Kids Research Express

Brain

2010-10-26T21:26:00.000-07:00

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

2009-10-09T03:40:00.001-07:00

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 Eye Disorder and Diseases.

Eye

2009-10-09T03:39:00.010-07:00

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.

See Human Eye.

Fallopian Tube

2009-10-09T03:39:00.009-07:00

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

2009-10-09T03:39:00.007-07:00

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

2009-10-09T03:39:00.005-07:00

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.

Foot

2009-10-09T03:39:00.003-07:00

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

2009-10-09T03:39:00.001-07:00

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 functioning normally, empties through the biliary ducts into the duodenum to aid digestion by promoting peristalsis and absorption, preventing putrefaction, and emulsifying fat. See Digestive System.

Gene

2009-10-09T03:38:00.002-07:00

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. Alleles are alternate forms of the same gene. For example, a pea plant has one gene that determines height, but that gene appears in more than one form—the gene that produces a short plant is an allele of the gene that produces a tall plant. The behavior of alleles and how they influence inherited traits follow predictable patterns. Austrian monk Gregor Mendel first identified these patterns in the 1860s (see Mendel’s Laws).

In organisms that use sexual reproduction, offspring inherit one-half of their genes from each parent and then mix the two sets of genes together. This produces new combinations of genes, so that each individual is unique but still possesses the same genes as its parents. As a result, sexual reproduction ensures that the basic characteristics of a particular species remain largely the same for generations. However, mutations, or alterations in DNA, occur constantly. They create variations in the genes that are inherited. Some mutations may be neutral, or silent, and do not affect the function of a protein. Occasionally a mutation may benefit or harm an organism and over the course of evolutionary time, these mutations serve the crucial role of providing organisms with previously nonexistent proteins. In this way, mutations are a driving force behind genetic diversity and the rise of new or more competitive species that are better able to adapt to changes, such as climate variations, depletion of food sources, or the emergence of new types of disease (see Evolution).

Hemoglobin

2009-10-09T03:38:00.001-07:00

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 regulating the amount of nitric oxide to which the vessels are exposed.

Urine

2009-10-08T07:25:00.000-07:00

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 becomes too great. The volume, acidity, and salt concentration of urine are controlled by hormones. Measurements of the composition of urine are useful in the diagnosis of a wide variety of conditions, including kidney disease, diabetes, and pregnancy.

Tendon

2009-10-08T07:23:00.000-07:00

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 jumping. Rupture of a tendon in the finger can result in deformity of the hand.

Tears

2009-10-08T07:20:00.000-07:00

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.

Musculoskeletal System

2009-10-08T07:19:00.000-07:00

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

2009-10-08T07:18:00.000-07:00

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

2009-10-08T07:15:00.001-07:00

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 of sunlight that is absorbed by the eye—darkness causes the hypothalamus to stimulate melatonin release while light suppresses its release.

Meiosis

2009-10-08T07:10:00.000-07:00

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

2009-10-08T07:05:00.000-07:00

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 membrane surrounds nutritional molecules like polysaccharides, complex lipids, nucleic acids, or proteins. In a process called endocytosis, these molecules are broken down for reuse. Phagosomes form when the cell membrane engulfs large objects, like debris from sites of injury or inflammation or disease-causing bacteria, in a process called phagocytosis. Autophagosomes form when the endoplasmic reticulum wraps around spent cell structures, such as mitochondria, that are destined for recycling. In all cases the digestive enzymes supplied by the lysosomes digest the membrane-bound objects into simple compounds that are delivered to the cytoplasm as new cell-building materials.

Lymph

2009-10-08T07:01:00.000-07:00

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

2009-10-07T19:41:00.000-07:00

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

2009-10-07T19:38:00.000-07:00

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

2009-10-07T03:40:00.000-07:00

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

2009-10-07T03:37:00.000-07:00

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 testes and ovaries, placenta (the sac of nutritive tissue that supports and protects the fetus), and adrenal glands. Their chief function is to regulate the development of the secondary sex characteristics such as deepening of the voice in males and distribution of body hair. The third group, the lactogens, are secreted by the pituitary gland and are necessary for the secretion of milk in the mammary glands of mammals. They are also believed to affect maternal behavior patterns.

Epinephrine

2009-10-07T03:34:00.000-07:00

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 function of norepinephrine seems to be the maintenance of normal blood circulation. It is also the chemical agent responsible for transmission of nerve impulses in the autonomic nervous system. Large amounts of epinephrine and norepinephrine are produced by some tumors of the adrenal glands, resulting in a great increase in blood pressure.

Bile

2008-10-19T04:14:00.000-07:00

Bile, bitter, neutral, or slightly alkaline fluid secreted by the liver and passed through a duct into the gallbladder, where it is stored and, as necessary, released into the duodenum. As formed in the liver, bile is a thin, watery fluid to which the gallbladder adds a mucous secretion, forming a complex thickened and stringy substance consisting of salts and bile salts, proteins, cholesterol, hormones, and enzymes. The gallbladder returns water containing salts and other materials to the circulation and concentrates the complex further by a tenfold reduction of the bile salts, which the liver synthesizes from cholesterol. Such foods as fats, egg yolk, and foods rich in cholesterol cause concentrated bile, together with secretions from the pancreas, to be discharged into the duodenum to promote digestion, to stimulate peristalsis and absorption, and to carry off excess cholesterol and the disintegration products of overage red blood cells. The hemoglobin of such disintegrating cells degrades rapidly into reddish-yellow bilirubin, predominant in the bile of carnivorous and omnivorous animals, and biliverdin, a green pigment that appears in the bile of herbivores. Under normal conditions, the liver efficiently clears these pigments.

Certain conditions create an inability to excrete bile, and this may create serious disabilities, such as jaundice. In obese and immobilized persons, in pregnant women, and in cases of obstruction of flow of bile, gallstones may be formed by precipitation of bilirubin in combination with calcium and cholesterol. Stasis frequently coexists with inflammation and infection of the gallbladder; this may alter the concentration of bile constituents and create debris around which bile and its components may precipitate to form gallstones, which may then block the common bile duct to reduce or stop the flow of bile. Inflammation and infection, together with the consequent regurgitation of bile into the liver, may damage that organ, sometimes causing cirrhosis.

Thyroxine

2008-09-28T06:50:00.000-07:00

Thyroxine, also tetraiodothyronine, main hormone secreted by the thyroid gland. The function of this and the other thyroid hormone, triiodothyronine, is to increase the cellular rate of carbohydrate metabolism and of protein synthesis and breakdown. The hormones are synthesized in the thyroid by combining iodine with the amino acid tyrosine and are secreted into the blood as a complex with the protein plasma globulin. Both synthesis and secretion are regulated by, and in turn regulate the formation of, a hormone secreted by the pituitary gland. Thyroxine was first isolated in 1919 by the American biochemist Edward Calvin Kendall and was synthesized in 1927 by the British biochemist Charles Harington. Synthetic thyroxine is now used in treating thyroid-deficiency conditions such as cretinism and goiter.

Adrenocorticotropic Hormone (ACTH)

2008-09-28T05:56:00.000-07:00

Adrenocorticotropic Hormone (ACTH), also known as corticotropin, hormone secreted by the anterior part of the pituitary gland. The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layer) of the adrenal gland. One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolism. ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis. ACTH is a complex protein molecule containing 39 amino acids. Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increase in response to stress, disease, and decreased blood pressure.

Testosterone

2008-09-27T00:07:00.001-07:00

Testosterone, principal male hormone, or androgen, produced mainly in the Leydig cells in the male testes. The Leydig cells also produce two other androgens of less potency and in much smaller quantities.

