EYE ANATOMY
The structure of the human eye is so complex that it’s hard to believe that it’s not the product of intelligent design. But by looking at the eyes of other animals, scientists have shown that it evolved very gradually from a simple light-dark sensor over the course of around 100 million years.
It functions in a very similar way to a camera, with an opening through
which the light enters, a lens for focusing and a light-sensitive membrane at
the back.
The amount of light that enters the eye is controlled by the circular and
radial muscles in the iris, which
contract and relax to alter the size
of the pupil. The light first passes through a tough protective sheet called
the cornea, and then moves into the lens. This adjustable structure bends the
light, focusing it down to a point on the retina, at the back of the eye.
The retina is covered in millions of light-sensitive receptors known as
rods and cones. Each receptor contains pigment molecules, which change shape
when they are hit by light, triggering an electrical message that travels to
the brain via the optic nerve.
Fovea : This
pit at the centre of the back of the eye is rich in light receptors and is
responsible for sharp central vision.
Optic nerve : Signals
from the retina travel to the brain via the optic
nerve, a bundle of fibres that exits through the back of the eye.
Blind spot : At
the position where the optic nerve leaves the eye, there is no space for light
receptors, leaving a natural blind
spot in our vision.
Retina : The
retina is covered in receptors that detect light. It is highly pigmented,
preventing the light from scattering and ensuring a crisp image.
Iris : This
circular muscle controls the size of the pupil, allowing it to be closed down
in bright light, or opened wide in
the dark.
Lens : The
lens is responsible for focusing the light, and can change shape to accommodate
objects near and far from the eye.
Pupil : The
pupil is a hole that allows light to reach the back of the eye.
Cornea : The
pupil and iris are covered in a tough, transparent membrane, which provides
protection and contributes to focusing
the light.
Sclera : A
tough white membrane known as the sclera helps to maintain the eye’s spherical shape.
Ciliary body : This
tissue surrounds the lens and contains the muscles responsible for changing its shape.
Seeing In Three Dimensions
Each eye sees a slightly different image, allowing the brain to perceive
depth. Our eyes are only able to produce
two-dimensional images, but with some clever processing, the brain is able to build these flat pictures into a
three-dimensional view. Our eyes are positioned about five centimetres (two
inches) apart, so each sees the world from a slightly different angle. The
brain compares the two pictures, using
the differences to create the illusion of depth.
Individual image
Due to the positioning of our eyes, when objects are closer than about
5.5m (18ft) away, each eye sees a slightly different angle.
Combined image
The incoming signals from both eyes are compared in the brain, and the
subtle differences are used to create a three-dimensional image.
Try it for yourself
By holding your hand in front of your face and closing one eye at a time,
it is easy to see the different 2D
views perceived by each eye.
Protection
The eyes are shielded by several layers of protection. They are almost completely encased in bone
at the back and insulated from shock by layers of muscle and connective tissue.
The front is kept moist with tears and are constantly wiped by the blinking of
the eyelids, while the hairs of the eyebrows and eyelashes catch any debris that might fall in.
Lachrymal gland
Tears are produced here and wash across to the inner corner of the eye,
helping to clean and nourish the
surface.
Eyelashes
Eyelashes not only catch dust before it enters the eye, they are also
sensitive, like whiskers, and the slightest unexpected touch triggers a
protective blink.
Eyebrows
The arch of the eyebrows helps to keep sweat and rain away from the eyes,
channelling it down the sides of the
face.
Vision Problems
The most common problems with our
eyesight
Farsightedness (hyperopia)
If the eye is too short, the cornea is too flat, or if the lens sits too
far back, incoming light is focused behind the retina, making nearby objects
appear blurry, particularly in the dark.
Nearsightedness (myopia)
If the eye is too long, or the cornea and lens are too curved, the light
is focused before it hits the back of the eye, and then starts to defocus again
as it reaches the retina, making distant objects
difficult to see.
Colour-blindness
This rare condition is often linked to a gene on the X-chromosome and
occurs more commonly in men than in women. A defect in the cone cells of the
eye reduces the number of colours that
can be detected.
