EYE

specialized sensory organ capable of light reception and, in vertebrate animals, able to receive visual images that are then carried to the visual centre in the brain. Eyes fall broadly into two categories: direction (non-image-forming) and image-forming. The term ocellus is broadly applied to any simple eye and also to the eyespot, an eyelike structure of certain one-celled organisms. Direction eyes are found in many worms, mollusks, cnidarians, echinoderms, and other invertebrates. Such eyes have in common the possession of one or more (usually many) light-sensitive cells and a partial shield of pigment cells. Often the shield is cup-shaped and nearly surrounds the sensors. In bilaterally symmetrical animals the eyes are usually paired. Image-forming eyes are found in certain mollusks (cephalopods, some bivalves), most arthropods, and nearly all vertebrates. Two basic types of image-forming eyes occur, simple and compound. The basic components of the simple eye are the lens and the retina, the former focusing impinging light as an image on the latter. Light-sensitive cells of the retina transmit to the brain signals of varying intensity, depending on the intensity of light on their part of the image. The compound eye, found only in arthropods, is most highly developed in insects and crustaceans. It consists of a number of closely packed elements, called ommatidia, each of which is essentially a separate eye. Each ommatidium consists of a hexagonal, rectangular, or circular tube with a lens and a sensory element (rhabdomere). The rhabdomere may receive light through its own lens alone or through several adjacent lenses. In either case, the overlap of the fields of adjacent lenses results in the arthropod's seeing a multiple image, which is partially integrated in the brain. Horizontal section of the eye The human eye (see illustration) is roughly spherical with opaque sides and back and with a transparent front, through which the light rays pass. The eyeball houses the retina, a layer of nerve tissue made up of millions of light receptors, and all of the structures needed to focus light onto it. The sclera, the tough outer shell of the eyeball, is composed of dense fibrous tissue; a segment of the sclera, the cornea, is modified to allow light to pass into the eye and to aid in focusing. The eyeball is filled with a transparent jellylike material, the vitreous humour, which maintains its spheroidal shape. Immediately beneath the sclera is an underlying vascular layer, the choroid, which supplies nutrients to all parts of the eye and which also contains the ciliary body, a muscular structure that alters the shape of the lens in focusing. Within the choroid lies the retina. The scleral surface on exposed areas of the eye is covered with a mucous membrane, the conjunctiva, which protects the eye from becoming dry. The transparent cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. Behind the cornea is the anterior chamber, filled with a watery fluid called the aqueous humour, and the iris, a muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, an opening at the iris' centre. The aqueous humour flows through the pupil into the posterior chamber, a small space between the iris and the lens. The shape of the lens is controlled by the action of the ciliary body, altering the focal length of the lens as needed. The lens focuses an image onto the rods and cones (receptor cells) of the retina at the back of the eye. Much of the image is projected onto the fovea, a structure near the centre of the retina where large numbers of cones give maximum sharpness of vision. The receptors record the image and transmit it to the optic nerve, which exits from the eyeball behind the optic disk on its way to the visual centres of the brain. Some common disorders of vision are myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. In myopia the lens' point of focus falls in the vitreous body, so that when light reaches the retina it is out of focus. Conversely, the point of focus in hyperopia would fall behind the retina. Astigmatism results from defects in the corneal curvature. The effects of these disorders can be ameliorated with the aid of eyeglasses or contact lenses. Other disorders include night blindness and colour blindness (qq.v.). There are many different diseases that can affect the eyes, including ones that attack eye muscle control and those that cause the degeneration of the retina (resulting in a tunnellike, narrowing field of vision, or a field of vision where the periphery is clear but the centre dark). Glaucoma is a more common disease, especially among older people. It is the result of an increase in the pressure of the fluids in the eye and can lead to loss of vision.