StemCell Therapy

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.[1]

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.[2]

The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit. The smaller frontal unit, more curved, called the cornea is linked to the larger unit called the sclera. The corneal segment is typically about 8mm (0.3in) in radius. The sclerotic chamber constitutes the remaining five-sixths; its radius is typically about 12mm. The cornea and sclera are connected by a ring called the limbus. The iris the color of the eye and its black center, the pupil, are seen instead of the cornea due to the cornea's transparency. To see inside the eye, an ophthalmoscope is needed, since light is not reflected out.

The dimensions differ among adults by only one or two millimeters; it is remarkably consistent across different ethnicities. The vertical measure, generally less than the horizontal distance, is about 24mm among adults, at birth about 1617 millimeters (about 0.65inch). The eyeball grows rapidly, increasing to 22.523mm (approx. 0.89 in) by three years of age. By age 13, the eye attains its full size. The typical adult eye has an anterior to posterior diameter of 24 millimeters, a volume of six cubic centimeters (0.4 cu. in.),[3] and a mass of 7.5 grams (weight of 0.25 oz.).[citation needed]

The eye is made up of three coats, enclosing three transparent structures. The outermost layer, known as the fibrous tunic, is composed of the cornea and sclera. The middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, and iris. The innermost is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels, which can be seen in an ophthalmoscope.

Within these coats are the aqueous humour, the vitreous body, and the flexible lens. The aqueous humour is a clear fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than the aqueous humour present behind the lens, and the rest is bordered by the sclera, zonule, and lens. They are connected via the pupil.[4]

The approximate field of view of an individual human eye is 95 away from the nose, 75 downward, 60 toward the nose, and 60 upward, allowing humans to have an almost 180-degree forward-facing horizontal field of view.[citation needed] With eyeball rotation of about 90 (head rotation excluded, peripheral vision included), horizontal field of view is as high as 270. About 1215 temporal and 1.5 below the horizontal is the optic nerve or blind spot which is roughly 7.5 high and 5.5 wide.[5]

The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and geometrically by adjusting the iris which regulates the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) is mostly complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light merely starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like tobacco.[citation needed]

The human eye can detect a luminance range of 1014, or one hundred trillion (100,000,000,000,000) (about 46.5 f-stops), from 106 cd/m2, or one millionth (0.000001) of a candela per square meter to 108 cd/m2 or one hundred million (100,000,000) candelas per square meter.[6][7][8] This range does not include looking at the midday sun (109 cd/m2)[9] or lightning discharge.

At the low end of the range is the absolute threshold of vision for a steady light across a wide field of view, about 106 cd/m2 (0.000001 candela per square meter).[10][11] The upper end of the range is given in terms of normal visual performance as 108 cd/m2 (100,000,000 or one hundred million candelas per square meter).[12]

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Human eye - Wikipedia, the free encyclopedia

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