Testosterone stimulates the development of the male secondary sex characteristics after puberty, causing growth of the beard and pubic hair, development of the penis, and change of voice. The hormone also aids in the growth, muscular development, and masculine body contour of the adult male. See Hormone.

If, before puberty, little or no testosterone secretion occurs, secondary sexual characteristics fail to develop. In addition, the long bones continue to grow abnormally and give the patient a characteristic tall but effeminate build. If testicular failure follows puberty, less obvious changes occur, although gradual recession of beard, weakening of muscles, increased deposition of fat, and change in voice may develop slowly, with infertility usually present and decreased libido and sexual potential common. The hormone is often useful in treating certain types of breast cancer in women.

Androgen

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Androgen, term embracing any of the male sex hormones, substances that induce and maintain secondary sex characteristics in males. The principal androgen is testosterone, produced in the testes of males and in the adrenal glands of both sexes. Testosterone circulates in the blood and is excreted in the urine. Androgens function principally, beginning at puberty, in the stimulation of such male secondary sex characteristics as development of the genital organs and maturation of sperm, growth of body hair, and changes in the larynx that lower the voice. Androgens have an anabolic effect—that is, they stimulate protein synthesis and inhibit protein breakdown, enhancing the growth of muscle and bone tissue in the developing male. An excess of androgens can lead to a variety of disorders, including the accentuation of male secondary sex characteristics in both males and females. Less frequently, an excess of androgens may cause testicular or ovarian tumors. For a discussion of the related female sex hormones, see Estrogen.

Composition of Blood

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About 55 percent of the blood is composed of a liquid known as plasma. The rest of the blood is made of three major types of cells: red blood cells (also known as erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

Plasma

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Plasma consists predominantly of water and salts. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. In extreme conditions this can result in seizures, coma, or even death. The pH of plasma, the common measurement of the plasma’s acidity, is also carefully controlled by the kidneys within the neutral range of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated.

Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body.

Plasma carries a large number of important proteins, including albumin, gamma globulin, and clotting factors. Albumin is the main protein in blood. It helps regulate the water content of tissues and blood. Gamma globulin is composed of tens of thousands of unique antibody molecules. Antibodies neutralize or help destroy infectious organisms. Each antibody is designed to target one specific invading organism. For example, chicken pox antibody will target chicken pox virus, but will leave an influenza virus unharmed. Clotting factors, such as fibrinogen, are involved in forming blood clots that seal leaks after an injury. Plasma that has had the clotting factors removed is called serum. Both serum and plasma are easy to store and have many medical uses.

Red Blood Cell

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Red Blood Cell or Erythrocyte, the oxygen-carrying component of the blood. Also referred to as red corpuscles, they are by far the most numerous type of blood cell.

Red blood cells make up almost 45 percent of the blood volume. Their primary function is to carry oxygen from the lungs to every cell in the body. Red blood cells are composed predominantly of a protein and iron compound, called hemoglobin, that captures oxygen molecules as the blood moves through the lungs, giving blood its red color. As blood passes through body tissues, hemoglobin then releases the oxygen to cells throughout the body. Red blood cells are so packed with hemoglobin that they lack many components, including a nucleus, found in other cells.

The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble, and is able to bend in many directions without breaking. This is important because the red blood cells must be able to pass through the tiniest blood vessels, the capillaries, to deliver oxygen wherever it is needed. The capillaries are so narrow that the red blood cells, normally shaped like a disk with a concave top and bottom, must bend and twist to maneuver single file through them.

Blood Type

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There are several types of red blood cells and each person has red blood cells of just one type. Blood type is determined by the occurrence or absence of substances, known as recognition markers or antigens, on the surface of the red blood cell. Type A blood has just marker A on its red blood cells while type B has only marker B. If neither A nor B markers are present, the blood is type O. If both the A and B markers are present, the blood is type AB. Another marker, the Rh antigen (also known as the Rh factor), is present or absent regardless of the presence of A and B markers. If the Rh marker is present, the blood is said to be Rh positive, and if it is absent, the blood is Rh negative. The most common blood type is A positive—that is, blood that has an A marker and also an Rh marker. More than 20 additional red blood cell types have been discovered.

Blood typing is important for many medical reasons. If a person loses a lot of blood, that person may need a blood transfusion to replace some of the lost red blood cells. Since everyone makes antibodies against substances that are foreign, or not of their own body, transfused blood must be matched so as not to contain these substances. For example, a person who is blood type A positive will not make antibodies against the A or Rh markers, but will make antibodies against the B marker, which is not on that person’s own red blood cells. If blood containing the B marker (from types B positive, B negative, AB positive, or AB negative) is transfused into this person, then the transfused red blood cells will be rapidly destroyed by the patient’s anti-B antibodies. In this case, the transfusion will do the patient no good and may even result in serious harm. For a successful blood transfusion into an A positive blood type individual, blood that is type O negative, O positive, A negative, or A positive is needed because these blood types will not be attacked by the patient’s anti-B antibodies.

White Blood Cells

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White blood cells only make up about 1 percent of blood, but their small number belies their immense importance. They play a vital role in the body’s immune system—the primary defense mechanism against invading bacteria, viruses, fungi, and parasites. They often accomplish this goal through direct attack, which usually involves identifying the invading organism as foreign, attaching to it, and then destroying it. This process is referred to as phagocytosis.

White blood cells also produce antibodies, which are released into the circulating blood to target and attach to foreign organisms. After attachment, the antibody may neutralize the organism, or it may elicit help from other immune system cells to destroy the foreign substance. There are several varieties of white blood cells, including neutrophils, monocytes, and lymphocytes, all of which interact with one another and with plasma proteins and other cell types to form the complex and highly effective immune system.

Platelets and Clotting

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The smallest cells in the blood are the platelets, which are designed for a single purpose—to begin the process of coagulation, or forming a clot, whenever a blood vessel is broken. As soon as an artery or vein is injured, the platelets in the area of the injury begin to clump together and stick to the edges of the cut. They also release messengers into the blood that perform a variety of functions: constricting the blood vessels to reduce bleeding, attracting more platelets to the area to enlarge the platelet plug, and initiating the work of plasma-based clotting factors, such as fibrinogen. Through a complex mechanism involving many steps and many clotting factors, the plasma protein fibrinogen is transformed into long, sticky threads of fibrin. Together, the platelets and the fibrin create an intertwined meshwork that forms a stable clot. This self-sealing aspect of the blood is crucial to survival.

See also Blood Diseases: Anemia; leukemia, hemophilia

Blood Type

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Blood Type, in medicine, classification of red blood cells by the presence of specific substances on their surface. Typing of red blood cells is a prerequisite for blood transfusion. In the early part of the 20th century, physicians discovered that blood transfusions often failed because the blood type of the recipient was not compatible with that of the donor. In 1901 the Austrian pathologist Karl Landsteiner classified blood types and discovered that they were transmitted by Mendelian heredity (see Mendel's Laws).

The four blood types are known as A, B, AB, and O. Blood type A contains red blood cells that have a substance A on their surface. This type of blood also contains an antibody directed against substance B, found on the red cells of persons with blood type B. Type B blood contains the reverse combination. Serum of blood type AB contains neither antibody, but red cells in this type of blood contain both A and B substances. In type O blood, neither substance is present on the red cells, but the individual is capable of forming antibodies directed against red cells containing substance A or B. If blood type A is transfused into a person with B type blood, anti-A antibodies in the recipient will destroy the transfused A red cells. Because O type blood has neither substance on its red cells, it can be given successfully to almost any person. Persons with blood type AB have no antibodies and can receive any of the four types of blood; thus blood types O and AB are called universal donors and universal recipients, respectively.

