EYE

EYE . The eye is one of the most widespread symbols in all religious representation. As the active organ of visual perception, it is closely linked with light. Without light, the eye could neither see nor discern clearly. It is therefore only natural that in most cultures the eye is the symbol of intellectual perception and the discovery of truth. The eye knows because it sees. As early as the fifth century bce, Democrites thought that certain images exist already in the body and that they emerge from the gaze of certain persons. Pliny the Elder explained that the small image inscribed in the pupil is a sort of miniature soul (Natural History 21.12.51). Similarly, the Bambara of West Africa say that the image perceived by the eye is, in fact, the double of the object or being that is seen: "Man's world is his eye." Thus the eye is often considered the mirror of the soul, the body's window, which reveals each person's deep thoughts by means of his gaze. As the mirror of the interior, the eye is the place where the mysterious life of the soul is glimpsed. In seeking to discover the reality behind the physical appearances it perceives, the eye becomes the locus of inner revelation. The expression "His eyes were opened" means that a rational or religious truth has been unveiled.

According to the symbolic conception of man as microcosm, which is found in the most ancient cultures, the eye is likened to the sun. Like the luminous star, it sees all, shines, and sparkles. Its glances are rays that pierce like arrows. Among the Semang, the Boshiman, and the Fuegians, the sun is the eye of the supreme god. This isomorphism of the eye and the sun reveals moral and religious values according to which all vision introduces clairvoyance, justice, and righteousness. Just as the sun illuminates by projecting its light everywhere, the eye seeks to discover and see everything, even faults and crimes. It thus becomes the emblem of a superior being who punishes and takes vengeance. To be all-seeing is to become omnipotent. Such valorization of the eye sometimes leads to its sacrificial oblation, which results in a supernatural second sight that replaces and sublimates simple, corporeal vision. This second sight is like the inner eye, or the "eye of the heart," so common among mystics who perceive the divine light.

The Eye as Divine Attribute

In Egypt, as in the most ancient cultures of the eastern Mediterranean Basin, the presence of a symbolic eye signifies the power of the supreme divinity to see and know everything. Thus "the eye of Horus" appears on the stelae of Memphis, and eyes are engraved on a Cretan ring in the Ashmolean Museum (Oxford University), the symbol of an anonymous divinity who looks at and listens to men. The sacerdotal myths and traditions of ancient Egypt testify that the eye has a solar nature and is the fiery source of light and knowledge. Re, the sun god, is endowed with a burning eye and appears in the form of a rearing cobra with dilated eyes. In the cult of Harmerti, Re and Thoth are the two eyes of Horus, the god of the sky. One Harmerti story relates the struggle between Horus and Seth; Seth pokes out the eye of Horus, who is later cured by Thoth. The healed, healthy eye is the oudjat eye that shines in the dark and vanquishes death. The Book of Going Forth by Day (17.29ff.) recapitulates that mythical episode in a formula that is said to the dead person by the one playing the role of Thoth: "I restore to the eye the fullness it possessed on the day of the fight between the two adversaries." This means that light and darkness, life and death, are reconciled in the beyond. This oudjat eye was painted on the inner sides of the coffin, on each side of the head, and an inscription affirmed that they were hereafter the eyes by which the dead person would see in the afterlife and which would permit him to follow the spectacle of the exterior world while remaining in his tomb. The oudjat eye was provided for the dead person "in order that he be animated by it" (Pyramid Texts 578). This is why Horus's eye made an excellent amulet.

At Tell Brak in eastern Syria, the excavation of a sanctuary going back to 2500 bce has revealed the worship of a divinity with a thousand eyes. In this temple consecrated to Inanna (Ishtar), hundreds of statuettes with multiple eyes have been found, votive offerings or apotropaic images attesting that the eye was the emblem of that all-seeing and omnipresent feminine divinity. Analogous finds have been made at Ur, Lagash, and Mari. Inanna's brother Shamash is the sun god, whose eye sees everything and who knows the most secret thoughts. He can reward or punish deliberately. In Babylon, Shamash was invoked before every divination, for he was the one who wrote the signs in the entrails of sacrificial victims. Without his help, the diviner could not see them.

