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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Sugar Metabolism: GLYCEMIA AND GLYCOSURIA

If the body produces too much pituitary hormone or too little insulin, the amount of sugar in the blood rises abnormally, producing a condition known as hyperglycemia. In hyperglycemia the blood may contain as much as four times the normal amount of sugar. Hyperglycemia in itself is not lethal, but it is a symptom of a serious disease, diabetes mellitus. Diabetes is sometimes caused by a tumor or other condition in the pancreas that prevents the formation of insulin. Diabetic patients do not die of hyperglycemia, but if they are not given injections of insulin they may die from such causes as the accumulation of poisons in the body, produced by altered metabolism of fats; the body of the diabetic consumes fats as a substitute for the sugar that it cannot use.

If an excessive amount of insulin is injected into the body, the amount of sugar is reduced to a dangerously low level, a condition known as hypoglycemia or insulin shock. Controlled insulin shock is sometimes used in the treatment of certain types of mental illness.

In a normal individual, if the amount of sugar in the blood rises abnormally, the excess is removed from the blood by the kidneys and excreted in the urine. The presence of sugar in the urine is called glycosuria, and although it is an important symptom of diabetes, it is not always found in diabetic patients; moreover, glycosuria may appear in normal individuals immediately after a large meal. The critical test for diabetes is neither hyperglycemia nor glycosuria, but blood-sugar tolerance: after ingesting sugar, both normal and diabetic individuals show an increased percentage of blood sugar; the percentage remains high in the diabetic, whereas in the normal individual the excess glucose is rapidly converted into glycogen.

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Sugar Metabolism: ENZYMES AND HORMONES

The interconversion between glucose and glycogen is catalyzed by a number of different enzymes. Phosphorylase is responsible for the release of glucose-1-phosphate from glycogen; the reaction is enhanced by the hormones adrenaline and glucagon. Glucose-1-phosphate is converted to glucose-6-phosphate, which can either be metabolized or converted to free glucose, which enters the bloodstream. The uptake of glucose by cells is stimulated by insulin. Before glucose is used it is converted to glucose-6-phosphate (by hexokinase), which may be metabolized or (in the liver and in muscle) converted to uridine diphosphate glucose. From the latter compound glucose is transferred to glycogen, in a reaction catalyzed by glycogen synthetase and stimulated by insulin. By as yet unknown mechanisms, cortical and pituitary hormones as well as thyroxin are also involved in the control of carbohydrate metabolism.

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Sugar Metabolism: DIGESTION, ASSIMILATION, AND STORAGE

Carbohydrates such as starch, dextrin, glycogen (animal starch), sucrose (cane sugar), maltose (malt sugar), and lactose are broken down in the digestive tract into simple, six-carbon sugars that pass easily through the intestinal wall. Fructose (fruit sugar) and glucose are unchanged in the digestive tract and are absorbed as such. Cellulose, a common constituent of many foods, is an important nutritional element for some animals, notably cattle and termites, but has no value in human nutrition (see Nutrition, Human).

The digestion of carbohydrates is performed by various enzymes (see Enzyme). Amylase, found in saliva and in the intestine, breaks starch, dextrin, and glycogen into maltose, a 12-carbon sugar. Other sugar-converting enzymes in the small intestine break 12-carbon sugars into 6-carbon sugars. Maltase breaks maltose into glucose; sucrase, or invertase, breaks cane sugar into glucose and fructose; lactase breaks milk sugar into glucose and galactose.

The six-carbon sugars, which become the end products of carbohydrate digestion, pass through the wall of the small intestine into minute blood vessels and thence into the portal vein, which carries them to the liver. They are then converted into a single compound, glycogen (see Starch), which is stored there. This glycogen is available at all times and is converted to glucose and released into the bloodstream as required by the body. One of the end products of glucose metabolism in the muscles is lactic acid, which is carried by the bloodstream back to the liver and partly reconverted into glycogen.

