Kids Research Express

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 continuo...

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 . Related Topics: Human Eye ,  Functioning of the Eye Eye Protective Structures Eye Comparative Anatomy Eye Diseases

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 n...

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, 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 huma...

Foot (anatomy), in human anatomy, lowest structure on the leg, consisting of 26 bones and resembling the hand. Seven thick, short, tarsal bones compose the heel and back of the instep; five parallel metatarsal bones, which form the front of the instep, spread toward the front of the foot to form the ball. Fourteen smaller phalanges make up the toes; the large toe is composed of two, and each smaller toe is made up of three. All the bones are firmly connected by tough bands of tissue called ligaments; the plantar ligament runs from the heel bone to the metatarsals, keeping the bones in place. Movements of the foot are controlled by leg muscles. The tarsal and metatarsal bones form the two arches of the foot: the plantar arch running from the heel to the ball and normally touching the ground only at each end, and the metatarsal arch across the ball of the foot. With the thick layer of fatty tissue under the sole, these flexible arches absorb pressure and the shocks of walking and jumping...

Gallbladder, muscular organ that serves as a reservoir for bile , present in most vertebrates. In humans, it is a pear-shaped membranous sac on the undersurface of the right lobe of the liver just below the lower ribs. It is generally about 7.5 cm (about 3 in) long and 2.5 cm (1 in) in diameter at its thickest part; it has a capacity varying from 1 to 1.5 fluid ounces. The body (corpus) and neck (collum) of the gallbladder extend backward, upward, and to the left. The wide end (fundus) points downward and forward, sometimes extending slightly beyond the edge of the liver. Structurally, the gallbladder consists of an outer peritoneal coat (tunica serosa); a middle coat of fibrous tissue and unstriped muscle (tunica muscularis); and an inner mucous membrane coat (tunica mucosa). The function of the gallbladder is to store bile, secreted by the liver and transmitted from that organ via the cystic and hepatic ducts, until it is needed in the digestive process. The gallbladder, when functio...

Gene, basic unit of heredity found in the cells of all living organisms, from bacteria to humans. Genes determine the physical characteristics that an organism inherits, such as the shape of a tree’s leaf, the markings on a cat’s fur, and the color of a human hair (see Heredity ). Genes are composed of segments of deoxyribonucleic acid (DNA), a molecule that forms the long, threadlike structures called chromosomes. The information encoded within the DNA structure of a gene directs the manufacture of proteins, molecular workhorses that carry out all life-supporting activities within a cell (see Genetics ). Chromosomes within a cell occur in matched pairs. Each chromosome contains many genes, and each gene is located at a particular site on the chromosome, known as the locus. Like chromosomes, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene in the other chromosome of the pair, and these two genes are called alleles. All...

Hemoglobin, iron-protein compound in red blood cells that gives blood its red color and transports oxygen, carbon dioxide, and nitric oxide. Hemoglobin is present in all but the least complex of animals. It carries oxygen from the lungs or gills, where blood is oxygenated, to body cells. When saturated with oxygen, hemoglobin is called oxyhemoglobin. After releasing oxygen to the body tissues, hemoglobin reverses its function and picks up carbon dioxide, the waste product of cellular respiration, for transport to the lungs, where it is expired. When saturated with carbon dioxide, hemoglobin is known as carboxyhemoglobin. In 1996 scientists discovered that, in addition to oxygen and carbon dioxide, hemoglobin takes up and releases a third gas, nitric oxide. Nitric oxide plays an important role in regulating blood pressure by relaxing the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regul...

Urine, pale yellow fluid produced by the kidneys , composed of dissolved wastes and excess water or chemical substances from the body. It is produced when blood filters through the kidneys, which remove about 110 liters (230 pints) of watery fluid from the blood every day. Most of this fluid is reabsorbed into the blood, but the remainder is passed from the body as urine. Urine leaves the kidneys, passes to the bladder through two slender tubes, the ureters, and exits the body through the urethra. A healthy adult can produce between 0.5 to 2 liters (1 to 4 pints) of urine a day, but the quantity varies considerably, depending on fluid intake and loss of fluid from sweating, vomiting, or diarrhea. Water accounts for about 96 percent, by volume, of the urine excreted by a healthy person. Urine also contains small amounts of urea, chloride, sodium, potassium, ammonia, and calcium. Other substances, such as sugar, are sometimes excreted in the urine if their concentration in the body beco...