The Indo-European world attached the same value to the eye as to the sun and to the gods, that is, the quality of being able to see everything. In the Ṛgveda, the god Sūrya, son of Dyaus, is called "the eye of the sky" (10.37.2) and "the eye of Mithra and of Varuna" (1.115.1, 7.61.1, 10.37.1). He sees from afar and everywhere, and spies on the whole world. Varuṇa, the celestial god, is described as sahasrāta, god of a thousand eyes, for he sees everything. According to the hymn of Puruṣa (Ṛgveda 10.90), the sun was born from the eye of the cosmic giant Puruṣa so that at death, when a man's soul and body return to the cosmic primordial man, his eye will go back to the sun. It is understandable why, in the Bhagavadgītā as well as in certain Upaniṣads, eyes are identified with the two celestial lights, the sun and moon, which are the eyes of Viśvakarman, the divine architect with multiple eyes who ordained everything. The Tamil caste of the Kammalans, who claim to descend from him, have as their main task painting the eyes of divine statues according to a ritual as sacred as that of the oudjat eye, the eye of Horus, in Egypt. In Iranian tradition, the sun is also the eye of Ahura Mazdā (Yasna 1.11), whereas in the tenth Yasht of the Avesta, Mithra is called the "master of vast plains who has a thousand ears and ten thousand eyes," thus assimilating him to the sun.

For the Greeks, the gods "with piercing gaze" saw everything belonging to the past, present, and future in a single, unified vision. This panopteia is the very mark of their divinity. Thus there is Kronos, who has four eyes, two in front and two in back; Zeus, whose "vast gaze" pierces through to the most secret things (Hesiod, Works and Days 240, 265); Apollo, the solar god, who sees everything (Iliad 3.277, Odyssey 11.109); and Dionysos, whose Bacchic hymn repeats that he "shines like a star with his eye of fire that darts its rays over the whole earth" (Diodorus of Sicily, 1.11.3). All the Greek gods cast a sovereign and pure gaze on man. The gaze of Athena Glaukopis shines and fascinates; the eye she fixes on her enemies is "a sharp one, an eye of bronze." When Achilles, driven by rage, tries to kill Agamemnon, Athena seizes him by his hair and forces him to look at her. The hero cries out, "It is terrifying to see the light of your eyes" (Iliad 1.200), for the light in her eyes is the light of reason. In Greek poetry, the image of the eye or the pupil is used to mark the quality of a person and the affection one feels for him: "Where is the eye of my beloved Amphiaraos, this hero who was both a seer and a valiant warrior?" (Pindar, Olympian Odes 6.16.7). The Greek religious experience consisted primarily of a vision. Since Homeric times, an indissoluble relationship has existed between knowing and seeing: knowledge is based on sight, on an optical intuition. In his Metaphysics, Aristotle speaks of that joy of seeing, which makes a better basis for knowing than any other perception. The joy of sensitive contemplation is the climax of the initiation into the great mysteries of Eleusis, the epopteia, and the very source of all philosophy, repeats Plato (Timaeus 47a). For the knowledge of truth rests on a vision that moves upward from tangible realities to timeless and eternal things: "Holy is the man who has the gods before his eyes" (scholium on Pindar, Pythian Odes 4.151b).

The Germanic and Celtic worlds also valued the magical power of the eye. Óðinn (Odin), god of war and magic, is a one-eyed god, for he voluntarily gave an eye in payment to the sorcerer-giant Mímir, or Memory. In return, Óðinn was permitted to drink every day at the spring of knowledge and thus to learn the science of the runes. The loss of an eye is therefore the means of acquiring superior vision and the supernatural powers that flow from it. After the Vanir killed Mímir, Óðinn practiced divination by interrogating the head, which he succeeded in preserving (Ynglingasaga 4, 7). This theme of knowledge acquired as a result of blindness—even ocular mutilation—is found elsewhere in other Indo-European traditions. Thus, the Greek diviner Tiresias attains the ability to see the future by becoming blind; Oedipus learns the will of the gods by blinding himself; and the blind king Dhṛtarāṣṭra in the Mahābhārata has special powers. Blindness, or voluntary mutilation of the eyes, becomes the sign of the superior sight possessed by the druids and the diviners. Indeed, this quasi-magical power of the eye is found again in Celtic myths. The god Lugh keeps only one eye open. He makes a tour of the enemies' camp, hopping and singing in an act similar to Óðinn's during the battle between the Æsir and the Vanir. The same attitude is found in the Celtic hero Cú Chulainn, who, when seized by furor, closes one eye and enlarges the other, or sometimes swallows one eye and places the other on his cheek to frighten his adversary. Many Gallic coins show a hero's head with an eye disproportionately enlarged.