Other hereditary blood-group systems have subsequently been discovered. The hereditary blood constituent called Rh factor is of great importance in obstetrics and blood transfusions because it creates reactions that can threaten the life of newborn infants. More than twenty additional blood types have been discovered. Their primary importance is in legal cases involving proof of paternity. See also Genetics.

Serum

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Serum, clear, yellowish fluid that remains as the liquid portion of blood after clotting has taken place. Before clotting occurs, the liquid of the blood is called plasma. Both plasma and serum contain proteins, salts, sugars, waste products, vitamins, minerals, fats, and hormones. But plasma also contains the protein fibrinogen and certain other elements necessary for clotting, and it therefore clots as easily as whole blood. Serum, which lacks these elements, does not clot.

In order to isolate serum for chemical tests and other medical uses, whole blood is placed in a glass tube and allowed to clot. The clotted blood is then placed in a centrifuge, a machine that spins the blood at high speeds, causing the clot and the cellular parts of the blood to separate from the serum and sink to the bottom of the tube. The serum remains at the top of the tube.

Serum from animals like horses and sheep can be used to provide human beings with protection against infections or poisons. Animals are given specific protein molecules derived from living organisms like bacteria to stimulate animal immune systems to produce antibodies. Once injected into a human patient, a serum containing animal antibodies can provide rapid immunity against specific diseases. Such a serum is called an antitoxin, and is used to treat such diseases as diphtheria in people who have not been previously immunized, or vaccinated. A serum with antibodies that combat snake venom is called an antivenin. An injection of antivenin is the only drug treatment available for snakebites, and it is normally administered if the patient displays life-threatening signs and symptoms.

Rh Factor

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Rh Factor, term applied to any of the 30 or more substances, called agglutinogens, found on the surfaces of red blood cells. They are distinct from the main blood types, but their composition is unknown. The Rh factors were named by the American pathologists Karl Landsteiner and Alexander Solomon Wiener, who discovered the first of them in the blood of the rhesus monkey in 1937. This first Rh agglutinogen, corresponding to the one now designated Rh0, is present in the blood of about 85 percent of all humans. Blood reactions involving Rh factors are now chiefly of interest in obstetrics.

The presence of Rh factors in the blood is controlled by the laws of heredity. An individual who possesses one gene for the Rh factor will express the factor on the red blood cells. If a woman is Rh-negative, that is, if she has two recessive genes for the Rh0 factor, and a man has two genes that express the Rh-positive factor, then all of their children will be Rh-positive. But if the Rh-positive man has one recessive gene, then each child has a 50 percent chance of being Rh-positive. When carrying an Rh-positive child, the mother will build up antibodies to the Rh0 factor in about 5 percent of all cases. These antibodies will usually be too weak to harm the first child. But during labor and delivery some of the baby's Rh-positive blood may get into the mother's bloodstream and trigger or sensitize her immune system. Her antibodies will then attack the red blood cells of any subsequent Rh-positive children. This reaction produces erythroblastosis fetalis, or Rh disease, which results in jaundice, anemia, brain damage, and often death, either before or shortly after birth.

Cerebrum

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Cerebrum (the Latin word for brain)—two large, almost symmetrical hemispheres which make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex to fit within the cranium.

The cerebrum receives information from all the sense organs and sends motor commands (signals that result in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, a parallel strip of cerebral cortex just in back of the central sulcus, receives input from the sense organs.

Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortex are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.

Cerebellum

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Cerebellum (“little brain”)—two smaller hemispheres located at the lower back of the brain beneath the occipital lobes, which coordinates body movements. It is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum, the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem—the midbrain, the pons, and the medulla oblongata.

The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.

Brain Stem

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Brain stem, the lowest part of the brain, a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rate, as well as reflexes like eye movement and vomiting. The brain stem has three main parts: the medulla, pons, and midbrain. A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.

Midbrain

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Midbrain, the topmost structure of the brain stem that contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers—pathways carrying sensory (input) information and motor (output) commands. Relay and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei, called the inferior colliculus, control auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.

Pons

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Pons, the prominent bulge in the brain stem, continuous with and below the midbrain and directly in front of the cerebellum. It consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.

Medulla Oblongata

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Medulla Oblongata, the long, stalklike lowermost portion of the brain stem. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body, and the right half of the brain with the left half of the body.

Creatine

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Creatine, substance in the bloodstream of vertebrate animals that is used for supplementary energy transport in certain body systems. (Arganine serves a similar role in invertebrate animals.) Adenosine triphosphate, or ATP, is the main immediate source of usable energy in the body, but brain, heart, and muscle cells require enormous amounts of energy that creatine also helps to supply. Produced in the kidneys and liver, the chemical is absorbed into cells and picks up the high-energy phosphate group from excess ATP at cellular bodies called mitochondria, then shuttles back and forth to other cells to supply the energy.

Fertilization

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Fertilization is the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring (see Reproduction).

Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo.

Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed—a sperm cell or an egg cell. Each gamete contains only half the genetic material of the original cell. During sperm and egg fusion in fertilization, the full amount of genetic material is restored: half contributed by the male parent and half contributed by the female.

For information on plant fertilization see the articles on Seed, Pollination, and Plant Propagation.

Skeleton

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Skeleton (anatomy), term applied to all the rigid or semirigid structures supporting the soft tissues of an animal's body and providing leverage for muscular action. In vertebrates, the skeleton is known as the endoskeleton and is formed within the body. Some invertebrate animals, such as insects and crustaceans, have skeletons known as exoskeletons on the outside of the body.

EXOSKELETON

A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.

ENDOSKELETON

Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages

In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.

The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.

Pregnancy and Childbirth

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Pregnancy and Childbirth, terms for the gestation period of the human reproductive cycle (see Reproduction).

PREGNANCY

Pregnancy starts when a male’s sperm fertilizes a female’s ovum (egg), and the fertilized ovum implants in the lining of the uterus (see Fertilization; Reproductive System). Because pregnancy changes a woman’s normal hormone patterns, one of the first signs of pregnancy is a missed menstrual period (see Menstruation). Other symptoms include breast tenderness and swelling, fatigue, nausea or sensitivity to smells, increased frequency of urination, mood swings, and weight gain. Some women also experience cravings for unusual substances such as ice, clay, or cornstarch; this condition, called pica, can indicate a dietary deficiency in iron or other nutrients. By the 12th week of pregnancy many of these symptoms have subsided, but others appear. For example, a woman’s breasts usually increase in size, and her nipples darken. The most obvious symptom is weight gain; most physicians now recommend a gain of about 9 to 12 kg (about 22 to 26 lb) by the end of pregnancy.

The first few months of pregnancy are the most critical for the developing infant, because during this period the infant’s brain, arms, legs, and internal organs are formed. For this reason a pregnant woman should be especially careful about taking any kind of medication except on the advice of a physician who knows that she is pregnant. X rays should also be avoided, and pregnant women should avoid smoking and alcohol consumption.

LABOR AND CHILDBIRTH

A normal pregnancy lasts about 40 weeks, or 280 days, after the beginning of the last menstrual period. Occasionally women go into labor before the expected date of birth, resulting in a premature infant. About 7 percent of all infants are premature—that is, born before the 37th week of pregnancy. Babies born just a few weeks early usually develop normally. Recent advances in the care of premature infants now allow many babies who are born after only 25 to 26 weeks of pregnancy to survive.

