Understanding Your Eye
The structure and functioning of the eye are, in many
ways, similar to that of an autofocus camera.
The Cornea
At the front of the eye is the
cornea. This is the clear
transparent curved surface that allows light rays and
images into the eye. It also plays a significant role,
together with the lens, in focusing these images onto the
retina. Hence, it is this
corneal curvature that
LASIK seeks to modify to
correct for refractive errors in the eye. The clarity
of the cornea is also affected by disease, infection
and degenerations and in severe cases, corneal
transplant surgery may be needed to restore clarity to
the cornea and improve eyesight.
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The Anterior Chamber,
Iris and Pupil
Just behind the cornea is the
anterior chamber, a space filled with a clear liquid known as the
aqueous. This is produced by
the ciliary body and its
function is to provide nutrients to the cornea and the
lens. The iris is the
coloured part of the eye with an aperture (or hole) in
the middle called the pupil. It controls the size/diameter of the pupil thereby
adjusting the amount of light that enters the eye.
This is similar to the function of the diaphragm of a
camera controlling the aperture size.
Inflammation of this iris/ciliary body structure
causes a condition called
Iritis or Uveitis and
this can be the result of various local or systemic
diseases
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The Anterior
Chamber Angle
The
anterior chamber angle is
the area where the iris is attached to the peripheral
corneal limbal region. It has the tissues (trabecular meshwork) that are responsible for the reabsorption of the
aqueous from the anterior chamber. Abnormalities in
this area can cause the condition called
Glaucoma. If the
trabecular meshwork is physically blocked by the root
of the iris, Closed Angle Glaucoma results. If it is
the malfunction of the trabecular meshwork itself that
results in poor reabsorption of aqueous in the
presence of an open angle, then Open Angle Glaucoma
can occur.
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The Lens
The clear lens plays a major
role together with the cornea in focusing images onto
the retina. It is held in place by suspensory
ligaments which are attached to the
ciliary body. Contractions of the ciliary body exert forces onto
the suspensory ligaments which will in turn affects
the curvature of the lens to adjust accordingly and
focus the images, whether near or far, onto the
retina. As one ages, the clarity of the lens starts to
change and cloudiness of the lens results in
Cataract formation. If
vision is significantly affected, the removal of the
lens surgically and replacing it with an artificial
intraocular lens implant (IOL) will help restore
sight.
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Vitreous
Body
The vitreous is a
transparent gel like substance that fills the globe of
the eye. As one ages, the gel like nature of the
vitreous becomes more liquid and it is at this stage
that precipitates can sometimes form within the
vitreous causing one to see
Floaters. The
liquification and contraction of the vitreous can also
cause one to see flashes and in some cases cause
retinal holes, tears or detachment.
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The Retina
The retina is the nerve
tissue of the eye that contains photoreceptors that
respond to light and images (this is equivalent to the
film in a camera.) These responses are converted to
electrical impulses and transmitted to the brain via
the optic nerve. There are 2 types of
photoreceptors; Rods, which are more widely
distributed in the retina, are responsible for
peripheral and night vision, and Cones, which are
mostly concentrated in and around the central portion
of the retina called the
macula. The cones are mainly responsible for sharp central
vision and our appreciation of colour. It is here that
a condition known as
Age Related Macular Degeneration
can affect the central vision of a person as he grows
old.
The
Retinal Blood Vessels
The
retinal blood vessels are
fine vessels comprising both arteries and veins. They
carry blood and provide oxygen and nourishment to the
retina. Diseases like diabetes and hypertension can
affect these blood vessels, resulting in a group of
conditions known as
Retinal Vascular Disorders.
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The Optic
Nerve
The optic nerve transmits
the electrical impulses generated by the retina, in
response to images, to the brain. In glaucoma, the
optic nerve head may be compromised due to the
increased intraocular pressure and ‘cupping’ of the
optic nerve results. This may lead to loss of
peripheral vision and if left untreated, the visual
loss will progress until central vision is also
affected. Inflammations of the optic nerve can also
occur (optic neuritis) leading to loss of vision.
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