Sugar Metabolism

Sugar Metabolism, process by which the body uses sugar for energy. Carbohydrates, one of the three principal constituents of food, form the bulk of the average human diet. The end product of the digestion and assimilation of all forms of carbohydrate is a simple sugar, glucose, commonly called grape sugar when found in food, or blood sugar when found in the human body. The metabolism of fats and of certain protein substances also sometimes leads to the production of glucose. Glucose is the principal fuel that the muscles and other portions of the body consume to produce energy. It is present in every cell and almost every fluid of the body, and its concentration and distribution are among the most important processes in human physiology. A few other sugars are of comparatively minor importance in human physiology, notably lactose, or milk sugar, which is formed in the mammary glands of all lactating animals and is present in their milk. See Metabolism; Sugar.

Thalamus

Thalamus, a brain part, consists of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus is the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.

Phagocytosis

Phagocytosis (Greek -phagos, “one that eats”; kytos, “cell”), process of ingestion of matter by cells known, in this context, as phagocytes. Single-celled life forms that bodily engulf and ingest foreign matter—whether other cells, bacteria, or nonliving material—display phagocytosis. In multicellular organisms the process is relegated to specialized cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders.

In humans and other higher animals, phagocytes are wandering cells that occur throughout the body. Larger phagocytes, called macrophages, are particularly important in the lymph system, liver, and spleen; amoeboid macrophages also travel throughout the body's tissues, feeding on bacteria and other foreign matter. Smaller phagocytes, which are known as granular leukocytes—a type of white blood cell—are carried throughout the body by the bloodstream (see Blood). Attracted to sites of infection by chemicals which are emitted by the invading bacteria, they can pass through blood-vessel walls to reach the invaders. The successfulness of the process is related to the nature of the alien material. Proteins in the blood normally coat foreign particles, attracting the phagocytes to adhere and feed. If more-active bacterial forms invade the body, however, they may not be ingested until physically trapped or until coated by particular proteins called antibodies (see Antibody; Immune System). If still uningested, they may actually be spread throughout the body by the phagocytes.

Brain

Brain, portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions—including love, hate, fear, anger, elation, and sadness—are controlled by the brain. It also receives and interprets the countless signals that are sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.

THE HUMAN BRAIN

The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex.

Functions of the Cerebral Cortex

Many motor and sensory functions have been “mapped” to specific areas of the cerebral cortex, some of which are indicated here. In general, these areas exist in both hemispheres of the cerebrum, each serving the opposite side of the body. Less well defined are the areas of association, located mainly in the frontal cortex, operative in functions of thought and emotion and responsible for linking input from different senses. The areas of language are an exception: both Wernicke’s area, concerned with the comprehension of spoken language, and Broca’s area, governing the production of speech, have been pinpointed on the cortex.

Left and Right Brain Functions

Although the cerebrum is symmetrical in structure, with two lobes emerging from the brain stem and matching motor and sensory areas in each, certain intellectual functions are restricted to one hemisphere. A person’s dominant hemisphere is usually occupied with language and logical operations, while the other hemisphere controls emotion and artistic and spatial skills. In nearly all right-handed and many left-handed people, the left hemisphere is dominant.

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The Human Eye

Structure of the Eye
The amount of light entering the eye (right) is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. A mesh of blood vessels, the choroid, supplies the retina with oxygen and sugar. Lacrimal glands (left) secrete tears that wash foreign bodies out of the eye and keep the cornea from drying out. Blinking compresses and releases the lacrimal sac, creating a suction that pulls excess moisture from the eye’s surface.
The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.

The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye.

Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended.

The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.

See: Functioning Of The Eye; Eye Protective Structures; Comparative Anatomy Of The Eye; Eye Diseases

Eye

Eye (anatomy), light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.