Tendon, in anatomy, extremely strong cord that is flexible but does not stretch, made of large bundles of white, fibrous protein known as collagen. A tendon joins a muscle with a bone or another muscle. A tendon delivers the pulling force of a muscle to a bone, which makes body motion possible. A tendon attached to a bone is cylindrical while one connected to a flat muscle on the wall of the abdomen is a wide sheet of fibers. The tendons in the hands, wrists, and feet are enclosed in protective membranes and lubricated by a fluid to prevent excessive friction. A larger tendon includes a system of nerves that registers pain when the tendon is squeezed, and triggers a reflex contraction in the adjoining muscle when the tendon is stretched. The Achilles tendon , which extends from midcalf to the heel, is the thickest and strongest tendon in the human body. Rupture of this tendon is a serious injury that most commonly affects athletes participating in events involving sprinting and jumpin...

Tears, fluid produced in the lacrimal glands above the outside corner of each eye . The fluid lubricates the eyes and protects them from foreign matter and infection—the latter because it contains salt and lysozyme, an enzyme that kills microorganisms. During normal flow, tears constantly wash over the exterior of the eye and collect at its inside corner, where they drain through two small lacrimal ducts into the nasal cavity. During heavy tear flow, such as that caused by irritation, the excess fluid that is not taken up by the lacrimal ducts spills over the eyelids, carrying off foreign bodies too large to be flushed through the lacrimal ducts.

The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hingelike action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles , to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system.

Menopause, permanent ending of menstruation in women. Menopause marks the end of a woman’s natural ability to bear children. Menopause is usually preceded by 10 to 15 years during which the ovaries gradually stop producing eggs and sex hormones, a period called the climacteric. Perimenopause encompasses this period of changing ovarian activity and also the first few years without menstrual cycling, typically characterized by hormonal and physical changes and sometimes emotional and psychological changes as well.

Melatonin, naturally occurring hormone that is released into the bloodstream during the hours of darkness. While scientists continue to study the function of melatonin in humans, they believe the hormone plays a role in the body’s circadian rhythm. This rhythm regulates the physiological functions that occur in the body within a 24-hour period, such as sleep-wake cycles, fluctuations in body temperature, heart rate, and blood pressure. More recently melatonin has become a popular nonprescription dietary supplement with the alleged ability to combat a variety of health problems. Melatonin is produced by the pineal body, a cone-shaped gland about the size of a pea that is situated near the center of the brain . The release of melatonin from the pineal body into the bloodstream is orchestrated by the hypothalamus, a region of the brain that governs the body’s internal environment to maintain temperature, water balance, and hormone balance. The hypothalamus receives cues about the amount ...

Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance. Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell.

Lysosome, membrane-bound sac found in nucleated cells that contains digestive enzymes that break down complex molecules in the body. Lysosomes are numerous in disease-fighting cells, such as white blood cells, that destroy harmful invaders or cell debris. Lysosomes vary greatly in size, typically ranging from 0.05 to 0.5 micrometers in diameter. Each lysosome is surrounded by a membrane that protects the cell from the lysosome’s digestive enzymes—if the lysosome breaks open, the enzymes would destroy the cell. Proteins embeded in the lysosome membrane protect the activity of the enzymes by maintaining the proper internal acidity. Membrane proteins also transport digested products out of the lysosome. Lysosome enzymes are manufactured in the rough endoplasmic reticulum and processed in the Golgi apparatus . They are delivered by sacs known as transport vesicles to fuse with three types of membrane-bound structures: endosomes, phagosomes, and autophagosomes. Endosomes form when the cell...