Delivery, the process by which the baby is expelled from the uterus through the birth canal and into the world, begins with irregular contractions of the uterus that occur every 20 to 30 minutes. As labor progresses, the contractions increase in frequency and severity. The usual length of labor for a first-time mother is about 13 to 14 hours, and about 8 or 9 hours in a woman who has given birth previously. Wide variations exist, however, in the duration of labor.

Most women prefer some kind of anesthesia to alleviate the pain associated with childbirth. Natural (unmedicated) childbirth, however, is becoming more popular, in part because many women are aware and concerned that the anesthesia and medication given to them is rapidly transported across the placenta to the unborn baby. Heavy doses of anesthesia can make the newborn baby less alert after birth.

Other options available regarding childbirth include regional (local) anesthesia, in which only those areas of the mother that are affected by the pain of childbirth are numbed. Such anesthesias include a lower spinal block and epidural anesthesia, in which the pelvic region is anesthetized. Another option is cesarean section, in which the baby is surgically removed from the uterus. Cesarean section is usually performed only for a specific medical reason.

Progesterone

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Progesterone, hormone formed by the granulosa cells of the corpus luteum of the ovary. The corpus luteum is a structure in the ovary that develops at the site where a mature egg was released at ovulation. Therefore the level of progesterone rises in the second half of the menstrual cycle (see Menstruation). If the released ovum is not fertilized, production of progesterone falls just before the onset of the next menstrual cycle and the corpus luteum degenerates. Progesterone was isolated and crystallized by three independent groups of investigators in 1934. It is a steroid hormone, a compound possessing the same chemical nucleus as the female estrogenic hormones and the male androgenic hormones, as well as cholesterol and adrenal steroid hormones. The principal function of progesterone is the preparation of the mucous membrane of the uterus for the reception of the ovum. The hormone also stimulates the formation of saclike structures in the lacteal glands in preparation for their function of producing milk, and inhibits the release in the pituitary of prolactin hormone.

Mammary Glands

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Mammary Glands, source of milk for the young and a characteristic of all mammals, including the egg-laying monotremes. Although the mammary glands evolved recently in the development of vertebrates, their origin is uncertain. In monotremes, they resemble sweat glands in structure, leading to speculation that they are sweat or sebaceous glands modified through evolution. In higher orders, the glands at maturity are developed only in females; in monotremes they evolve equally in both sexes. The organ in human males occasionally develops functional ability, giving support to the belief that lactation occurred in both sexes in prehistoric mammals.

Hip

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Hip, one of two projecting portions on opposite sides of the body between the lowest ribs and the thigh. The hip is made up of the hipbone, or innominate bone (a part of the pelvis), and of the skin, fat, muscles, and membranes overlying it.

The hip is a ball-and-socket joint; the ball is the spherical head of the thighbone, or femur, and the socket is a region on the side of the hipbone known as the acetabulum (Latin, “vinegar cup”). Friction between the bones of the hip joint is reduced by a coating of cartilage and by a lubricating agent known as synovial fluid. The center of gravity of the human body is located behind the hip joint, tending to throw the body backward. With the aid of various muscles from the pelvis and thigh, the femur is able to rotate in the acetabulum and move in any direction, the extent of its movement being limited only by certain supporting ligaments.

The hip and hip joint are subject to a variety of disorders and injuries. Congenital disorders of the hip are occasionally seen in children. Among the most important of these is congenital dislocation of the hip, where the end of the femur, which normally fits into the acetabulum, is not properly developed and does not lie in its proper relationship to the acetabulum. Injuries to the hip joint are common. In athletic injuries involving severe trauma, the head of the femur may be torn out of its normal position in the acetabulum by the force of the injury, causing dislocation. In older people, injuries, even relatively minor ones, may cause a fracture of the neck of the femur, the small portion that lies just below the head. In addition, the hip joint is subject to tuberculosis and to a variety of inflammations and degenerative changes in arthritis. Hip joints severely damaged by falls or arthritis are now often removed and replaced with an artificial hip joint that allows nearly normal activity in most persons.

Hydrocortisone

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Hydrocortisone, also cortisol, common names for 17-hydroxy-corticosterone, the principal hormone secreted by the outer layer, or cortex, of the adrenal gland. Hydrocortisone affects the metabolism of carbohydrate, protein, and fat; the maturation of white blood cells; the retention of salt and water in the body; the activity of the nervous system; and the regulation of blood pressure. Secretion of hydrocortisone from the adrenal cortex is stimulated by the pituitary hormone ACTH.

Because of their widespread effects, hydrocortisone and related compounds, called corticosteroids, or corticoids, are employed for many medical purposes. They are used to treat a deficiency of adrenal cortical hormones, a condition called congenital adrenal hyperplasia, rheumatoid disease that is not helped by milder drugs, and to counter severe non-infectious inflammations. Corticosteroids suppress the immune response, so they are used to increase acceptance of transplants (Medical Transplantation).

Other conditions for which they are helpful are asthma, collagen diseases, and eye inflammations. Because corticosteroids affect so many body processes, they must be used carefully. Corticosteroids dispose persons to infection and can lead to swelling of the face and limbs, muscle weakness, weight gain, high blood pressure, and diabetes.

Another naturally occurring corticosteroid hormone, called cortisone, was the first corticosteroid to be isolated, in 1935. It was synthesized in 1944 and subsequently became available for widespread medical use. Cortisone is rapidly converted to hydrocortisone in the body. Synthetic corticosteroids with more specific activity have now been made and are preferred in many situations.

Paranasal Sinuses

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Paranasal Sinuses, air-filled, mucus-lined cavities in the head and cheekbones that drain into the nasal cavity. The largest are the two maxillary sinuses found directly under the eye sockets. Others are the two frontal sinuses, found in the frontal bone just above and between the eye sockets, and the sphenoid and ethmoid sinuses, located deeper in the skull over the nasal cavity. The sinuses produce mucus, which is propelled by small, hairlike processes called cilia into the nasal cavity. Their function is not clearly understood, but they are believed to help warm and moisten inhaled air.

Prostate Gland

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Prostate Gland, chestnut-shaped male organ located next to the bladder and surrounding the urethra (the tube that carries urine from the bladder to the penis). The prostate gland produces a secretion known as prostate fluid that makes up most of the liquid part of semen, which is discharged from the penis during sexual orgasm. Measuring about 3 cm (about 1.2 in) across, the prostate gland is composed of both glandular tissue that produces prostate fluid and muscle tissue that helps in male ejaculation. Prostate fluid also helps to keep sperm, which is found in semen, healthy and lively, thereby increasing the chances that fertilization will occur

Pelvis

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Pelvis, lower part of the trunk of the body, bounded at the front and on either side by the hipbone, and at the back by the sacrum and the coccyx, the lowest part of the spinal column (see Hip; Sacroiliac Joint). The hipbone is composed of three separate bones: the ilium, the ischium or lower part of the hipbone, and the pubis, the central pubic bone that unites with the ischium at either side. In early life, the three bones are separate, but when a person is in the late teens or early 20s, they unite to form a single structure called the innominate bone. At the lower end of the hipbone is a cup-shaped depression called the acetabulum in which the femur, or thighbone, rotates. The pelvis thus acts as a unit in all bodily movements. The weight of the trunk is transferred from the spine through the sacrum and then through the hipbone to the thighbone and the lower extremities. Conversely, all forces acting on the lower limbs are transmitted to the trunk by the same route.

The cavity formed by the pelvic bones contains the lower portion of the intestines and rectum, the urinary bladder, and the internal organs of reproduction. In females, the pelvis is rounder and wider than in males, and the bones are lighter, reflecting the greater capacity required for expansion of the uterus in pregnancy and the emergence of the fetus in childbirth.

