Home' Trinidad and Tobago Guardian : May 26th 2013 Contents 4
HOW ARE OUR EYES MADE UP?
"The human eye is indeed an intricate work of art. The
eye is about the size of a ping pong ball and is made up
of the sclera which we commonly call 'the white of the
eye.' The sclera is tough, opaque tissue that covers and
protects the eye. Six minute muscles connect to it
around the eye and control the eye's movements. In ad-
dition, the optic nerve is attached to the sclera at the
back of the eye.
The cornea is the clear window of the eye. It transmits
and focuses light into the eye, however it is difficult to
see since it is really clear tissue. Just behind the cornea
are the iris, the pupil and the anterior chamber. The iris is
the coloured part which helps to regulate the amount of
light that enters the eye.
At the centre of the iris is the pupil (the black circle in
the eye). This is in fact an opening in the iris which al-
lows light to enter the eye. The pupil changes sizes to
allow a certain amount of light to enter the eye. Pupils
tend to get smaller when light shines brightly and open
wider when it is darker.
The anterior chamber is a fluid filled space between
the cornea and the iris. This fluid nourishes the eye and
keeps it healthy.
The light will enter through the pupil hitting the clear
and colourless lens which is suspended by a bunch of fi-
bres. The lens must now focus the rays of light on the
back of the eyeball to what is known as the retina. This
is a nerve layer that lines the back of the eye. The retina
senses light and creates impulses that are sent through
the optic nerve to the brain so that the brain can under-
stand what the eye is seeing.
The fibres which suspend the lens are attached to a
muscle called the ciliary muscle. This muscle is responsi-
ble for changing the shape of the lens.
The vitreous, the biggest part of the eye, is located be-
hind the lens.
This gives the eye its shape and it is filled with a clear,
jelly-like material called the vitreous humor. When light
passes through the lens, it shines through the vitreous
humor to the back of the eye.
The aqueous humor is located between the lens and
the cornea and helps to maintain correct pressure bal-
ance in the eye chamber," explained the Optometrist.
HOW DO WE SEE COLOUR?
Let's break down how our eyes see colours. Just as we
would use a knife and a fork when eating, the retina uses
special cells called rods and cones to process light. How-
ever, we only use one knife and one fork. The retina in
each eye uses about 120 million rods and 7 million cones.
Rods, which are extremely sensitive, help us to see in
black, white, and shades of grey. They also help us to tell
the form or shape of something.
Cones, on the other hand, sense colour. Most cone
cells are found in the macula, which is the central part of
the retina. They need more light to work than rods.
There are three types of cones and each type is sensitive
to one of three different colours --- red, green, or blue. To-
gether, they help you to see millions of colour. The optic
nerve at the back of the eye carries messages from the
eye to the brain at high speeds this way allowing you to
discern what is being viewed.
As the Optometrist clarified, "We have different photo
receptors in the eyes -- short, medium and long wave
length receptors (or cones). Different colours have dif-
ferent wave lengths and using each type of photo recep-
tor helps us to tell colours.
There are some people who are born with one or two
of these receptors and this leads to colour deficiency
which is the politically correct term for colour blindness.
Such persons who are born with less than three recep-
tors have red- green colour deficiency. People who have
been under radiation and who have had malaria may
have blue-yellow deficiency and this would be referred to
as Acquired Colour Vision Deficiency. Diabetes can also
affect a person's perception of colour -- usually red-green
is affected and so too can medication."
So can this be fixed? The Optometrist confirmed, "If
the receptors are damaged, no, it cannot be fixed." A
test, called the Ishihara test, can be done to measure
colour deficiencies and according to the south-based Op-
tometrist, "It is of utmost importance to check for these
deficiencies in children since it can determine career
choices. For example, becoming a pilot would be impossi-
ble if one is colour blind."
Thus, we can see why our eyes are truly important.
Caring for our eyes is an important job, especially if you
want to continue to enjoy the beauty all around us.
"One in every 12 men and one in every 200 women will suffer from colour deficiency."
These were the words of one Optometrist as he spoke to Health and Fitness about the
How many of us know what "colour deficiency" is, and generally, how our eyes work?
The south-based optometrist simplified things for us thusly.
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