has a chemical composition similar
to blood plasma but lacking the high protein content of the latter. This
liquid also feeds the cornea and is continually manufactured by specialized
Tissues in the choroid. The aqueous humor serves to keep the globe reasonably
firm. It is secreted continuously by the ciliary body into the posterior chamber, and flows as a gentle stream through the pupil into the anterior chamber, from which it is drained by the way of a channel at the limbus; that is, the juncture of the cornea and the sclera. It finally leaves the eye in the anterior ciliary veins.
The fluid also provides nutrition for the lens and also for the cornea, both
of which are devoid of blood vessels; the steady renewal and drainage serve
to bring into the eye various nutrient substances, including glucose and amino
acids, and to remove waste products of metabolism. |
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| Capsule
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This is the transparent membrane that surrounds and
encloses the lens. |
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| Choroid
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This is a vascular layer beneath the sclera which
supplies the nutrients to all parts of the eye. But it does more than
that. The chorid contains a substance called melanin, the same substance
that gives colour to our skin and hair. The melanin colours the choroid
black and traps stray light. If it wasn't for the melanin, images we see
would be faded on our retina like when the house lights go on at the end
of a movie in a dark movie theater. It also contains the ciliary body,
a muscular structure that alters the shape of the lens in focusing. |
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| Ciliary Body & Crystalline Lens
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The lens of an eye is similar to the lens in a camera.
It is a flexible structure about the size and shape of an aspirin tablet.
Like the cornea, the lens is transparent because it contains no blood vessels
and is relatively dehydrated.
The lens is a mass of tightly packed transparent fibrous cells, the lens
fibres, enclosed in an elastic capsule. The epithelium, covering the anterior
surface of the lens under the capsule, serves as the origin of the lens fibres.
It provides the focusing ability of the eye. It fine-tunes the focus on light
received from the pupil. The muscles of the ciliary body make constant
adjustments in the shape of the lens. It has the capacity to be bent forward
or slightly flattened out by the ciliary body to focus the light rays at the
correct point on the retina.
In its relaxed state, the eye is able to discern distant objects clearly.
As an object gets closer, the eye must change its light-bending power to
accommodate the nearness of the object. Thus, the ciliary body is arranged
so that when they contract, the zonule (connects the lens capsule to the
ciliary body) relaxes, allowing the lens to relax and become thicker in
the center. This increased thickness bends the light further so that near
objects are seen clearly. These adjustments produce a sharp visual image at
all times as the eye shifts focus between nearby and distant objects.
The lens become more rigid with age, so that its ability to be bent and
adjusted decreases and focusing on near objects tends to be more difficult
as a person reaches 40 to 50 years of age. This symptom is known as presbyopia.
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| Ciliary Nerves
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These are sensory nerve fibres that run underneath the
endothelium. They belong to the ophthalmic branch of the fifth cranial
nerve, the large sensory nerve in the head. |
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| Conjunctiva
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This thin transparent membrane lines and seals
the eye's opening from lid to lid. It acts like a barrier to prevent
foreign objects to find its way to the back of our eyes. This very same
membrane is the part of the eye which lets us open our eyes underwater
without flooding our eye socket. The lacririmal gland supports the
conjuctiva in two ways. First, it helps keep the conjunctiva moist.
Second, when the sensitive conjunctiva becomes irritated by an foreign
body, the lacririmal gland springs into action by letting go a flood
of tears to wash our the intrusion. |
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| Cornea
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The cornea is the clear outer surface of the eye,
which provides 60% of the focusing power in the eye. It can be compared
to the anterior window of a camera. It consists of the Epithelium or
outer covering, Bowman's membrane, Stroma or supporting structure,
Descemet's membrane, and the endothelium or inner layer (this is the
tissue that is ablated during laser surgery). The outer layer, the
epithelium is also highly regenerative and heals quickly from superficial
injuries. The four inner layers provide rigidity and protect the eye from
infection. The cornea is very sensitive to pain because of the ciliary
nerves that run just underneath the endothelium. |
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| Eyelids
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The eye is protected from mechanical injury by being enclosed
in a socket, or orbit, which is made up of portions of several bones of the skull
to form a four-sided pyramid the apex of which points back into the head. The
eyeball and its functional muscles are surrounded by a layer of orbital fats that
acts as a cushion permitting smooth rotation of the eyeball about the center of
rotation. The protrusion of the eyeballs-proptosis- in exophthalmic goitre is
caused by the collection of fluid in the orbital fatty tissue. |
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| Glandular Apparatus
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Secretions of the Lachrimal Glands keep the
eye moist. Tears don't evaporate as they flow across the eye
because the secretion of oily mucous material by other glands
prevents it from happening. As the oil secreted emerges onto the
surface of the lid margin, it acts as a barrier for the tear fluid,
which accumulates in the grooves between the eyeballs and the lid barriers. |
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| Iris
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Overlying the lens is the iris that controls
the amount of light entering the eye. It can be compared to the
stop or diaphragm of a camera. It is a structure made of thin
elastic tissue with and opening in the center, surrounded by a
circular muscle called a sphincter. The structure is the iris;
the opening is the pupil.
