The human eye is an organ that reacts to light and has several purposes. As a sense organ, the mammalian eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision, including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.
Similar to the eyes of other mammals, the human eye’s non-image-forming photosensitive ganglion cells in the retina receive light signals which effect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and internment of the body clock.
The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit. The smaller frontal unit, transparent and more curved, called the cornea is linked to the larger white unit called the sclera. The corneal segment is typically about 8 mm (0.3 in) in radius. The sclerotic chamber constitutes the remaining five-sixths; its radius is typically about 12 mm. The cornea and sclera are connected by a ring called the limbus. The iris is the colored circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris dilator and sphincter muscles. A device known as an ophthalmoscope is used to see inside the eye.
Light enters the eye through the cornea, then the pupil and then through the lens controlled by ciliary muscles. Light falling on the light-sensitive cells of the retina is converted into electrical signals that are carried to the brain by the optic nerves.