Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...

, the eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 allows vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

. Rod
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

 and cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

 cells in the retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

In common with the eyes of other mammals
Mammalian eye
-Dimensions:Dimensions vary only 1–2 mm among humans. The vertical diameter is 24 mm; the transverse being larger. At birth it is generally 16–17 mm, enlarging to 22.5–23 mm by three years of age. Between then and age 13 the eye attains its mature size...

, the human eye's non-image-forming photosensitive ganglion cell
Photosensitive ganglion cell
Photosensitive ganglion cells, also called photosensitive Retinal Ganglion Cells , intrinsically photosensitive Retinal Ganglion Cells or melanopsin-containing ganglion cells, are a type of neuron in the retina of the mammalian eye.They were discovered in the early 1990sand are, unlike other...

s in the retina receive the light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

 and entrainment
Entrainment (chronobiology)
Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by...

 of the body clock
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

The eye is not properly a sphere, rather it is a fused two-piece unit.