Other myths also valorize the magical power of the one-eyed person, as if the reduction of vision to a single eye increased the intensity of the gaze. Thus a glance from Medusa's single eye petrifies anyone who crosses its path, for it is the glance of death that leads to Hades. To overcome it, Perseus must first escape the other two Gorgons and hide the one eye they share between them. He conquers Medusa only by making use of Athena's mirror, which allows him to see the monster without being seen by her.

The Evil Eye and Magic

The belief in the unlucky influence of the evil eye is universal. It rests on a valorization of the gaze reputed to be harmful because the eye is abnormal (eyes of different colors, double pupils, squinting); such a gaze magically reveals the malevolent intention of the soul whose window the eye is. The evil eye, cast for vengeance or out of envy, is an invisible threat against which one must protect oneself with countermagic, as, for example, in this Babylonian incantation: "Take the eye, attach its feet to a bush in the desert, then take the eye and break it like a pottery vase!" In Egypt it was common practice to bear apotropaic names, wear amulets, and recite formulas. In the Roman world, fear of the fascinum was constant. Many mosaics have as a central motif a dangerous eye pierced by an arrow, surrounded by animals, and defended by an owl (the bird of evil omen) perched on its eyelid. Against this danger, people resorted to amulets picturing an eye or a phallus. Eyes were even painted on the prows of boats.

Ever since the time of the apostle Paul (Gal. 3:1), Christian preachers never ceased to raise their voices, in vain, against the belief in the evil eye. Some rituals of the Greek Orthodox church as found in the Mikron Euchologion contain a formula for exorcism against baskania ("witchcraft") similar to that found in certain Babylonian curses. The same belief is found in pre-Islamic Arabia and in the Muslim world. Muḥammad himself recited incantations to preserve his grandson from the evil eye, reviving the formulas that Abraham made use of in order to protect Ishmael and Isaac, the legend says. One Arabic proverb states that "the evil eye empties houses and fills tombs." The eye frequently occurs in the magical preparations of certain African ethnic groups as well as in the Eastern Orthodox world, where the eyes of the figures in icons were poked out and crushed and then made into a magical powder.

Religious and Mystical Values of the Eye

The word eye recurs 675 times in the Hebrew scriptures (Old Testament) and 137 times in the New Testament; this indicates the richness of its symbolic meanings. It designates, first, the organ of vision fashioned by the creator for the good of man (Ps. 94:9, Prv. 20:12), for "it is a joy for the eye to see the sun" (Eccl. 11:7). But the eye is also a privileged organ of knowledge that scripture always associates with the characteristics of the wise and learned: for example, Balaam, the diviner whose eye is closed to the terrestrial realities that surround him but is open to the hidden and the invisible once he meets the All-Powerful (Nm. 23–24). The Targum Yerushalmi makes Balaam a one-eyed seer, thus taking up again the theme of a second sight superior to normal corporeal vision. Symbolically, the eye designates the consciousness of man that Yahveh opens to the knowledge of his law and therefore of good and evil (Dt. 29:3, Is. 6:10). For Yahveh sees all (Ps. 14:2); he is the witnessing God from whom nothing escapes (Ps. 139:7–8). This ability to see all is an essential characteristic of his transcendent sovereignty, and the divine eye is the administrator of justice: before it the just man can find grace (Dt. 31:29, Jb. 11:4). But it is also the paternal eye of Providence, "who turns toward those who fear him" (Ps. 33;18), like Nehemiah praying night and day for the people of Israel (Neh. 1:6, Lv. 16:2, Nm. 4:20).

But however great his desire, man may not see God face to face (Ps. 42:3), for no one can see Yahveh without dying (Ex. 19:21, Lv. 16:2, Nm. 4:20). Even Moses saw only the back of the glory of God (Ex. 33:20–23). If some prophets have had a vision of divine glory, it is in a fugitive and symbolic fashion, through a cloud or in human shape. Thus Zachariah (c. 520 bce) saw Yahveh put before the high priest "a single stone decorated with seven eyes," symbol of God's vigilant presence in his temple. They are the seven planets, or seven divine eyes that sweep over the earth without resting. Likewise Ezekiel, in his vision of the chariot evoking Yahveh's throne, sees wheels whose rims are decorated with open eyes (Ez. 1:18), signs of Yahveh's omniscience.

