People can arrange to have their eyes donated to such organizations after their death. Eyes that are used in various ways for surgical repairs are supplied by eye banks. Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. In addition to errors of refraction ( astigmatism, farsightedness, and nearsightedness), the human eye is subject to various types of injury, infection, and changes due to systemic disease. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.Īccessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears, keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue. The pupil is the round opening in the center of the iris it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. A delicate membrane, the conjunctiva, covers the visible portion of the sclera. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. The sclera is the outermost layer of eye tissue part of it is visible as the “white” of the eye. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock (see rhythm, biological). This information stimulates the hypothalamus, which passes the information on to the pineal gland. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other arthropods. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation.
Eye, organ of vision and light perception.
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