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How the eye works

The eye is an elaborate photoreceptor, which detects information in the form of light from the environment and transmits this information by a series of electro-chemical changes to the brain. The part of the brain that processes this information is called the visual cortex.

If you look at the cross section below of an eye, you will see that it consists of a protective globe, the eyeball, with a central cavity that is divided into three parts. Information in the form of light, which represents what you 'see', is transmitted to the brain by the optic nerve.

Click on the names to see a description of each part of the eye.



Test your own field of vision

You can test your own field of vision. Although this is not a substitute for the optician's examination, it can be a useful indicator.

1. Close or cover your left eye and stare straight ahead at a point on a wall with your right eye

2. Hold your right hand about 30 cm (1 foot) away from your right ear over your shoulder

3. Gradually move your hand forwards, whilst at the same time moving (wiggling) your fingers

4. You will become aware of the movement at the edge of your vision

5. Repeat the manoeuvre, bringing your hand up from your waist, then down from your head and across from your left hand side

6. You should see farthest out to your right hand side and below (the view to the left and above will be restricted by your nose and eyebrows)

7. If you cannot see your moving fingers until you bring them directly in front of your eyes, you have lost your peripheral vision

8. If your field of vision is normal, there is one point at which you will not see your moving fingers - this is your blind spot

Find your blind spot

You should now see a spot and a cross on the screen.



Close your left eye and look at the spot with your right eye.

Gradually move closer to the screen whilst looking at the spot.

You will notice that the cross disappears when you get to between 15 cm and 30 cm (6 and 12 inches) from the screen.

Repeat by closing your right eye and looking at the cross with your left eye.

The spot will disappear from your view at about the same distance as before

Want to know more about eyes?

Anteria Chamber

The Anteria Chamber is located between the cornea in front and the pupil and iris behind. It contains aqueous humour.

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Aqueous Humour

Aqueous Humour is a transparent fluid.

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The Choroid is a sheet of tissue which lines the posteria 2 thirds of the sclera.

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Ciliary Body

The Ciliary Body is a ring of tissue which extends back from the sclera. The muscles of the ciliary body change the shape of the lens for focussing light.

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The Cornea

The Cornea is part of the Outer Coat of the eye. It is transparent, colourless and covers one sixth of the eye. As it is supplied with very fine nerve fibres, it is exceptionally sensitive to pain.

It is transparent in order to let light in, and from the front is almost circular. Most of the focussing power of the eye is provided by the Cornea, with the lens providing only the 'fine' adjustment. There is no blood supply to the Cornea, with most of the nutrition being supplied by the aqueous humour.

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The Fovea is a depression within the retina which contains the cones (see retina for details of cones)

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The Iris is a muscular diaphragm which contracts or relaxes to control the size of the pupil in bright or low light. It is visible as the coloured part of the eye.

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The lens provides some of the focussing power of the eye, so that light rays can be focussed on to the retina.

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Optic Nerve

The Optic Nerve carries electro-chemical messages from the retina, along the visual pathway, to the visual cortex in the brain.

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The Papilla is the small bump at the entrance to the eye where the optic nerve enters and leaves.

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Posteria Chamber

The Posteria Chamber is the narrow space between the iris and the pupil in front, and the lens behind. It contains aqueous humour.

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The Pupil is a round black hole through which light reaches the retina. In bright light the pupil closes its size and at night or in low light, it enlarges to allow as much light in as possible.

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The Retina is light sensitive and lines the inside of the back of the eye. It sees by means of cones (6 million) and rods (100 million). The cones identify colour and are used for direct vision such as reading. The rods are used for peripheral (side) vision, especially in low light.

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The Sclera is opaque and makes up 5 sixths of the outer layer of the eyeball. It is visible between the eyelids as the white of the eye.

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Vitreous Humour

Vitreous Humour is a transparent jelly-like structure which is made up of a network of fibres suspended in fluid. It is contained in the Vitreous Cavity, the largest chamber of the eye.

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