Eye

 The human eye is an Template:Wiki which reacts to Light for several purposes. As a conscious sense organ, the mammalian eye allows vision. Rod and cone Template:Wiki in the Template:Wiki allow conscious Light Perception and vision including Template:Wiki differentiation and the Perception of depth. The human eye can distinguish about 10 million colors.

In common with the eyes of other Template:Wiki, the human eye's non-image-forming photosensitive ganglion Template:Wiki in the Template:Wiki receive the Light signals which affect adjustment of the size of the pupil, regulation and suppression of the Template:Wiki Template:Wiki and entrainment of the Body clock'

General properties

The eye is not shaped like a Template:Wiki 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 8 mm (0.3 in) in Template:Wiki. The sclerotic chamber constitutes the remaining five-sixths; its Template:Wiki is typically about 12 mm. The cornea and sclera are connected by a ring called the limbus.
A child's eye showing Template:Wiki limbal ring.

The iris – the Template:Wiki 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 fundus (area opposite the pupil) shows the characteristic pale optic disk (papilla), where vessels entering the eye pass across and optic Template:Wiki fibers depart the Template:Wiki.
Dimensions
See also: mammalian eye

The Template:Wiki differ among adults by only one or two millimeters. The vertical measure, generally less than the horizontal distance, is about 24 mm among adults, at birth about 16–17 millimeters (about 0.65 inch). The Template:Wiki grows rapidly, Template:Wiki to 22.5–23 mm (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 Template:Wiki of 24 millimeters, a volume of six cubic centimeters (0.4 cu. in.), and a Template:Wiki of 7.5 grams (Template:Wiki of 0.25 oz.).
Components

The eye is made up of three coats, enclosing three transparent structures. The outermost layer is composed of the cornea and sclera. The middle layer consists of the choroid, ciliary Body, and iris. The innermost is the Template:Wiki, 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 humor, the vitreous Body, and the flexible lens. The aqueous humor 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 humor, Template:Wiki behind lens and the rest, and is bordered by the sclera, zonule, and lens. They are connected via the pupil.
Template:Wiki vessels can be seen within the sclera.
Dynamic range

The Template:Wiki 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 Template:Wiki; full adaptation through adjustments in retinal Template:Wiki (the Purkinje effect) is mostly complete in thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 f-stops) is possible. The process is nonlinear and multifaceted, so an interruption by Light merely starts the adaptation process over again. Full adaptation is dependent on good Template:Wiki flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like Template:Wiki.

The eye includes a lens not dissimilar to lenses found in optical instruments such as cameras and the same Template:Wiki can be applied. The pupil of the human eye is its aperture; the iris is the Template:Wiki that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil Template:Wiki. The entrance pupil is typically about 4 mm in Template:Wiki, although it can range from 2 mm (f/8.3) in a brightly lit place to 8 mm (f/2.1) in the dark. The latter value Template:Wiki slowly with age; older people's eyes sometimes dilate to not more than 5-6mm.
Field of view

The approximate field of view of an Template:Wiki human eye is 95° away from the Template:Wiki, 75° downward, 60° toward the Template:Wiki, and 60° upward, allowing humans to have an almost 180-degree forward-facing horizontal field of view. With Template:Wiki rotation of about 90° (head rotation excluded, peripheral vision included), horizontal field of view is as high as 270°. About 12–15° Template:Wiki and 1.5° below the horizontal is the optic Template:Wiki or blind spot which is roughly 7.5° high and 5.5° wide