Although, as the apostle John says, "No one has ever seen God" (1 Jn. 4:12), Jesus promised the pure in heart that they shall see God (Mt. 5:8). This beatitude makes of the eye a symbol of inner purity (Mt. 6:22–23); otherwise the eye, as the opportunity for scandal, ought to be plucked out and thrown far away. Early Christian preaching insists on the opposition between the eyes of flesh and those of the spirit; in Paul's case, physical blinding symbolically preceded the opening of the eyes of the heart (Acts 9:18). Furthermore, the function of the Son is to render visible his Father: "Whoever sees me has seen the Father," says Jesus to Philip (Jn. 14:9). But at the end of time, full vision will be given to everyone and "man's eyes will contemplate the glory of God just as he is" (1 Jn. 3:2). Gnosticism especially retains the Pauline theme of "the eye of the heart," an image already frequent in the writings of the Greek philosophers and the Hebrew rabbis. For the gnostic, "the eye is the inner light to the man of light" (Gospel of Thomas, logion 24), and the prototype of the man of light is "the eye of light" (Sophia of Jesus Christ).

In numerous philosophical and religious traditions, the inner eye allows access to wisdom. Plotinus explains that the eye of the soul dazzled by the light of understanding is fixed on pure transparency; the soul therefore sees the light found at the interior of his own gaze, and the eye of understanding contemplates the light of nous by participating in the very light of this sun-spirit (Enneads 5.3[17].28). Recalling that the wise man is he who sees and that the fool is blind, Philo Judaeus explained that formerly the prophets were called "seers" (1 Sm. 9:8). For him, wisdom is not only what is obtained by the vision of the inner eye, just as light is perceived by the carnal eye; but wisdom also sees itself, and this is the splendor of God, who, in opening the soul's eye to wisdom, shows himself to man (De migratione Abraham 38).

In Hinduism, the god Śiva is endowed with a third eye, the frontal eye that gives him a unifying vision. His look of fire expresses the pureness of the present without any other temporal dimension, as well as the simultaneity of beings and events, which he reduces to ashes in revelation of the all. Likewise Buddha, the "awakened one," received inner enlightenment through the celestial eye, which permitted him to see the life of all beings simultaneously and gave him the knowledge of the chain of the fundamental forces of existence as well as its previous forms. This eye of wisdom, prajñācakṣus, is found at the limit of unity and multiplicity, of emptiness and creation; it permits the wise man to grasp them simultaneously. The organ of inner vision, it is the very sign of Buddhist wisdom.

But the inner eye is the organ of wisdom only because it is capable of actually experiencing the divine. Every revelation presents itself as a veil that has been pulled back before the gaze of religious man, for whom the beatific vision and the contemplation of God are the very essence of eternal life. The eye of the heart is thus a frequent theme in spiritual and mystical literature. Just as the eye can neither see nor discern its object without light, so the soul cannot contemplate God without the light of faith, which alone opens the eyes of the heart. "Man must therefore become entirely eye"; such is the teaching of the Desert Fathers reiterated by Symeon the New Theologian (Hymns of the Divine Loves 45), for the soul's eye, relieved of carnal passions, can perceive the divine light that opens up on the heavens. Following Origen and his theory of spiritual senses, Gregory of Nyssa, Augustine, Bernard of Clairvaux, and all the Fathers state that it is God who, by opening the heart's eye, makes one see. Meister Eckhart again picks up this teaching (Sermons), and Teresa of Ávila specifies that what we know otherwise than by faith, the soul recognizes at sight, although not by eyesight. Leon Bloy writes in his Journal (June 6, 1894) that "we must turn our eyes inward" in order to speak our desire for a vision of truth, for the carnal eye only allows us to see "in enigma and as in a mirror." The eye of the heart is therefore man seeing God and, at the same time, God looking at man; it is the instrument of enlightenment and inner unification: "We shall find the pearl of the kingdom of heaven inside our hearts if we first purify the eye of our spirit" (Philotheus of Sinai, Forty Chapters on Spiritual Sobriety 23).

See Also

Sun; Visions.

Bibliography

Bleeker, C. Jouco. The Sacred Bridge. Leiden, 1963.

Boyer, Régis. La religion des anciens scandinaves. Paris, 1981.

Crawford, O. G. S. The Eye Goddess. London, 1957.

Durand, Gilbert. Les structures anthropologiques de l'imaginaire. 3d ed. Paris, 1969.

Hocart, Arthur M. "The Mechanism of the Evil Eye." Folk-Lore 49 (June 1938): 156–157.

Pettazzoni, Raffaele. The All-Knowing God. Translated by H. J. Rose. London, 1956.

Seligman, Siegfried. Der böse Blick. Berlin, 1910.

Vries, Jan de. Altgermanische Religionsgeschichte. 2d ed. 2 vols. Berlin, 1956–1957.

Vries, Jan de. Keltische Religion. Stuttgart, 1961.

