eye

I -INTRODUCTION
Eye (anatomy), light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.

II -THE HUMAN EYE
The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.
The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye.
Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended.



The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.

Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea, a small round area of the retina exists that has no light-sensitive cells. This optic disk forms the blind spot of the eye.

III -FUNCTIONING OF THE EYE
In general the eyes of all animals resemble simple cameras in that the lens of the eye forms an inverted image of objects in front of it on the sensitive retina, which corresponds to the film in a camera.
Focusing the eye, as mentioned above, is accomplished by a flattening or thickening (rounding) of the lens. The process is known as accommodation. In the normal eye accommodation is not necessary for seeing distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament. A young child can see clearly at a distance as close as 6.3 cm (2.5 in), but with increasing age the lens gradually hardens, so that the limits of close seeing are approximately 15 cm (about 6 in) at the age of 30 and 40 cm (16 in) at the age of 50. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.

Structural differences in the size of the eye cause the defects of hyperopia, or farsightedness, and myopia, or nearsightedness. See Eyeglasses; Vision.
As mentioned above, the eye sees with greatest clarity only in the region of the fovea; due to the neural structure of the retina. The cone-shaped cells of the retina are individually connected to other nerve fibers, so that stimuli to each individual cell are reproduced and, as a result, fine details can be distinguished. The rodshaped cells, on the other hand, are connected in groups so that they respond to stimuli over a general area.

The rods, therefore, respond to small total light stimuli, but do not have the ability to separate small details of the visual image. The result of these differences in structure is that the visual field of the eye is composed of a small central area of great sharpness surrounded by an area of lesser sharpness. In the latter area, however, the sensitivity of the eye to light is great. As a result, dim objects can be seen at night on the peripheral part of the retina when they are invisible to the central part.

The mechanism of seeing at night involves the sensitization of the rod cells by means of a pigment, called visual purple or rhodopsin, that is formed within the cells. Vitamin A is necessary for the production of visual purple; a deficiency of this vitamin leads to night blindness. Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. Hence a person who steps from sunlight into a darkened room cannot see until the pigment begins to form. When the pigment has formed and the eyes are sensitive to low levels of illumination, the eyes are said to be dark-adapted.

A brownish pigment present in the outer layer of the retina serves to protect the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This action, called light adaptation, has the opposite effect to that of dark adaptation.
Subjectively, a person is not conscious that the visual field consists of a central zone of sharpness surrounded by an area of increasing fuzziness.

The reason is that the eyes are constantly moving, bringing first one part of the visual field and then another to the foveal region as the attention is shifted from one object to another. These motions are accomplished by six muscles that move the eyeball upward, downward, to the left, to the right, and obliquely. The motions of the eye muscles are extremely precise; the estimation has been made that the eyes can be moved to focus on no less than 100,000 distinct points in the visual field. The muscles of the two eyes, working together, also serve the important function of converging the eyes on any point being observed, so that the images of the two eyes coincide. When convergence is nonexistent or faulty, double vision results. The movement of the eyes and fusion of the images also play a part in the visual estimation of size and distance.