The iris is what people refer to when they speak about the
color of their eyes. Melanin, a dark brownish-black substance
inside the cornea, determines eye color: the more melanin there
is and the closer it is to the surface of the tissue, the
darker the color of the iris. The Melanin also absorbs strong or
bright light that might otherwise be too overwhelming or cause
blurred vision. Melanin is the same substance that gives skin and
hair their color. |
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Macula |
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The macula is a small area located at the
posterior portion of the retina that functions as the area of
most acute (central) vision. Diseases that affect this area can
cause a decrease in vision. |
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| Nerves
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The visual pathways of the eyes are equivalent
to the co-axial cable that connects your video camera to the VCR.
These nerves transmit visual information from the retina to the
occipital lobe of the brain where the information is processed. |
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| Optic Nerve
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The nerve is similar to the RCA cable that
connects your video camera to the VCR. The image seen in the
eye or the electrical impulses from the eye is transmitted by
the optic nerve to the optic chiasm where signals are directed
to the appropriate places for visual processing. The eyes don't
actually do the visualization itself; it's really the brain. When
your optometrist looks into your dilated eyes, the optic nerve head
is what is seen. |
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| Orbit
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The eye is protected from mechanical injury
by being enclosed in a socket, or orbit, which is made up of
portions of several bones of the skull to form a four-sided
pyramid the apex of which points back into the head. The eyeball
and its functional muscles are surrounded by a layer of orbital
fats that acts as a cushion permitting smooth rotation of the
eyeball about the center of rotation. The protrusion of the
eyeballs-proptosis- in exophthalmic goitre is caused by the
collection of fluid in the orbital fatty tissue. |
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| Posterior Chamber
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This is the space behind the iris, lens and ciliary
body as well as in front of the vitreous humor. |
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| Pupil
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When people say "the black of your eye", they
are talking about your pupil. The pupil really isn't black, what it
is really is the contractile opening in the
middle of the iris, the colored portion of the eye that contracts
or expands, depending on the amount of light striking it. The pupil
is able to regulate the amount of light entering the eye with the
help of the iris. The pupil expands to allow more light into the
eye during low light conditions and contracts to reduce the amount
of light entering the eye during intense light conditions. |
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| Retina
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Behind the pupil, covering the Sclera is a
thin layer of nervous tissue - an extension of the optic nerve -
called the retina, a screen of photoreceptor cells. It is similar
to the silver pieces in the film of a camera that react to light.
It is here that the physical image's light is converted
into electrical impulses and sent along the optic nerve to
the brain. The retina is made of cells called rods and cones and it
has 10 layers. Bits of pigment fill the rods and cones and
absorb particles of light that strike the retina. The pigments
in the rods is called rhodopsin or visual purple, and enables
the eye to discern shades of gray and see in dim light. There
are three types of pigment in the cones that enable the eye to
see colors and to see sharp images in bright light. Cyanolabe
absorbs blue light. Chlorabe absorbs green light and Erythrolabe
absorbs red light. These pigments enable us to see and distinguish
more than 200 colors. |
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| Sclera
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This is the rigid, white, fibrous outer coat
of the eye, continuous with the cornea. |
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| Vitreous Humor
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The vitreous humor is a transparent jelly-like
substance filling the cavity of the eyeball, behind the lens and
in front of the retina. It is continuously secreted by the ciliary body.
This fluid nourishes and lubricates both the cornea and the lens.
Fluid that has already nourished both these parts of the eye flows
into a drainage system that takes the contents away from the eye
via the veins. It also gives the shape to the eyeball and does
not hamper the light rays entering the eyeball before focusing on
the retina. |
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