The eye is the organ of sight in humans and animals. It transforms light waves into visual images and provides 80% of all information received by the human brain. These remarkable organs are almost spherical in shape and are housedin the orbital sockets in the skull. Sight begins when light waves enter theeye through the cornea (the transparent layer at the front of the eye), passthrough the pupil (the opening in the center of the iris, the colored portion of the eye), then through a clear lens behind the iris. The lens focuses light onto the retina, which functions like the film in a camera. Photoreceptorneurons in retinas, called rods and cones, convert light energy into electrical impulses, which are then carried to the brain via the optic nerves. At the visual cortex in the occipital lobe of the cerebrum of the brain, the electrical impulses are interpreted as images.

Many invertebrate animals have simple light-sensitive eye spots, consisting of a few receptor cells in a cup-shaped organ lined with pigmented cells, which detect only changes in light and dark regimes. Arthropods (insects, spiders, and crabs) have complex compound eyes with thousands of cells that construct composite pictures of objects. They are very sensitive to detecting movement.

The human eyeball is about 0.9 in (24 mm) in diameter and is not perfectly round, being slightly flattened in the front and back. The eye consists of three layers: the outer fibrous or sclera, the middle uveal or choroid layer, andthe inner nervous layer or retina. Internally the eye is divided into two cavities: the anterior cavity filled with the watery aqueous fluid, and the posterior cavity filled with gel-like vitreous fluid. The internal pressure inside the eye (the intraocular pressure) exerted by the aqueous fluid supports the shape of the anterior cavity, while the vitreous fluid holds the shape ofthe posterior chamber. An irregularly shaped eyeball results in ineffective focusing of light onto the retina and is usually correctable with eye glassesor contact lenses. The ophthalmic arteries provide the blood supply to the eyes, and the movement of the eyeballs is facilitated by six extraocular muscles that run from the bony orbit and insert into the sclera, part of the fibrous tunic.

The outer fibrous layer encasing and protecting the eyeball consists of two parts--the cornea and the sclera. The front one-sixth of the fibrous layer isthe transparent cornea, which bends incoming light onto the lens inside the eye. A fine mucus membrane, the conjunctiva, covers the cornea and lines the eyelid. Blinking lubricates the cornea with tears, providing the moisture necessary for its health. The cornea's outside surface is protected by a thin film of tears produced in the lacrimal glands located in the lateral part of orbit below the eyebrow. Tears flow through ducts from this gland to the eyelidand eyeball, and drain from the inner corner of the eye into the nasal cavity. A clear watery liquid, the aqueous humor, separates the cornea from the iris and lens. The cornea contains no blood-vessels or pigment and gets its nutrients from the aqueous humor. The remaining five-sixths of the fibrous layerof the eye is the sclera, a dense, tough, opaque coat visible as the white ofthe eye. Its outer layer contains blood vessels, which produce a "blood-shoteye" when the eye is irritated. The middle or uveal layers of the eye are densely pigmented, well supplied with blood, and include three major structures--the iris, the ciliary body, and the choroid. The iris is a circular, adjustable diaphragm with a central hole (the pupil), sited in the anterior chamberbehind the cornea. The iris gives the eye its color, which varies dependingon the amount of pigment present. If the pigment is dense, the iris is brown,if there is little pigment the iris is blue, if there is no pigment the irisis pink, as in the eye of a white rabbit. In bright light, muscles in the iris constrict the pupil, reducing the amount of light entering the eye. Conversely, the pupil dilates (enlarges) in dim light, so increasing the amount ofincoming light. Extreme fear, head injuries, and certain drugs can also dilate the pupil.

The iris is the anterior extension of the ciliary body, a large, smooth muscle that also connects to the lens via suspensory ligaments. The muscles of theciliary body continually expand and contract, putting on suspensory ligaments changing the shape of the lens, thereby adjusting the focus of light onto the retina, facilitating clear vision. The choroid is a thin membrane lying beneath the sclera and is connected the posterior section of the ciliary body.It is the largest portion of the uveal tract. Along with the sclera the choroid provides a light-tight environment for the inside of the eye, preventing stray light from confusing visual images on the retina. The choroid has a goodblood supply and provides oxygen and nutrients to the retina.

The front of the eye houses the anterior cavity, which is subdivided by the iris into the anterior and posterior chambers. The anterior chamber is the bowl-shaped cavity immediately behind the cornea and in front of the iris, whichcontains aqueous humor. This is a clear watery fluid that facilitates good vision by helping maintain eye shape, regulating the intraocular pressure, providing support for the internal structures, supplying nutrients to the lens and cornea, and disposing of the eye's metabolic waste. The posterior chamberof the anterior cavity lies behind the iris and in front of the lens. The aqueous humor forms in this chamber and flows forward to the anterior chamber through the pupil.

The posterior cavity is lined entirely by the retina, occupies 60% of the human eye, and is filled with a clear gel-like substance called vitreous humor.Light passing through the lens on its way to the retina passes through the vitreous humor. The vitreous humor consists of 99% water, contains no cells, and helps to maintain the shape of the eye and support its internal components.

The lens is a crystal-clear, transparent body that is biconvex (curving outward on both surfaces), semi-solid, and flexible, shaped like an ellipse or elongated sphere. The entire surface of the lens is smooth and shiny, contains no blood vessels, and is encased in an elastic membrane. The lens is sited inthe posterior chamber behind the iris and in front of the vitreous humor. Thelens is held in place by suspensory ligaments that run from the ciliary muscles to the external circumference of the lens. The continual relaxation and contraction of the ciliary muscles cause the lens to either fatten or became thin, changing its focal length, and allowing it to focus light on the retina.With age, the lens hardens and becomes less flexible, resulting in far-sighted vision, which necessitates glasses, bifocals, or contact lenses to restoreclear, close-up vision. Clouding of the lens also often occurs with age, creating a cataract that interferes with vision. Clear vision is restored by a relatively simple surgical procedure in which the entire lens is removed and an artificial lens implanted.

The retina is the innermost layer of the eye. It is a thin, delicate, extremely complex sensory tissue composed of layers of light sensitive nerve cells.The retina begins at the ciliary body and encircles the entire posterior portion of the eye. Photoreceptor cells in the rods and cones convert light firstto chemical energy and then electrical energy. Rods function in dim light, allowing limited nocturnal (night) vision: it is with rods that we see the stars. Rods cannot detect color, but they are the first receptors to detect movement. There are about 126 million rods in each eye and about 6 million cones.Cones provide acute vision, function best in bright light and allow color vision. Cones are most heavily concentrated in the central fovea, a tiny hollowin the posterior part of the retina and the point of most acute vision. Dense fields of both rods and cones are found in a circular belt surrounding thefovea, the macula lutea. Continuing outward from this belt, the cone densitydecreases and the ratio of rods to cones increases. Both rods and cones disappear completely at the edges of the retina.

The optic nerve connects the eye to the brain. The fibers of the optic nerverun from the surface of the retina and converge and exit at the optic disc (or blind spot), an area about 0.06 in (1.5 mm) in diameter located at the lower posterior portion of the retina. The fibers of this nerve carry electricalimpulses from the retina to the visual cortex in the occipital lobe of the cerebrum. If the optic nerve is severed, vision is lost permanently.

The last two decades have seen an explosion in ophthalmic research. Today, 90% of corneal blindness can be rectified with a corneal transplant, the most frequently performed of all human transplants. Eye banks receive eyes for sight-restoring corneal transplantation just as blood banks receive blood for life-giving transfusions. Many people remain blind, however, because of the lackof eye donors.

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