Michel Meslin (1987)

Translated from French by Kristine Anderson

Eye

Evolution of the eye

Anatomy and function of the human eye

Retina

Optic nerve

Resources

The eye is the organ of sight in humans and animals that 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 housed in the orbital sockets in the skull. Sight begins when light waves enter the eye through the cornea (the transparent layer at the front of the eye) pass through 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 that functions like the film in a (non-digital) camera. Photoreceptor neurons 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 brain, the electrical impulses are interpreted as images.

Evolution of the eye

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 constrict composite pictures of objects. They are very sensitive to detecting movement.

Anatomy and function of the human eye

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, and the 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 of the posterior chamber. An irregularly shaped eyeball results in ineffective focusing of light onto the retina and is usually correctable with glasses or contact lenses. An abnormally high intraocular pressure, due to overproduction of aqueous fluid or to the reduction in its outflow through a duct called the canal of Schlemm, produces glaucoma, a usually painless and readily treatable condition, which may lead to irreversible blindness if left untreated.

Elevated intraocular pressure is easily detectable with a simple, sight-saving, pressure test during routine eye examinations. 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 which insert 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 is the transparent cornea, which bends incoming light onto the lens inside the eye. A fine mucus membrane, the conjunctiva, covers the cornea and, also, 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 eyelid and 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 layer of the eye is the sclera, a dense, tough, opaque coat visible as the white of the eye. Its outer layer contains blood vessels that produce a blood-shot eye when the eye is irritated. The middle or uveal layers of the eye is densely pigmented, well supplied with blood, and includes 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 chamber behind the cornea. The iris gives the eye its color, which varies depending on 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 iris is 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 of incoming 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 the ciliary 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 good blood supply and provides oxygen and nutrients to the retina.

The front of the eye houses the anterior cavity that 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 that contains 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 chamber of 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), semisolid, 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 in the posterior chamber behind the iris and in front of the vitreous humor. The lens 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 causes 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 that necessitates glasses, bifocals, or contact lenses to restore clear, 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.

Retina

The retina is the innermost layer of the eye. The retina is 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 first to chemical energy and then electrical energy. Rods function in dim light, allowing limited nocturnal (night) vision: it is with rods that humans 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 six 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 hollow in 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 the fovea, the macula lutea. Continuing outward from this belt, the cone density decreases and the ratio of rods to cones increases. Both rods and cones disappear completely at the edges of the retina.

Optic nerve

The optic nerve connects the eye to the brain. These fibers of the optic nerve run from the surface of the retina and converge at 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.

KEY TERMS

Aqueous humor— Clear liquid inside the anterior and posterior chambers.

Choroid— Light-impermeable lining behind the sclera.

Cornea— The outer, transparent lens that covers the pupil of the eye and admits light.

Crystalline lens— Focusing mechanism located behind the iris and pupil.

Fovea— Tiny hollow in the retina and area of acute vision.

Iris— Colored portion of the eye.

Ophthalmology— Branch of medicine dealing with the eye.

Pupil— Adjustable opening in the center of the iris.

Retina— An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

Sclera— White of the eye.

Vitreous body— Opaque, gel-like substance inside the vitreous cavity.

The fibers of this nerve carry electrical impulses 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 three 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.

According to the Eye Bank Association of America, as of 2005, over 100,000 corneal transplants are performed around the world each year, with over 37,000 performed in the United States. Since 1961, over one million corneal transplants have been performed in the United States. 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 lack of eye donors. Although over 37,000 corneal transplants are performed in any given year in the United States, corneal tissue is always in demand. Promising medical advances are being made with artificial corneas, but currently these costly substances are reserved for the most serious corneal cases. Success rates with donor (natural) corneas are still higher than with artificially made corneas.

See also Vision disorders.

Resources

BOOKS

Denniston, Alistair. Oxford Handbook of Ophthalmology. Oxford, UK, and New York: Oxford University Press, 2006.

Fekrat, Sharon and Jennifer S. Weizer, eds. All About Your Eyes. Durham, NC: Duke University Press, 2006.

Goldberg, Stephen, M.D. Ophthalmology Made Ridiculously Simple. Miami, FL: MedMaster, 2005.

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

PERIODICALS

Malhotra, R. “Nasolacrimal Duct Obstruction Following Chicken Pox.” Eye 16, no. 1 (2002): 88-89.

Marie L. Thompson

Eye

The eye is the organ of sight in humans and animals which 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 housed in the orbital sockets in the skull. Sight begins when light waves enter the eye through the cornea (the transparent layer at the front of the eye) pass through 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. Photoreceptor neurons 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 brain, the electrical impulses are interpreted as images.

