The eye

The eye is one of the most complex and remarkable structures in the human body. In some ways it is the most sensitive of all our sense organs and can perceive things, such as distant galaxies, that none of our other senses can.

Structure

The eye is rather like a sphere with a bulge at the front. It has a tough, white outer layer, called the sclera, and where the sclera rounds off the bulge at the front of the eye, it forms the cornea, which is clear.

The conjunctiva is a very thin protective layer and is fairly loosely connected to the eyeball, except at the point where the cornea and the sclera meet; the conjunctiva covers the sclera, but not the cornea.

For a description of the image place your cursor over it (certain browsers only), or click here for an audio transcript of the Anatomy of the Eye.

Seeing

"…the eye sees not itself, But by reflection…" - Act I, Julius Caesar, William Shakespeare

In order to function, the eye needs light. Light, usually reflected by objects, enters the front of the eye through the cornea. Behind the cornea is the lens. It is the cornea which does most of the focusing, but the lens which fine-tunes the focusing and enables the eye to vary its focus. An image is formed by the cornea and lens on the retina, at the back of the eye. The most light-sensitive part of the retina is a tiny spot called the macula (which is actually Latin for "spot") and this is where your vision is at its sharpest. Due to the laws of optics, the image formed on the retina is actually upside-down; it is the brain which adjusts it to make it appear the right way up.

For a description of the image place your cursor over it (certain browsers only), or click here for an audio transcript of the passage of light from an object to the eye.

Between the cornea and the lens, light passes through the pupil. (the black spot in the middle of the eye). The iris, which is the coloured part of your eye, is actually a ring of very fine muscles which make the pupil larger or smaller, depending on how much light the eye needs to make an image. In bright conditions, the pupil shrinks to let in less light, while in darker conditions it lets in more light by getting bigger.

Eye colour

Eye colour is one of our distinguishing features. One example of this can be found in passports - it seems to be one of the things people most want to know about us. But what makes our eyes the colour they are, and just how different can eyes be?

When we talk about eye colour, we’re really referring to the colour of the iris. This muscular ring, which adjusts the size of the pupil, varies in colour from person to person, depending on how much melanin is in it.

For a description of the image place your cursor over it (certain browsers only).

Generally speaking, the darker the eye colour, the more melanin there is in the iris. So, dark brown eyes have a lot of melanin in them, hazel eyes have less, green eyes even less and grey and then blue eyes have the least of all. Melanin is a pigment that plays a part in protection against the sun, which is why paler-coloured eyes have only evolved in cooler, less sunny climates.

Some people have eyes that are different colours. The scientific name for this is heterochromia iridium, and it’s relatively rare in humans. It’s thought to result from an alteration to one of the genes that control eye colour.

Blind spots

Curiously, one of the very structures that enables you to see also gives you a blind spot. At the point on the retina where light passes as electrical impulses down the optic nerve, there are no light-sensitive cells. This means that any light falling on this part of the retina is "lost", giving you a gap in your "picture". Usually, your brain compensates for this gap, but the test below can show you what you’re missing!

• Close your right eye, or cover it with your hand
• Stare at the black circle (below)
• While looking at the circle, use 'side' (peripheral) vision to see the cross
• Slowly move your head towards the screen
• The cross should completely disappear

Move closer, and it will re-appear!

For a description of the image place your cursor over it (certain browsers only).

3D Vision

Each eye has its own field of vision, and where those fields of vision overlap, we have stereoscopic, or 3D vision. Some animals, such as ducks, have eyes set round the sides of their heads. This gives them great peripheral vision, but poor 3D vision. Those animals with eyes right in the front of their heads, however, such as humans, have very good 3D vision.

Stereoscopic vision is very good for judging distances - vital for our ancestors, whether they were tree-dwellers or spear-hunters, and still important for us today. Playing a ball-game with one eye shut is all that’s needed to convince most people of the benefits of 3D vision!

Keeping clean

In order for the eye to remain healthy and in good working order, it has to keep clean, and it does this via a number of mechanisms.

Perhaps the most obvious of these is the action of the eyelids. It’s thought that the average human blinks around twelve times a minute - 4 million times a year! - and this action keeps the eye clean and lubricated.

Tears, produced by the lacrimal gland, are an essential cleaning fluid for the eye. Not only can they wash the eye clean of any foreign bodies, but they also contain enzymes which protect against attack from bacteria.

For a description of the image place your cursor over it (certain browsers only), or click here for an audio transcript of the Anatomy of the Tear Duct.

The conjunctiva, the thin layer that covers the outside of the eyeball, also plays an important role in the defence of the eye. It forms a natural barrier to external dangers such as smoke, bacteria, allergens and the damaging effects of the sun and the wind.

For more information on how to protect your eyes from the effects of the sun, visit - sight and the sun.