How the eye works
The outside of the eye
The white of the eye, round the whole eyeball is called the
sclera, meaning ‘hard’ because it's tough like the
outside of a football. Over the front surface of the white of
the eye and up under the eyelids is a thin, clear skin called
the conjunctiva. That's the part that gets infected and red
when we get conjunctivitis or pink eye. Hidden back behind the
eyelids, underneath the conjunctiva, around the edges of the
eye are the muscles that are used to move the eye. Each muscle
has an individual name according to the direction it pulls the
eye in.
Tears are made in the lacrimal (tear) gland which is tucked
up below the brow bone, above the outside half of the eye.
Tears from the tear gland keep the front of the eye moist and
clean. The eye lids blink to move the tears across the eye. If
you stare without blinking very much at this computer screen
you will feel your eyes become sore and dry. At night time your
eyelids need to fully close to stop your eye surface from
becoming dry as you do not blink in your sleep. If in a deep
sleep have your eyes open they will feel dry and sore in the
morning.
Tears are not connected to the watery liquid inside the eye
at all, they drain away from the eye down two tiny holes called
the upper and lower punctum, one in each of the upper and lower
lids, near the corner nearest the nose. From there, they drain
away into the nose itself via the tear ducts. This is why your
nose runs when you cry and why if your nose is blocked with a
cold your tears run over your eyelids onto your cheeks. As the
back of your nose connects with your throat it is why we taste
eye drops.
The front part of the eye
Light comes into the eye through the clear surface at the
front called the cornea. The cornea or window of the eye is
curved like a shallow dome, like the watchglass in front of the
face of a watch. Behind the cornea is a pocket called the
‘anterior chamber’ or front chamber that's full of
watery fluid called the ‘aqueous humour’ which
means ‘watery fluid’ (usually shortened to just
‘aqueous’'). The part that sits in front of the
iris is called the anterior chamber and the part behind,
between the iris and the lens, which is much smaller, is called
the posterior (or back) chamber.
The iris is the coloured part of the eye and it's actually
a ring of muscle that opens and closes a gap (the black dot in
the centre of the eye) called the pupil. When the light is
bright, the iris automatically makes the pupil smaller and when
it's darker the iris opens up the pupil to allow more light to
go in. This means that the right amount of light comes into the
eye for it to work well whether it's a candle-lit room or a
bright sunny day outdoors. This is just like the diaphragm on
a camera.
Just behind the pupil is the lens, shaped like a lentil or
a Smartie sweet (although a bit flatter at the front and a bit
rounder at the back). It's made of strands of protein laid down
smoothly, like the layers in laminated glass. In a child the
lens proteins form a jelly that gradually becomes thicker and
hard in an older adult.
The cornea bends the light a lot as it comes into the eye,
but the lens bends it a little more so that the picture is
crisply focussed on the retina at the back of the eye.
The lens is held in place by ‘suspensory’
ligaments (called the ‘zonules of Zinn’) and is
attached to a ring of muscle, called the ciliary muscle that
can tighten and relax, allowing the lens to get rounder or
flatter. The natural lens changes shape all the time as your
eye focuses, being more like a ball when you look at close
things and more like a disc or spectacle lens when you look at
things far away. Artificial lenses stay the same shape which is
why they are not as good at focussing at different distances
without the aid of glasses as the natural lens. This change of
shape of the natural lens allows it to focus light crisply
whether it's coming from a mountain in the distance or from a
needle and thread just a few centimetres away. Older people
usually lose this flexibility in the lens so it becomes harder
for them to focus on near objects and they need reading glasses
to see things that are near.
Around the lens proteins is a clear bag called the lens
capsule. The capsule has some specialist cells called
epithelial cells that make new lens material so that the lens
can grow as a child is growing. When a cataract is removed in
a child usually as much lens protein is removed as possible,
but some lens capsule is deliberately left as this may be
useful to secure an artificial lens at the same operation or
in the future. The lens capsule continues to produce lens
proteins that can cause problems in the eye if they cover the
pupil (after cataract or lens pearls) or push up the iris to
block the angle of the eye (angle closure glaucoma).
