| Main
Menu
| A brief history
of spectacles | Parts of the
eye | The workings of the
eye | Eye Health
The Eye
Iris – The iris
is the coloured part of the eye. There are two major muscles in
the iris, the sphincter muscle which cause it to contract in bright
light and the dilator muscle which cause it to dilate in low light,
controlling light levels in the eye, very much like the aperture
of a camera.
Pupil – The pupil
is the dark centre of the eye. The pupil allows light through
the eye and its size is controlled by the dilator and sphincter
muscles of the iris. The pupils reaction to light is often used
to evaluate a patient’s neurological function.
Lens – Situated
behind the iris, the lens is transparent and flexible. It also
provides about a third of the eye’s power. The inner part
of the eye is called the nucleus and is surrounded by a softer
material, the cortex. The lens is suspended in place by zonules.
As you get older, the lens gradually hardens, reducing the ability
to accommodate or focus.
Cornea – The cornea
is the transparent area at the front of the eye which allows light
to pass through the eye. By acting as a powerful refracting surface,
it provides 70% of the eye’s total focusing power, the remaining
30% provided by the lens. With no blood vessels present in the
cornea, it is clear and shiny but with more nerve endings than
anywhere else in the body, the cornea is extremely sensitive.
Retina – The retina
consists of light sensitive cells known as rods and cones or photoreceptors
that convert light into electrical impulses. The impulses travel
along the optic nerve to the brain where they are turned into
images. There are approximately 6 million cones in the retina
which function best in bright light and help us to see colours,
and approximately 125 million rods which help us with night vision
and peripheral vision.
Sclera – The Sclera
is the white part of the eye. It is opaque and very tough and
protects the rest of the eye. There are six small muscles connecting
it around the eye which provide the eye’s movements. White
in younger people, the sclera does tend to yellow with age.
Conjunctiva – The
conjunctiva is a thin transparent tissue that covers the outer
surface of the eye. Its is fed by virtually invisible blood vessels.
It also secretes mucous and oil to help lubricate the eye.
Macula – Located
at the centre of the retina, this highly sensitive part of the
eye is responsible for detailed central vision. It allows us to
perform more detailed tasks such as reading.
Fovea
– The fovea is located at the very centre of the macula, as
the central point of the retina and consist entirely of cones.
Other articles of interest on the Glasses2you web site:
A
brief history of spectacles | Designer
glasses | Glasses
| Glasses Direct
| Glasses
for computer work | Guide
to buying glasses online | Reading
Glasses
|