Fallopian Tube

Fallopian Tube, one of two ducts in female mammals leading from the ovaries to the upper part of the uterus. They are also known as oviducts. In the human female the fallopian tubes are about 2 cm (about 0.75 in) thick and 10 to 13 cm (4 to 5 in) long. As the ovum leaves the ovary it passes into the mouth of the adjoining fallopian tube and is propelled toward the uterus by hairlike projections called cilia on the inner surface of the tube. If the ovum is fertilized inside the tube, where most fertilization takes place, it usually implants in the uterus. Some fertilized ova, however, implant in the fallopian tube itself and must be surgically excised. The condition is called an ectopic pregnancy. Many cases of infertility in women are due to blocked fallopian tubes, which can result from infection, especially that which is contracted from sexually transmitted disease. Surgical severing and sealing of the fallopian tubes is a common method of preventing pregnancy. These tubes were named after their discoverer, the Italian anatomist Gabriello Fallopio. See also Reproductive System.

Feces

Feces, also stool, excreta, or residual waste materials, evacuated from the bowels. Through peristalsis (involuntary intestinal contractions) and digestion, partly digested food begins to assume the aspects of feces when it passes from the small intestine to the large intestine. In a healthy digestive system, feces consist of undigested and indigestible food products such as mucous secretions and cellulose; traces of intestinal juices from the liver, the pancreas, and other digestive glands; undestroyed enzymes; leucocytes; epithelial cells; cellular debris from the intestinal walls; fat globules; nitrogenous protein products; mineral salts; water; and large numbers of bacteria.

Fetus

Fetus, term applied to an animal embryo after a definite period has elapsed following conception. In human reproduction, for example, the period is eight weeks; for early embryonic development, see Embryology.

In the first half of the second month of gestation, the human embryo closely resembles that of other mammals, but in the latter part of the month the head becomes disproportionately large, principally because of development of the brain. The external genitalia also appear in the latter part of the second month. The extremities become more developed, and the fetus attains a length of about 3 cm (about 1.2 in).

By the end of the third month, centers of ossification appear in most of the bones, the fingers and toes become differentiated, and the external genitalia begin to show definite sex differentiation. After the fourth month the average fetus is almost 15 cm (almost 6 in) long and weighs about 113 g (about 4 oz). The sex of the fetus is easily identifiable. The face looks human, and movement is usually discernible. During the fifth and sixth months a downy covering called lanugo develops on the body, and the body becomes increasingly larger in proportion to the head. The fetus attains a length of about 30 cm (about 12 in) and weighs about 624 g (about 1 lb 6 oz).

During the seventh month the skin, which is red and wrinkled, is covered with a white substance called the vernix, or vernix caseosa, which protects the skin. The vernix is a mixture of epithelial cells, lanugo hairs, and secretions from the glands of the skin. By then the fetus is about 40 cm (about 15 in) and has attained a weight of more than 1 kg (more than 2 lb). The pupillary membrane disappears from the eyes. The body organs are sufficiently developed to sustain life outside the uterus; the more developed the fetus, the greater are its chances for extrauterine life. A fetus born at this period moves its limbs quite energetically and cries with a weak voice. After this period, during the eighth and ninth months, the fetus loses its wrinkled appearance due to the deposition of subcutaneous fat. The fingers and toes have well-developed nails.

Full term is reached at the end of the tenth lunar month of pregnancy. Most of the fetal hair has been shed, and the fetus is ready for birth, having attained a length of about 50 cm (about 20 in) and a weight of approximately 3 kg (approximately 7 lb). The vernix covers the entire surface of the body. When the infant is born before the full term and weighs less than 2.4 kg (5 lb 8 oz), it is considered premature.

Respiratory activity occurs in the fetus as early as the twelfth week of gestation and continues throughout its intrauterine life. The lungs do not function in any effective sense, however, because the fetus is enclosed in a sac that fills with a clear amniotic fluid early in the embryonic period. Oxygen and materials needed for nutrition are brought to the fetus from the placenta, a vascular organ which unites the fetus to the maternal uterus, by the umbilical vein. Conversely, the placenta is responsible for the conveyance of carbon dioxide and waste products from fetus to mother. The placenta has an increasing permeability as pregnancy advances. Metabolites, waste products of metabolism, gain access to the fetal circulation from the mother's blood by direct diffusion across the membranes and, in certain cases, by selective transfer of particles.