the eye how it works

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.