Breast

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Breast, one of a pair of glandular organs in mammals that secrete milk for newborns and infants. Also known as a mammary gland, particularly in nonhuman mammals, the breast is unique to mammals and is not found in any other type of animal. In humans, the female breast has a distinctive rounded shape while the male breast remains flat and undeveloped. Only mature females can produce milk, which is rich in nutrients, antibodies, and other substances that aid in a baby’s growth and development.

The surface of a human breast has a circular, pigmented area in the center called the areola. The nipple, a rounded protruding structure, lies in the center of the areola. In mature women the breast contains a collection of 10 to 15 tubes, called ducts, that connect to the nipple. These ducts branch out from the nipple into the interior of the breast, ending in clusters of rounded cells, called lobules, that produce the milk. In addition to the structures directly connected with the production and outflow of milk, the breast is composed of fatty tissue and ligaments that provide support and shape.

In humans, a woman’s breast size and shape vary widely, depending on her age and whether she is pregnant or going through a menstrual cycle. Breast development begins in girls about 10 to 12 years old, when the ovaries start to produce the hormone estrogen. Completion of breast development, which occurs around 16 to 18 years old, requires the interaction of other hormones such as progesterone, prolactin, and corticosteroids.

After breast growth is completed, the breasts typically undergo monthly cyclic changes in response to fluctuating hormone levels in the blood that occur during the menstrual cycle. Just before the onset of menstruation, the breasts are often swollen and tender because the ductal system expands in preparation for pregnancy. If pregnancy does not occur, the estrogen level falls and the breasts return to normal.

During pregnancy there is a remarkable growth of ducts and lobules in the breast along with a thickening of the nipples. After a baby is born, the hormone prolactin stimulates milk production in the breast. Initially, the breast produces a thick yellow liquid called colostrum, which is particularly rich in the disease-fighting substances called antibodies. Within three to five days, the breast produces milk as the suckling infant stimulates the release of another hormone called oxytocin. This hormone causes contractions in the network of cells that surround the ducts and lobules, so that milk readily flows from the breast and into the mouth of the hungry infant.

As a woman passes the childbearing age and enters menopause, the ovaries stop producing estrogen. The decreased stimulation from this hormone causes the milk-producing ducts and lobules in the breast to be replaced with extra fatty tissue.

Breasts are naturally lumpy, but any area of the breast that becomes more prominent than the surrounding tissue or feels unusually firm requires the attention of a physician. Breast lumps are a common problem in women and may range from a cyst, a fluid-filled structure that does not pose a cancer risk, to breast cancer, the most common type of cancer in women other than skin cancer. Also, in men between 20 and 40 years of age, breast enlargement can be a sign of testicular cancer, and breast lumps in men over 40, although rare, can be an indication of breast cancer.

Often one of the first diagnostic tests performed to evaluate a lump, an ultrasound is able to distinguish a cyst from a solid mass, a possible indication of cancer that requires further investigation. Solid lumps are evaluated with a special X-ray procedure called a mammogram, currently the best tool available for early detection of breast cancer. A mammogram may be followed by needle aspiration, a type of biopsy in which cells are removed from the lump and examined under a microscope. If these test results are normal, the lump is unlikely to be cancer. It may be removed or monitored carefully for signs of growth. If cancer is suspected, or if the lump shows signs of growth later, surgical removal is usually necessary.

As a preventive measure, physicians encourage regular breast self-examinations in which a woman uses her fingers to feel for changes in breast shape and fluid discharge from the nipple. Physicians also urge women to have regular mammograms, which can detect very small cancers before a lump begins to develop. These small cancers typically respond to treatment better than cancers that have grown large enough to produce a lump.

Uterus

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Uterus or Womb, flattened, pear-shaped, hollow organ in the pelvis of the human female and most other mammals. In pregnancy, it is the organ that holds the unborn developing child (see Pregnancy and Childbirth).

The uterus consists of a body, a base or fundus, a neck or cervix, and a mouth. Suspended in the pelvis, it lies with the base directed upward and forward and the cervix directed slightly backward. It is connected to the vagina by the cervix. On either side of the uterus lies an ovary. Eggs produced by the ovaries reach the uterus through the fallopian tubes. In the unimpregnated condition the uterus is about 7.6 cm (about 3 in) in length, 5 cm (2 in) in breadth, and 2.5 cm (1 in) in thickness. In pregnancy the fertilized egg implants itself in the lining of the uterus, where it grows to maturity; the walls of the uterus are elastic and stretch during pregnancy to hold the developing child. The uterus body consists of a firm outer coat of muscle, known as the myometrium, and an inner lining of soft, glandular material, known as the endometrium, that thickens with blood during ovulation, preparatory to receiving a fertilized ovum. If fertilization does not occur, this lining breaks down in menstruation.

The uterus is prone to infection. Endometritis, an inflammation of the mucous lining of the uterus, may affect either the neck or the body of the organ or both. Cancer and fibroid tumors of the uterus are comparatively frequent.

See also Reproductive System.

Tonsil

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Tonsil, name applied to any of a number of masses of lymphoid tissue forming a ring around the walls of the pharynx, or throat. These masses consist of cells, similar to the lymphocytes of the bloodstream, embedded in fibrous connective tissue and covered by a single layer of epithelium. The lymphoid cells are phagocytic (see Phagocytosis) and help protect the pharynx from invasion by disease-producing bacteria. Tonsils may become inflamed and acutely or chronically infected. Such a condition, called tonsillitis, is often caused by streptococcus infection. Tissues surrounding the tonsils frequently form pus during acute attacks of tonsillitis, particularly streptococcal, causing the tonsils to show white specks or even be coated with whitish exudate. This condition is called quinsy. Pharyngeal tonsils, located at the back of the throat, may become abnormally large when inflamed. They are called adenoids. Acute cases of tonsillitis are often treated by administration of such antibiotics as penicillin. Chronic recurrent tonsillitis may be treated by surgical removal of the tonsils, a procedure that is known as tonsillectomy.

Spinal Cord

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Spinal Cord, that part of the central nervous axis contained within the vertebral or neural canal. In the adult human it extends from the base of the skull to the second lumbar vertebra. Below this it tapers off as a narrow thread containing little nervous matter. Above the foramen magnum, in the base of the skull, it is continuous with the medulla oblongata. The cord, like the brain, is encased in a triple sheath of membranes and is partially divided into two lateral halves by a median fissure toward the front and a median partition toward the rear; 31 pairs of spinal nerves arise, each by an anterior and a posterior root, from each side of the cord.

The spinal cord functions in the transmission of ascending impulses to the brain and of descending impulses from the brain to the cord. Thus, it conveys to higher centers information brought to it by peripheral nerves from many parts of the body; in addition, it is acted on by impulses from the brain itself. The spinal cord relays impulses also to the muscles, blood vessels, and glands by means of outgoing nerves, either in response to incoming stimuli or to signals from higher levels. See Nervous System; Spinal Column.

Alimentary Canal

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Alimentary Canal, in anatomy, the principal part of the digestive system. It begins at the mouth and extends to the anus, having, in humans, an average length of about 9 m (about 30 ft). Passing through the head, neck, and body, it includes the mouth, pharynx, esophagus, stomach, small intestines, cecum, and large intestines.

Achilles Tendon

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Achilles Tendon, in the legs of mammals, powerful, cordlike band of connective tissue that joins the fused muscles of the calf to the bone of the heel. Because bones are moved by muscles pulling on the connecting tendon, severance or rupture of the Achilles tendon results in immediate loss of the normal use of the leg and foot. The name of the tendon is derived from the legendary Greek warrior Achilles.

