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Eyes detect light and are the organs of vision. The size and shape of  the human eye varies from person to person and depends on the race of the person. On an average, a human eye is about 2.5 cm wide and 2.5 cm deep. The eyes of a person are not symmetrical and one eye can be slightly larger than the other eye.

The extra ocular muscles that move the eye are connected to the outermost layer of the eye called the sclera. Part of  the sclera is clear, so that light can pass through and is called the cornea.

The iris is the pigmented membrane in the eye between the cornea and the lens. The colour of the eye depends on the pigment in the iris, if there is no pigment the eye is blue in colour, and if there is more pigment, the eye is black in colour.  The pupil is a hole in the centre of  the iris. The size of the pupil changes to increase or  decrease the amount of light entering the eye. The amount of light entering the eye can vary by about 30 times.

A transparent biconvex lens is located in the eye behind the iris. An image is formed on the retina by  focusing the light rays which enter the eye through the pupil. The light sensing part of the eye  - the retina contains the rod cells and the cone cells. The rod cells are responsible for seeing  black and white images (when there is less light). The cone cells are responsible for seeing color image and finer details.  The cone cells are located at the center of the retina, an area called the macula.

The nerve fibers from the retina and macula collect to form the optic nerve, which carries the electrical impulses to the brain, where the images are interpreted. The optic disk is the portion of  the eye which does not contain any rod and cone cells. The blood vessels and nerve fibers leave the eye at the optic disk.

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