Evolution of the eye

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 which constrict composite pictures of objects. They are very sensitive to detecting movement.

Anatomy and function of the human eye

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, and the 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 of the posterior chamber. An irregularly shaped eyeball results in ineffective focusing of light onto the retina and is usually correctable with glasses or contact lenses. An abnormally high intraocular pressure, due to overproduction of aqueous fluid or to the reduction in its outflow through a duct called the canal of Schlemm, produces glaucoma, a usually painless and readily treatable condition, which may lead to irreversible blindness if left untreated. Elevated intraocular pressure is easily detectable with a simple, sight-saving, pressure test during routine eye examinations. The ophthalmic arteries provide the blood supply to the eyes, and the movement of the eyeballs is facilitated by six extraocular muscles which run from the bony orbit which insert 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 is the transparent cornea, which bends incoming light onto the lens inside the eye. A fine mucus membrane , the conjunctiva, covers the cornea, and also 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 eyelid and 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 layer of the eye is the sclera, a dense, tough, opaque coat visible as the white of the eye. Its outer layer contains blood vessels which produce a "blood-shot eye" when the eye is irritated. The middle or uveal layers of the eye is densely pigmented, well supplied with blood, and includes 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 chamber behind the cornea. The iris gives the eye its color , which varies depending on 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 iris is 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 of incoming 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 which also connects to the lens via suspensory ligaments. The muscles of the ciliary 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 good blood 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 which contains aqueous humor. This is a clear watery fluid which 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 chamber of 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 which 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 in the posterior chamber behind the iris and in front of the vitreous humor. The lens 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 that necessitates glasses, bifocals, or contact lenses to restore clear, 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.

Retina

The retina is the innermost layer of the eye. The retina is 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 first to 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 six 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 hollow in 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 the fovea, the macula lutea. Continuing outward from this belt, the cone density decreases and the ratio of rods to cones increases. Both rods and cones disappear completely at the edges of the retina.

Optic nerve

The optic nerve connects the eye to the brain. These fibers of the optic nerve run from the surface of the retina and converge at 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 electrical impulses 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 lack of eye donors.

See also Vision disorders.

Resources

books

Davison, Hugh, ed. The Eye. San Diego: Academic Press, 1984.

Introduction to Ophthalmology. San Francisco: American Academy of Ophthalmology, 1980.

Moller, Aage R. Sensory Systems: Anatomy and Physiology. New York: Academic Press, 2002.

periodicals

Koretz, Jane F., and George H. Handelman. "How the Human Eye Focuses." Scientific American (July 1988): 92-99.

Malhotra, R. "Nasolacrimal Duct Obstruction Following Chicken Pox." Eye 16, no. 1 (2002): 88-89.

Marie L. Thompson

KEY TERMS

Aqueous humor

—Clear liquid inside the anterior and posterior chambers.

Choroid

—Light-impermeable lining behind the sclera.

Cornea

—The outer, transparent lens that covers the pupil of the eye and admits light.

Crystalline lens

—Focusing mechanism located behind the iris and pupil.

Fovea

—Tiny hollow in the retina and area of acute vision.

Iris

—Colored portion of the eye.

Ophthalmology

—Branch of medicine dealing with the eye.

Pupil

—Adjustable opening in the center of the iris.

Retina

—An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

Sclera

—White of the eye.

Vitreous body

—Opaque, gel-like substance inside the vitreous cavity.

Eye

The eye is the organ of sight (vision) in humans and animals. The eye works by transforming light waves into visual images. Eighty percent of all information received by the human brain comes from the eyes. These organs are almost spherical in shape and are housed in the eye (orbital) sockets in the skull.

Sight begins when light waves enter the eye through the cornea (the transparent layer at the front of the eye), pass through the pupil (the

Words to Know

Aqueous humor: Clear liquid filling the small cavities between the cornea and the iris and between the iris and the lens.

Astigmatism: Vision disorder caused by an uneven curvature in the cornea (sometimes the lens), resulting in indistinct or slightly out-of-focus images.

Cataract: A clouding of the lens of the eye.

Choroid: Delicate membrane between the sclera and the retina.

Cones: Light-sensitive nerve cells of the retina that function chiefly in bright light and are sensitive to color.

Cornea: Protective lens covering the iris.

Farsightedness: Vision disorder caused by an eyeball that is too short or a lens that is too weak; objects far away are seen easily while those up close appear blurry.

Glaucoma: Serious vision disorder caused by a buildup of aqueous humor, resulting in pressure against the retina.