Sacroiliac Joint

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Sacroiliac Joint, fibrous joint in the lower back between the lumbar vertebrae and the coccyx, the bones at the lower end of the spinal column. It is composed of five fused vertebrae that form a solid triangular bone (the sacrum), and the ilium parts of the two hip bones. Once believed to be the site of sprain and lower back pain, it is in fact bound by powerful ligaments that avert injury. Such pain is more generally caused by a displaced vertebral disk (Spinal Column).

The sacroiliac joint is subject to a number of diseases embraced by the term spondylitis, an inflammation of the vertebrae that is often rheumatic in character. Spondylitis is frequently accompanied by ankylosis, a chronic disease that progresses up the spine, fusing the vertebrae. The different forms of spondylitis are controlled by early recognition, diagnosis by X ray, and treatment by deep radiation therapy, cortisone (Hydrocortisone), exercises, and orthopedic procedures.

Spinal Column

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Spinal Column, common name applied to the structure of bone or cartilage surrounding and protecting the spinal cord in vertebrate animals (see Animal). It is also called a vertebral column, spine, or backbone,

ANATOMY AND PHYSIOLOGY

The spinal column forms the major part of the skeleton. To it are attached the skull, shoulder bones, ribs, and pelvis. In very primitive animals having a vertebral column, the spine consists of a solid cartilaginous rod known as the notochord. Although remnants of the notochord persist in the cartilages that form part of the apparatus connecting adjoining vertebrae, in higher animals the notochord is replaced by a series of separate bones called vertebrae.

The shape and number of vertebrae vary among different animals. In general, the vertebrae are stacked like a column of poker chips and are held together by ligaments, the connective tissue that holds bones together at a joint. In humans the spinal column contains 33 vertebrae: 7 cervical vertebrae in the neck; 12 thoracic, or dorsal, vertebrae in the region of the chest, or thorax, providing attachment for 12 pairs of ribs; 5 lumbar vertebrae in the small of the back; 5 fused sacral vertebrae forming a solid bone, the sacrum (see Sacroiliac Joint), which fits like a wedge between the bones of the hip; and a variable number of vertebrae fused together to form the coccyx at the bottom of the sacrum.

Before birth, the human spinal column forms a single curve with the convex surface toward the back; at birth, two primary curvatures are present, both of which are concave forward. The upper one is located in the thoracic and the lower one in the sacral region. If the child develops normally, two compensatory forward curvatures develop in the cervical and lumbar regions, just above the primary curvatures. These normal curvatures provide a degree of resilience that would not be possible in a series of rigid, straightly stacked bones.

Most of the individual vertebrae are shaped somewhat like rings; the body, or thick portion of the ring, is located toward the front portion of the body. Between each of the separate vertebrae is a thick, fibrous disk of cartilage—called an intervertebral disk—that forms the principal joint between the bodies of adjoining vertebrae; however, the vertebrae also move with each other at several other joints.

Sense Organs

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Sense Organs, in humans and other animals, faculties by which outside information is received for evaluation and response. This is accomplished by the effect of a particular stimulus on a specialized organ, which then transmits impulses to the brain via a nerve or nerves.

Aristotle classified five senses as follows:

1. Hearing, a sense by which sound waves are perceived by the organ of hearing—the ear—in vertebrate animals. The process of sound perception is called audition. The physical stimulus of auditory sensation is the vibration of some material object. The vibration is transmitted from the object to the ear, under ordinary conditions, by a wave movement of air particles.

2. Sight (vision), is the ability to see the features of objects we look at, such as color, shape, size, details, depth, and contrast. Vision is achieved when the eyes and brain work together to form pictures of the world around us. Vision begins with light rays bouncing off the surface of objects. These reflected light rays enter the eye and are transformed into electrical signals. Millions of signals per second leave the eye via the optic nerve and travel to the visual area of the brain. Brain cells then decode the signals into images, providing us with sight.

3. Smell, a sense by which odors are perceived. The nose, equipped with olfactory nerves, is the special organ of smell. The olfactory nerves also account for differing tastes of substances taken into the mouth, that is, most sensations that appear introspectively as tastes are really smells.

4. Taste, a sense by which four gustatory qualities (sweetness, sourness, saltiness, and bitterness) of a substance are distinguished. Taste is determined by receptors, called taste buds, the number and shape of which may vary greatly between one person and another. In general, women have more taste buds than men. A greater number of taste buds appears to endow a greater sensitivity to sweetness, sourness, saltiness, and bitterness. In humans, the taste buds are located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. The mucous membrane lining these areas is invested with tiny projections of papillae, each of which in turn is invested with 200 to 300 taste buds. The papillae located at the back of the tongue, and called circumvallate, are arranged to form a V with the angle pointing backward; they transmit the sensation of bitterness. Those at the tip of the tongue transmit sweetness, whereas saltiness and sourness are transmitted from the papillae on the sides of the tongue. Each flask-shaped taste bud contains an opening at its base through which nerve fibers enter. These fibers transmit impulses directly to the brain. In order for a substance to stimulate these impulses, however, it must be in solution, moistened by the salivary glands. Sensations of taste have been determined to be strongly interrelated with sensations of smell.

5. Touch, a sense by which the body perceives contact with substances. In humans, touch is accomplished by nerve endings in the skin that convey sensations to the brain via nerve fibers. Nerves end in or between the cells of the epidermis, the outer layer of the skin, in all parts of the body. In one complex form of nerve ending, the terminals form tiny swellings, or end bulbs; characteristic of this form are the Pacinian corpuscles found in the sensitive pad of each finger. Touch is the least specialized of the senses, but acuteness can be sharpened by use.

Mastoid Process

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Mastoid Process, conical prominence of the temporal bone of the human skull, situated behind the ear. It commonly becomes infected in cases of suppurative otitis media. The inner ear adjoins the hollow, spongy spaces within the mastoid process so that infection of the ear easily spreads to that area, causing pain and swelling. Surgical drainage of pus and injection of antibiotics usually eliminate mastoid infection and prevent its spread to nearby areas of the brain.

Cephalic Index

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Cephalic Index, basic measure of the shape of the skull, used by scientists of the 19th and early 20th centuries in classifying humans. Devised by Swedish anatomist Anders Retzius in the 1840s, the cephalic (or cranial) index expresses the width of the skull as a percentage of the length from front to back. Craniologists distinguished three basic classifications of skulls based on the cephalic index: dolichocephalic (long skulls), with widths of less than 75 percent of the length; mesocephalic (medium skulls), with indexes of 75 to 80; and brachycephalic (broad skulls), with indexes of more than 80 percent.

Skull

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Skull, term applied to the rigid cartilaginous or bony structure located anterior to the spinal column in all vertebrate animals, and serving to encase and protect the brain and to provide attachment for the muscles of the face and mouth. In lower vertebrates, such as cyclostomes and elasmobranchs, the skull is composed of cartilage throughout the lifetime of the organism; in teleost fishes and in more highly developed vertebrates, including humans, the skull is cartilaginous during embryonic and fetal life, becoming ossified early in postnatal life.

The adult human skull is divided into two regions, the cranial and the facial. The cranial region is the portion of the skull directly surrounding the brain; the facial region includes all the other bones of the skull.

