The eye is one of our five sensory organs and is probably the most used and easiest to understand. The basic structure of the eye is a sphere with a clear �bubble window�, the cornea in the front and the optic nerve connecting to the brain at the back. Layered behind the cornea is the iris and lens. The iris is a ring of coloured tissue that controls the amount of light entering the eye. In bright light the muscles in the iris contract reducing the size of the pupil at the centre of the iris, which restricts the amount of light coming into the eye. In darkness the iris expands to let in as much light as possible. Once through the pupil, light passes through the lens, a clear membrane that can change shape in order to focus the light onto the back of the eye. At the back of the eye is the retina, a photosensitive layer of cells that turns the light hitting it into electric signals which are passed via the optic nerve to the brain. The retina is made up of two different types of cells, rods and cones. The cone cells occupy the centre of the retina and are used for colour vision, the rods occupy the outer edges of the retina and are used for night vision.
The remaining space in the eye (the majority of the inside of the sphere) is filled with a clear liquid called the vitreous. This holds the eye in shape and allows for an unaffected transmission of light from the front to the back of the eye.
Thus the eye is much like a camera with the lens of the eye serving the same purpose as the lens of a camera and the retina doing the same job as the film.
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