Iris: Colored portion around the pupil that regulates the amount of light entering the eye.

Lacrimal gland: Tear-producing gland that lies immediately above each eyeball at the outer corner of the eye socket.

Nearsightedness: Vision disorder caused by an eyeball that is too long or a lens that is too strong; objects up close are seen easily while those far away appear blurry.

Pupil: Adjustable opening in the center of the iris through which light enters the eye.

Retina: Photosensitive lining inside the eye.

Rods: Light-sensitive nerve cells of the retina that function chiefly in dim light.

Sclera: Tough, fibrous outer covering (the "white") of the eyeball.

Vitreous humor: Clear, gel-like substance inside the large cavity in back of the lens (the center of the eyeball).

opening in the center of the colored portion of the eye, called the iris), then through a clear lens behind the iris. The lens focuses light onto the retina, which functions like the film in a camera. Nerve cells in retinas, called rods and cones, convert light energy into electrical impulses. These impulses are then carried via the optic nerve to the brain where they are interpreted as images.

The human eyeball is about 0.9 inch (2.3 centimeters) in diameter and is not perfectly round, being slightly flattened in the front and back. The eye consists of three layers: the sclera (pronounced SKLIR-a), the choroid (pronounced KOR-oid), and the retina.

Sclera

The sclera, the outer fibrous layer, encases and protects the eyeball. The visible portion of the sclera is seen as the "white" of the eye. When that portion is irritated, the small blood vessels contained in the layer enlarge, producing a "bloodshot eye." In the center of the visible portion of the sclera is the cornea, which projects slightly forward. A delicate membrane, the conjunctiva, covers the cornea and visible portion of the sclera.

Choroid

The choroid is a thin membrane lying underneath the sclera. It is composed of a dense pigment and numerous blood vessels that nourish the internal tissues of the eye. At the front end of the choroid is the ciliary body. Running like a ring around the visible portion of the eye, the ciliary body connects the choroid with the iris. The ciliary body contains muscles that are connected by ligaments to the lens behind the iris. The iris is the visible portion of the choroid. It gives the eye its color, which varies depending on the amount of pigment present in the choroid. Dense pigment makes the iris brown, while little pigment makes the iris blue. If there is no pigment the iris is 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, increasing the amount of light entering. Extreme fear, head injuries, and certain drugs can also dilate the pupil.

Lens

The lens is a crystal-clear, flexible body that is biconvex (curving outward on both surfaces). The entire surface of the lens is smooth and shiny, contains no blood vessels, and is encased in an elastic membrane. The lens sits behind the iris and focuses light on the retina. In addition to holding the lens in place, the muscles of the ciliary body contract and relax, causing the lens to either fatten or become thin. As the shape of the lens changes, so does its focus.

Retina

The retina is the innermost layer of the eye. The retina is thin, delicate, sensory tissue composed of layers of light-sensitive nerve cells. The retina begins at the ciliary body (not at the front of the eye) and encircles the entire interior portion of the eye. Rods and cones, nerve cells of the retina, convert light first to chemical energy and then electrical energy. Rods function chiefly in dim light, allowing limited night vision: it

is with rods that we see the stars. Rods cannot detect color, but they are the first cells to detect movement. Cones function best in bright light and are sensitive to color. In each eye there are about 126 million rods and 6 million cones.

Fluids of the eye

Between the cornea and the iris and between the iris and the lens are two small cavities. These cavities are filled with a clear watery fluid known as aqueous humor. This fluid aids good vision by helping maintain eye shape, providing support for the internal structures, supplying nutrients to the lens and cornea, and disposing of the eyes' cellular waste.

The large cavity in back of the lens (the center of the eyeball) 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 is 99 percent water and contains no cells. It helps to maintain the shape of the eye and support its internal components.

Other structures of the eye

Tears are produced by the lacrimal gland, which lies immediately above each eyeball at the outer corner of the eye socket. Tears flow through ducts from this gland to the area beneath the upper eyelid. Blinking spreads the tears across the cornea's outside surface, keeping it moist and clean. Tear fluid then either evaporates or drains from the inner corner of the eye into the nasal cavity.

Eyelashes, eyelids, and eyebrows all help to protect the eye from dust and dirt. Extending from the eye socket to the eyeball are six small muscles that contract and relax, allowing the eye to move in various directions.