Exteriorly the cranial bones include the two frontal bones, which constitute the forehead and which fuse together in adulthood; the two parietal bones, which constitute the top of the head and which in early childhood are separated from the frontal bones by a space called the anterior fontanel; the single occipital bone, constituting the back of the skull, which is pierced by a large opening through which the spinal cord enters the cranial cavity; and the two temporal bones, which constitute the temples and of the head and bear the zygomatic processes, or cheekbones. The human temporal bone represents the fusion of four bones found in lower mammals: the squamosal bone, which constitutes the side of the head and articulates with the jawbone; the petrosal bone, which contains the inner ear; the mastoid bone, which is behind the ear; and the tympanic bone, which surrounds the channel from the eardrum to the external ear.

Internally, the cranial bones include the ethmoid bone, which forms part of the septum of the nose and through which the olfactory nerves pass from the brain to the upper and middle turbinates; the sphenoid bone, which constitutes most of the floor of the cranial cavity and which houses the pituitary gland; and part of the occipital bone. The floor of the cranial cavity contains three terraced depressions, which contain the cerebellum and the frontal and temporal lobes of the cerebrum.

The facial bones include the two nasal bones, which constitute the upper portion of the bridge of the nose; the two lacrimal bones, which are located in each eye orbit next to the nose, close to the tear ducts; the maxillary bone, which constitutes the upper jaw; the mandible, which constitutes the lower jaw; the two palatine bones of the hard palate; the vomer, which, with a part of the ethmoid bone, constitutes the nasal septum; and the two inferior turbinates of the nose.

See: Disorders of the Human Skull

Periosteum

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Periosteum, tough, fibrous membrane that surrounds each bone. It is highly vascular and is the means by which the outer layers of the shafts and the greater part of the spongy portions of the bones are supplied with blood. It consists of an outer fibrous layer and an inner osteogenetic layer. The inner layer is very vascular and contains many protoplasmic cells called osteoblasts. Numerous experiments show that the formation of bone is essentially due to the action of the periosteum and that, by transplanting detached portions of periosteum into muscular or other tissues, bony tissue is generated in those parts. It appears to be the curative agent in the case of bone breakage. In most cases in which the periosteum has become detached as a result of a wound or of disease, the exposed bone perishes (except in the instance of the skull, which derives most of its nutrient matter from the dura mater, actually the periosteum of the inner surface of the skull). Inflammation of the periosteum results in a condition known as periostitis.

Appendix

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Appendix, a worm-shaped tube branching off the cecum, the first part of the large intestine. It is located on the lower right side of the abdomen and is usually about 9 cm (about 3.5 in) long, with a thick wall. Only humans and apes have an appendix. It has no known function in human biology, but it does contain a large amount of lymphoid tissue, which may provide a defense against local infection. Many scientists believe that the human appendix at one time served a useful purpose that has gradually been lost through evolution.

For reasons not fully understood, the appendix can become infected and filled with pus—particularly in children, teenagers, and young adults—resulting in appendicitis. Symptoms of appendicitis include pain and cramps in the area between the right hip bone and the navel, fever, nausea and vomiting, constipation, and diarrhea. The treatment is surgical removal of the appendix, known as appendectomy. If the appendix wall ruptures, infection may spread to the abdominal cavity, causing peritonitis, an inflammation of the abdominal lining. Acute peritonitis is often fatal if untreated.

Bladder

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Bladder, in anatomy, organ that stores urine after its formation by the kidneys. It consists of three layers: a lining of mucous membrane, called epithelium; an intermediate layer of involuntary muscle, with fibers arranged in three layers, each running in a different direction; and an outer layer of connective tissue, covered by the peritoneum above and to the back and blending with the connective tissues of the body wall in front and below. Urine enters the bladder from the kidneys through two ureters and is discharged through the urethra. The openings of the ureters lie in the basal part of the bladder, each about 3.8 cm (about 1.5 in) from the opening of the urethra, which is located in the midline of the bladder at its lowest point. Average normal capacity of the adult bladder is about half a liter (about 1 pt). See also Urinary System.

Spleen

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Spleen, flattened, oblong organ that removes disease-producing organisms and worn-out red blood cells from the bloodstream. The spleen is situated in the upper left abdominal cavity, in contact with the pancreas, the diaphragm, and the left kidney. It is supported by bands of fibers that are attached to the peritoneum (the membrane lining the abdominal cavity). The spleen varies in size but, in adult humans, it is about 13 cm (about 5 in) long, up to 10 cm (4 in) wide and 3.8 cm (1.5 in) thick, and weighs about 200 g (about 7 oz). The spleen is fed by the splenic artery, and venous blood from the spleen enters the liver.

The spleen removes iron from the hemoglobin of red blood cells for use in the body. It also removes such waste materials as bile pigments for excretion as bile by the liver. The spleen produces antibodies against various disease organisms and manufactures a variety of blood cells. In some mammals (but not in humans) it stores red blood cells and feeds them into the circulation to maintain the volume of blood in cases of hemorrhage. In the unborn child, the organ functions primarily to produce red blood cells, a function that is normally taken over by the bone marrow after birth. However, if diseases slow down this function in the bone marrow, the spleen may again start to produce red blood cells.

Many diseases affect the spleen. In splenomegaly, the spleen enlarges. This condition is usually an indication of bacterial, parasitic, and viral infections, such as infectious mononucleosis, tuberculosis, malaria, rheumatoid arthritis, and histoplasmosis. Splenomegaly may also be caused by cirrhosis of the liver. In hypersplenism, a malfunctioning or hyperactive spleen may so increase its normal rate of destroying red blood cells as to cause a variety of hemolytic anemias. Hemorrhage of an enlarged spleen may magnify anemia and also cause an iron deficiency. Small splenic abscesses may run a benign course without detection. Large splenic abscesses, however, may rupture into the bowel cavity to cause peritonitis (inflammation of the abdominal wall lining), and the infection may invade the bloodstream and be carried to the liver, where it may form additional abscesses. Splenic cysts are rare, and the spleen is seldom the site of a carcinoma, or cancer.

A hyperactive spleen may sometimes be treated with radiation or corticosteroids (steroids that are extracted from the adrenal cortex). Abscesses are treated with broad-spectrum antibiotics or surgical drainage. Surgical removal of the spleen is required if the spleen or the splenic artery ruptures.

Pancreas

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Pancreas, conglomerate gland lying transversely across the posterior wall of the abdomen. It varies in length from 15 to 20 cm (6 to 8 in) and has a breadth of about 3.8 cm (about 1.5 in) and a thickness of from 1.3 to 2.5 cm (0.5 to 1 in). Its usual weight is about 85 gm (about 3 oz), and its head lies in the concavity of the duodenum.

The pancreas has both an exocrine and an endocrine secretion. The exocrine secretion is made up of a number of enzymes that are discharged into the intestine to aid in digestion. The endocrine secretion, insulin, is important in the metabolism of sugar in the body (see Sugar Metabolism). Insulin is produced in small groups of especially modified glandular cells in the pancreas; these cell groups are known as the islets of Langerhans. The failure of these cells to secrete sufficient amounts of insulin causes diabetes (see Diabetes Mellitus).

Diseases of the pancreas are not common. Hemorrhage in the pancreas and acute pancreatitis are, however, serious conditions. If not relieved rapidly, they may cause death. The symptoms are not definite, resembling those of peritonitis or intestinal obstruction.

Thymus Gland

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Thymus Gland, name applied to a structure located just beneath the upper portion of the sternum in almost all vertebrates. The thymus gland consists chiefly of lymphatic tissue and contains a few small areas of epithelial tissue known as Hassall's corpuscles. The human thymus gland increases in weight in the first two years of life, and from then until puberty it grows slowly to a weight of about 43 g (about 1.5 oz). After puberty, it shrinks gradually and the lymphatic tissue of the thymus gland is replaced by fat. In the adult human the organ is chiefly composed of fatty tissue. Scientists generally agree that the thymus gland plays an important role in the development of immune responsiveness in early life (see Immune System). It is a site of formation of lymphocytes and a site of antibody production. Whether or not it has any other endocrine functions is uncertain. Clearly the adult animal is not affected by its removal. Abnormal enlargement of the thymus or development of tumors of the gland may occur in myasthenia gravis, but the reason is not known.

