Human eye

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 affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

Structure

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. A device known as an ophthalmoscope is used to see inside the eye.