In a ring around the lens, next to the ciliary muscle is
an area called the ‘ciliary body’ (ciliary means
‘fringed’ and it is called this because it has lots
of little hair-like bits that line the inside of the eye at the
front). The ciliary body makes liquid to keep the eye
‘pumped up’ to the right pressure. The flow of this
liquid around the eye is needed to supply nutrition (food) to
the clear cells inside the eye that don't have blood vessels
and to take away the cells waste products. The liquid goes into
the clear, soft jelly-like stuff called the ‘vitreous
humour’ which means ‘glassy fluid’.
The vitreous fills up the space in the inside of the eye to
keep it in shape and through the middle of it, from the back
of the lens through the vitreous to where the optic nerve
leaves the eye is the faintest line called the hyaloid (or
Cloquet's) canal. This is the remains of the end of hyaloid
artery that was attached to the lens but disappeared as the
eye formed in the foetus in the womb. The fluid then flows
through the pupil (the gap in the middle of the iris). Around
the edge of the iris are some drainage channels called
‘canal of Schlemm’ or ‘angle’, which
allows the liquid in the vitreous to drain out through the
veins around the eye.
The front part of the eye, with all of the structures above,
is often called the ‘anterior segment’, the back
part of the eye, described below, is often referred to as the
‘posterior segment’.
The back part of the eye
Lining the inside of the white sclera of the eye is the
choroidal layer, which contains lots of blood vessels. Lining
the choroid is the pigment epithelium or layer in which the
rod and cone light receptor cells of the retina dip. The
innermost layer of the eye is the retina which is a
light-sensitive film.
The retina is the part that is reflected when you see
‘red eye’ in a photo. It appears a bright red due
to all the blood vessels in the choroidal layer if the pigment
epithelium is lightly pigmented (in blond individuals) and is
a darker red in people with black skin and darkly coloured
eyes. It is the ‘red reflex’ that doctors check for
when they shine a light in the eye. If a child has a dense
cataract light reflects back off the white lens and most light
does not reach the red lining of the inside of the eye. The
pigment epithelium is important to reduce internal reflections
inside the eye. Very blond individuals, including those with
albinism have very little of this pigment and may suffer from
glare and dazzle in bright lighting.
The retina of the eye is sensitive to light like a film in
a camera. The edges of the retina are good at picking up
movement, but not very good at colour and detail. The middle
of the retina (called the macula which means ‘spot’
and found in the area directly behind the middle of the lens)
is the most important part for detecting detail. In the centre
of the macula is the most sensitive portion called the
‘fovea’ or dip.
The ‘pictures’ from the retina are passed along
the optic nerve in the form of electrical signals similar to
the way a digital camera sends picture information to our
computer or television set. The optic nerve is a data cable
that runs out of the back of the eye to the brain. It's
off-centre so the end of it and the hyaloid canal don't get in
the way of the visual axis. The point where the optic nerve
leaves the back of the eye forms a little disc with a slight
cup-shaped indentation in it. Where the nerve leaves the back
of the eye there's a little natural blind spot in our vision,
as there are no light-sensitive cells in the nerve itself. Our
brain learns to ignore this blind spot. The optic nerve takes
the information to the brain, where it's processed by several
different parts of the brain into a picture we can
understand.
The imaginary line that light travels through the cornea,
the aqueous, the pupil, the front of the lens capsule, the
lens itself, the back of the lens capsule, the vitreous to the
macula is called the ‘visual axis’ or sight line
and every part of it is clear in a healthy eye.
All of this must work perfectly for us too see normally.
Checked for medical accuracy by Miss Isabelle
Russell-Eggitt, Consultant Paediatric Ophthalmologist, Great
Ormond Street Hospital, London
Last updated: 05/08/08
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Childhood Cataract Network