Vision disorders

Farsightedness and nearsightedness are common vision disorders. They occur because of a defect in the shape of the eyeball or in the refractive power (ability to bend light rays) of the lens. In these cases, the image the eye perceives is distorted because the parallel rays of light that enter the eye do not fall perfectly on a tiny hollow (called the fovea) in the retina at the back of the eye. However, corrective eyeglasses can easily overcome these disorders.

With farsightedness, objects far away are seen easily while those up close appear blurry. The cause may be that the eyeball is too short or the lens is too weak.

With nearsightedness, objects up close are seen easily while those far away appear blurry. The cause may be that the eyeball is too long or the lens is too strong.

Astigmatism, another common vision disorder, can occur in combination with farsightedness or nearsightedness. Individuals with astigmatism see indistinct or slightly out-of-focus images. The condition is brought about by an uneven curvature in the cornea (sometimes the lens). As a result, some light rays entering the eye focus on the fovea while others focus in front or behind it. Like farsightedness and nearsightedness, astigmatism can be corrected with eyeglasses or contact lenses.

A cataract is the clouding of the lens, which alters the amount of light entering the eye. The most common cataracts are senile cataracts, a result of aging that occurs in almost all people over 65 years old. These cataracts grow slowly over months or years, cause no pain, usually affect both eyes, and gradually reduce vision. If not treated, they eventually cause blindness. Clear vision can be restored by a relatively simple surgical procedure in which the entire lens is removed and an artificial lens is implanted.

Glaucoma is a serious vision disorder caused by a buildup of aqueous humor, which is prevented for some reason from properly draining. The excessive amount of fluid causes pressure against the retina, affecting vision. Long-term diseases like diabetes or a malfunctioning thyroid gland can bring about glaucoma. If left untreated, glaucoma will result in permanent blindness. The condition can be controlled with drugs that either increase the outflow of aqueous humor or decrease its production.

[See also Radial keratotomy ]

eye / ī/ • n. 1. each of a pair of globular organs in the head through which people and vertebrate animals see, the visible part typically appearing almond-shaped in animals with eyelids. ∎  the corresponding visual or light-detecting organ of many invertebrate animals. ∎  used to refer to someone's power of vision and in descriptions of the manner or direction of someone's gaze: his sharp eyes had missed nothing. ∎  used to refer to someone's opinion or attitude toward something: in the eyes of his younger colleagues, Mr. Arnett was an eccentric.2. a thing resembling an eye in appearance, shape, or relative position, in particular: ∎  the small hole in a needle through which the thread is passed. ∎  Naut. a loop at the end of a rope, esp. one at the top end of a shroud or stay. ∎  a rounded eyelike marking on an animal, such as those on the tail of a peacock; an eyespot. ∎  a round, dark spot on a potato from which a new shoot can grow. ∎  a center cut of meat: eye of round. ∎  the center of a flower, esp. when distinctively colored. ∎  the calm region at the center of a storm or hurricane.• v. (eye·ing or ey·ing) [tr.] look at or watch closely or with interest: Rose eyed him warily.PHRASES: all eyes used to convey that a particular person or thing is currently the focus of public interest or attention: all eyes are on the hot spots of eastern Europe.be all eyes be watching eagerly and attentively.close (or shut) one's eyes to refuse to notice or acknowledge something unwelcome or unpleasant: he couldn't close his eyes to the truth—he had cancer.give someone the eye inf. look at someone in a way that clearly indicates one's sexual interest in them: this blonde was giving me the eye.have (or keep) one's eye on keep under careful observation. ∎  hope or plan to acquire: the county sheriff has his eye on retirement.have (or with) an eye to have (or having) as one's objective: with an eye to transatlantic business, he made a deal in New York. ∎  consider (or be considering) prudently; look (or be looking) ahead to: the charity must have an eye to the future. (only) have eyes for be (exclusively) interested in or attracted to: he has eyes for no one but you.keep an eye (or a sharp eye) on keep under careful observation: dealers are keeping an eye on the currency markets.keep one's eyes open be on the alert; watch carefully or vigilantly for something: visitors should keep their eyes peeled for lions.lay (or set or clap) eyes on inf. see: Harry has not laid eyes on Alice for twenty years.make eyes at someone look at someone in a way that indicates one's sexual interest.open someone's eyes enlighten someone about certain realities; cause someone to realize or discover something: the letter finally opened my eyes to the truth.see eye to eye have similar views or attitudes to something; be in full agreement: Mr. Trumble and I do not always see eye to eye.with one eye on giving some but not all one's attention to: I sat with one eye on the clock, waiting for my turn.DERIVATIVES: eyed / īd/ adj. [in comb.] a brown-eyed girl.eye·less adj.