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Cross-section through the eyeCross-section through the eyeSight is the most important of our senses: around three-quarters of all information processed in the brain comes in through the eyes alone. Each eyeball is about 25 millimetres (1 inch) across and well protected in a socket, called the orbit, inside the skull bone. Light rays enter the eye through its transparent domed front, the cornea. They pass through the pupil, a hole in a ring of muscle known as the iris. The rays are bent or focused by the lens, and shine a clear image on to the retina which lines the rear of the eyeball. When light hits the retina, light-sensitive cells produce nerve signals, which pass along the optic nerve to the brain.

How the eye works

Light enters the eye through a transparent, domed window called the cornea at the front of the eye. This does most of the work of focusing images on to the retina. Only half a millimetre thick at the centre, its outer surface is the epithelium, a transparent continuation of the skin. The cornea thickens to one millimetre where it joins the white of the eye, called the sclera.
How light rays are focused on to the retinaHow light rays are focused on to the retina
A pupil dilated (top) and contracted (above)A pupil dilated (top) and contracted (above)Light passes through an opening, called the pupil, in the iris, the eye’s coloured part. This is a ring of muscle that controls the size of the pupil. The iris works like the aperture on a camera: if too much light enters the eye, the pupil is reduced in size. If too little light enters, it grows larger. The pupil can vary in size from one to eight millimetres (0.04–0.3 inches). The adjustment is automatic—it works without any conscious effort.

Behind the iris there is a soft, elastic lens, which finely adjusts the sharpness of the image. The main body of the eye is filled with a clear, jelly-like substance called the vitreous humour. This gives the eye its shape and firmness. Light shines through this on to the retina.

The blood vessels in the retinaThe blood vessels in the retina


Eye colour is the colour of the iris, which can be green, blue, brown, grey and hazel (a combination of light brown, green and gold). The iris has a fine texture that—like fingerprints—is unique to every individual person. Even identical twins have completely different iris textures.

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