Lymph, common name for the fluid carried in the lymphatic system . Lymph is diluted blood plasma containing large numbers of white blood cells, especially lymphocytes, and occasionally a few red blood cells. Because of the number of living cells it contains, lymph is classified as a fluid tissue. Lymph diffuses into and is absorbed by the lymphatic capillaries from the spaces between the various cells constituting the tissues. In these spaces lymph is known as tissue fluid, plasma that has permeated the blood capillary walls and surrounded the cells to bring them nutriment and to remove their waste substances. The lymph contained in the lacteals of the small intestine is known as chyle. The synovial fluid that lubricates joints is almost identical with lymph, as is the serous fluid found in the body and pleural cavities. The fluid contained within the semicircular canals of the ear, although known as endolymph, is not true lymph.

Corticoid, also corticosteroid, any steroid hormone that derives from the outer layer, or cortex, of the adrenal gland . Among the corticoids are the glucocorticoids, such as cortisone and cortisol (see Hydrocortisone ), essential to metabolism and the body's reaction to stress; and the mineralocorticoids, which have other important regulatory functions, such as maintaining ion balance. The adrenal cortex also secretes sex hormones, although these are produced in much greater quantities by the testes and ovaries. See Endocrine System ; Steroids .

Heredity, process of transmitting biological traits from parent to offspring through genes , the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type . Heredity accounts for why offspring look like their parents: when two dogs mate, for example, they have puppies, not kittens. If the parents are both Chihuahuas, the puppies will also be Chihuahuas, not great Danes or Labrador retrievers. The puppies may be a little taller or shorter, a little lighter or a lot heavier than their parents are. Their faces may look a little different, or they may have different talents and temperaments. In all the important characteristics, however—the number of limbs, arrangement of organs, general size, fur type—they will share the traits of their parents. The principles of heredity hold true not only for a puppy but also for a virus, a roundworm, a pansy, or a human.

Adenoids, lymphoid tissue at the back of the throat, which usually shrinks and disappears by adolescence. Enlargement of this tissue, however, is fairly common in children and may interfere with breathing. Symptoms of enlarged adenoids include a nasal voice, persistent breathing through the mouth, snoring, and restless sleep. Formerly these tissues were routinely removed in children, because it was thought that inflamed adenoids led to recurrent infections and colds. More recently, medical science has recognized this condition as usually benign, and the number of adenoidectomies has consequently declined. See also Tonsil .

Sex Hormone, any of several chemical substances that affect the development and functioning of the reproductive system in vertebrates, or animals with a backbone. The sex hormones are divided into three major groups: gonadotropins, gonadal hormones, and lactogens. Gonadotropins stimulate the gonads, which are sperm- or egg-producing organs. The male gonads are the testes, which produce sperm, and the female gonads are the ovaries, which produce eggs. Gonadotropins are secreted by the pituitary gland, which is located in the center of the brain and is controlled by an area of the brain known as the hypothalamus. Gonadotropins, such as the leutinizing hormone (LH) and the follicle-stimulating hormone (FSH) in females and the interstitial cell-stimulating hormone (ICSH) in males, control the development and functions of the ovaries and testes, including menstruation and sperm production. Gonadal hormones such as estrogen, progesterone, and testosterone are secreted primarily by the teste...

Epinephrine, hormone secreted by the adrenal gland . The pure compound, first isolated by the Japanese chemist Jokichi Takamine, is also known as adrenaline. It was formerly prepared from adrenal glands, but it is now made synthetically. Epinephrine normally is present in the bloodstream in minute quantities. In times of excitement or emotional stress, however, large additional quantities are secreted, exerting a marked effect on body structures in preparation for physical exertion. For example, epinephrine stimulates the heart, constricts the small blood vessels, raises the blood pressure, liberates sugar stored in the liver, and relaxes certain involuntary muscles while contracting others. It is widely used as a drug to stimulate the heart in cases of shock, to prevent bleeding, and to expand lung bronchioles in acute asthma attacks. The adrenal gland also secretes a substance chemically related to epinephrine, called norepinephrine, noradrenaline, or levoarterenol. In general, the ...