Kids Research Express

Eye Diseases

Eye disorders may be classified according to the part of the eye in which the disorders occur.

The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection by staphylococci. Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands. Abscesses of the eyelids are sometimes the result of penetrating wounds. Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping of the upper lid. Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns. Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscular contractions resulting from chronic irritation. The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignant tumors. Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball. See Conjunctivitis; Trachoma.

Disorders of the cornea, which may result in a loss of transparency and impaired sight, are usually the result of injury but may also occur as a secondary result of disease; for example, edema, or swelling, of the cornea sometimes accompanies glaucoma.

The choroid, or middle coat of the eyeball, contains most of the blood vessels of the eye; it is often the site of secondary infections from toxic conditions and bacterial infections such as tuberculosis and syphilis. Cancer may develop in the choroidal tissues or may be carried to the eye from malignancies elsewhere in the body. The light-sensitive retina, which lies just beneath the choroid, also is subject to the same type of infections. The cause of retrolental fibroplasia, however—a disease of premature infants that causes retinal detachment and partial blindness—is unknown. Retinal detachment may also follow cataract surgery. Laser beams are sometimes used to weld detached retinas back onto the eye. Another retinal condition, called macular degeneration, affects the central retina. Macular degeneration is a frequent cause of loss of vision in older persons. Juvenile forms of this condition also exist.

The optic nerve contains the retinal nerve fibers, which carry visual impulses to the brain. The retinal circulation is carried by the central artery and vein, which lie in the optic nerve. The sheath of the optic nerve communicates with the cerebral lymph spaces. Inflammation of that part of the optic nerve situated within the eye is known as optic neuritis, or papillitis; when inflammation occurs in the part of the optic nerve behind the eye, the disease is called retrobulbar neuritis. When the pressure in the skull is elevated, or increased in intracranial pressure, as in brain tumors, edema and swelling of the optic disk occur where the nerve enters the eyeball, a condition known as papilledema, or chocked disk.

For disorders of the crystalline lens, see Cataract. See also Color Blindness.

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Eye Comparative Anatomy

The simplest animal eyes occur in the cnidarians and ctenophores, phyla comprising the jellyfish and somewhat similar primitive animals. These eyes, known as pigment eyes, consist of groups of pigment cells associated with sensory cells and often covered with a thickened layer of cuticle that forms a kind of lens. Similar eyes, usually having a somewhat more complex structure, occur in worms, insects, and mollusks.

Two kinds of image-forming eyes are found in the animal world, single and compound eyes. The single eyes are essentially similar to the human eye, though varying from group to group in details of structure. The lowest species to develop such eyes are some of the large jellyfish. Compound eyes, confined to the arthropods (see Arthropod), consist of a faceted lens, each facet of which forms a separate image on a retinal cell, creating a moasic field. In some arthropods the structure is more sophisticated, forming a combined image.

The eyes of other vertebrates are essentially similar to human eyes, although important modifications may exist. The eyes of such nocturnal animals as cats, owls, and bats are provided only with rod cells, and the cells are both more sensitive and more numerous than in humans. The eye of a dolphin has 7,000 times as many rod cells as a human eye, enabling it to see in deep water. The eyes of most fish have a flat cornea and a globular lens and are hence particularly adapted for seeing close objects. Birds’ eyes are elongated from front to back, permitting larger images of distant objects to be formed on the retina.

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Protective Structures of the Eye

Several structures, not parts of the eyeball, contribute to the protection of the eye. The most important of these are the eyelids, two folds of skin and tissue, upper and lower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury. The eyelashes, a fringe of short hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed.

Inside the eyelids is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera. Each eye also has a tear gland, or lacrimal organ, situated at the outside corner of the eye. The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away any small dust particles or other foreign matter on the surface of the eye.

Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blink more often and more tears are produced. On the edges of the eyelids are a number of small glands, the Meibomian glands, which produce a fatty secretion that lubricates the eyelids themselves and the eyelashes.

The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running into the eyes. The hollow socket in the skull in which the eye is set is called the orbit. The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye from mechanical injury by blows or collisions.

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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.

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 rod-shaped 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 (see Vitamin). 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.

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Glycosides

Glycosides, class of complex chemical compounds in plants. They are broken down by plant enzymes into sugars, among which glucose is generally included, and into other substances. The term glucoside is often used synonymously with glycoside, but in its more specific meaning it refers to glycosides that yield glucose.

Each glycoside in a plant is hydrolyzed (converted in a reaction with water) by an enzyme, usually a specific enzyme found in the same plant. The enzyme emulsin, however, causes hydrolysis of several glycosides. The enzymes and glycosides are stored in separate plant cells until the reaction products of the glycosides are needed and the enzymes are activated.

Glycosides are believed to serve several purposes in the plant. Glycosides are bitter tasting, and it is believed that they help keep birds and insects from eating seeds and fruit before they are fully grown, by which time the glycosides have been converted to sweet sugars. When a plant tissue is bruised, the enzymes hydrolyze the glycosides into products, such as phenol compounds and acids that have an antiseptic action and prevent decay of the damaged tissues.

Glycosides are soluble in water and are obtained from plants by water extraction. They are mostly colorless crystalline solids with a bitter taste. Simple glycosides have been synthesized in the laboratory, and several hundred glycosides have been extracted from plants and used for many purposes. Among the important glycosides are indican (see Indigo Plant), used for dyeing; digitalin (see Digitalis), used in medicine; and the saponins, foaming agents used industrially and medicinally.

Glucose

Glucose, monosaccharide sugar, C₆H₁₂O₆. It is found in honey and the juices of many fruits; the alternate name grape sugar is derived from the presence of glucose in grapes. It is the sugar most often produced by hydrolysis of natural glycosides. Glucose is a normal constituent of the blood of animals (see Sugar Metabolism).

Glucose is a white crystalline solid, less sweet than ordinary table sugar. Solutions of glucose rotate the plane of polarization of polarized light to the right; hence the alternative name dextrose (Latin dexter, “right”). Glucose crystallizes in three different forms. The degree of rotation of polarized light is different for each form.

Glucose is formed by the hydrolysis of many carbohydrates, including sucrose, maltose, cellulose, starch, and glycogen. Fermentation of glucose by yeast produces ethyl alcohol and carbon dioxide. Glucose is made industrially by the hydrolysis of starch under the influence of dilute acid or, more commonly, under that of enzymes. It is chiefly used as a sweetening agent in the food-processing industries. It is also used in tanning, in dye baths, in making tableted products, and in medicine for treating dehydration and for intravenous feeding.

Sugar Metabolism: FERMENTATION

The chemical reaction whereby plants such as yeast use sugar is remarkably similar to the metabolism of sugar in the human body. Yeast contains a mixture of 12 enzymes, which are collectively known as zymase. Most of these enzymes, including hexokinase, are identical to enzymes involved in the human metabolism of glucose. The principal difference occurs at the end of the chain of reactions; a glucose-decomposition product called pyruvic acid is converted in the body into lactic acid, but in plants it is converted by zymase into ethyl alcohol. See Fermentation.

Many problems in the physiology of sugar remain to be solved. Present work in this field has been accelerated since the discovery of tracer elements, especially radioactive carbon. Sugars, synthesized with radioactive carbon, can be followed through the body after ingestion.

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