Varicose Vein

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Varicose Vein, dilated (enlarged) and often twisted vein just below the skin that develops when the valves in the vein no longer function properly or when blood volume in the vein increases. Varicose veins develop most commonly in the legs, but also occur in the anus (hemorrhoids), esophagus, and testes in males (varicocele).

Varicose veins in the legs are characterized by a purplish-blue color. These veins may become prominent and readily visible. Varicose veins may be painful and cause swelling of the ankles and ulcerations on the skin of the legs. Venous thromboses (blood clots) may develop within varicose veins. If these blood clots break off from the vein and enter the circulatory system, they may cause a dangerous obstruction elsewhere, as in the smaller arteries of the lungs. Simple superficial varicose leg veins are treated by applying pressure all along their length with an elastic stocking. Larger varicose veins may be removed by a physician with a chemical solution or surgery.

Dilation and inflammation of the veins in the membranes of the anus or rectum produce hemorrhoids, also called piles. Mild hemorrhoids may only itch, but advanced cases are painful and cause bleeding. Topical ointments may provide temporary relief from discomfort. Doctors recommend a diet rich in high fiber foods to improve regularity of bowel movements. The most serious cases require surgical removal of the dilated veins.

Vein

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Vein, in anatomy, blood vessel that conducts the deoxygenated blood from the capillaries back to the heart. Three exceptions to this description exist: the pulmonary veins return blood from the lungs, where it has been oxygenated, to the heart; the portal veins receive blood from the pyloric, gastric, cystic, superior mesenteric, and splenic veins and, entering the liver, break up into small branches that pass through all parts of that organ; and the umbilical veins convey blood from the fetus to the mother's placenta. Veins enlarge as they proceed, gathering blood from their tributaries. They finally pour the blood through the superior and inferior venae cavae into the right atrium of the heart. Their coats are similar to those of the arteries, but thinner, and often transparent. See Circulatory System; Heart; Varicose Vein.

Tongue

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Tongue (anatomy), muscular organ in the mouth, the primary organ of taste and important in the formation of speech and in the chewing and swallowing of food. The tongue, which is covered by a mucous membrane, extends from the hyoid bone at the back of the mouth upward and forward to the lips. Its upper surface, borders, and the forward part of the lower surface are free; elsewhere it is attached to adjacent parts of the mouth. The extrinsic muscles attach the tongue to external points, and the intrinsic muscle fibers, which run vertically, transversely, and longitudinally, allow it great range of movement. The upper surface is covered with small projections called papillae, which give it a rough texture. The color of the tongue, usually pinkish-red but discolored by various diseases, is an indication of health.

The tongue serves as an organ of taste, with taste buds scattered over its surface and concentrated toward the back of the tongue. In chewing, the tongue holds the food against the teeth; in swallowing, it moves the food back into the pharynx, and then into the esophagus when the pressure of the tongue closes the opening of the trachea, or windpipe. It also acts, together with the lips, teeth, and hard palate, to form word sounds.

See also Palate.

Throat

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Throat, in the human body, passage leading to the lungs and the stomach, located in the neck in front of the spinal column. It contains the larynx, pharynx, and the upper parts of the esophagus and the trachea. Several large arteries and veins also pass through this region.

Tooth Development

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Humans are diphyodont—that is, they develop two sets of teeth during their lives. The first set of teeth are the deciduous teeth, 20 small teeth also known as baby teeth or milk teeth. Deciduous teeth start developing about two months after conception and typically begin to erupt above the gumline when a baby is six or seven months old. Occasionally a baby may be born with one or more deciduous teeth at birth, known as natal teeth. By the time a child is six years old, a second set of 32 larger teeth, called permanent teeth, start to erupt, or push out of the gums, eventually replacing the deciduous teeth.

Human tooth development occurs in stages. The hard tissue of the deciduous teeth, or the dentin, forms while the fetus is in the womb. After the child is born, tooth enamel develops in stages. Front tooth enamel, for example, is usually complete around one month after birth, while the enamel on the second molars is not completely developed until a child is about a year and a half old. When the enamel is fully developed the tooth erupts. Front teeth usually erupt when a child is from 6 to 12 months of age, second molars between 13 and 19 months old, and canines usually erupt at 19 months or older. The final stage of tooth development is root completion, a slow process that continues until the child is more than three years old.

Around the age of six, the roots of deciduous teeth slowly dissolve as the developing permanent teeth start to push them out. Deciduous teeth eventually fall out and are replaced by the erupting permanent teeth. This begins a transitional phase of tooth development that takes place over the next 15 years. As baby teeth are pushed out by permanent teeth, the entire mouth and jaw transform from their childhood shape to a more pronounced, adultlike structure. From age six to age nine, a child’s permanent incisors, canines, and first molars erupt. The bicuspids erupt from age 10 to age 12, and the second molars come in by age 13. The third molars, or wisdom teeth, usually erupt by the age of 21.

When human teeth grow to a certain size, the root essentially closes and the teeth stop growing. Closed-rooted teeth have narrow root openings that are only big enough for the periodontal ligament, blood vessels, and a nerve.

Permanent Teeth
The 20 deciduous teeth, also called baby teeth or milk teeth, begin to fall out when a child is about six years old. Deciduous teeth are gradually replaced by 32 permanent teeth illustrated here. The 8 incisors (4 on the upper jaw and 4 on the lower) have a straight, sharp edge for cutting and biting. The 4 pointed canines are specialized for tearing. The 8 bicuspids, or premolars, have grinding surfaces, as do the 12 more massive molars. The third molars, absent in some people, are called wisdom teeth.

Structure and Function of Urinary System

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Kidney
The kidneys filter the blood and rid the body of wastes. Approximately one million nephrons (right) compose each bean-shaped kidney (left). The filtration unit of the nephron, called the glomerulus, regulates the concentration within the body of important substances such as potassium, calcium, and hydrogen, and removes substances not produced by the body such as drugs and food additives. Cancers that originate in the filtration tissues of the kidney, called renal cell cancer, account for 85 percent of all cancers of the kidney. A small percentage of cancers originate in the renal pelvis, a cavity in the center of each kidney.

The kidneys lie embedded in fat tissue on either side of the backbone at about waist level. Each fist-sized kidney is reddish-brown, weighs 140 to 160 g (5 to 6 oz), and is similar in shape to the kidney beans.

On the inner border of each kidney is a depression called the hilum, where the renal artery, the renal vein, and the ureter connect with the kidney (the adjective renal is from the Latin term renalis, meaning of or near the kidneys). The renal artery delivers over 1700 liters (450 gal) of blood to the kidneys each day, which these organs filter and return to the heart via the renal vein. Each kidney contains about 1 million microscopic coiled channels, called nephrons, which perform this critical blood-filtering function and produce urine in the process.

The bulblike upper portion of the kidney’s nephrons filters water; urea, the nitrogen-containing breakdown product of protein; salts; glucose; amino acids, the building blocks of proteins; yellow bile compounds from the liver; and other trace substances from the blood. As this material moves through a long, looped tubule, many of these filtered materials are reabsorbed into the blood to be reused by the body to maintain normal body functions. Less than 1 percent of the water and other materials remain behind to be excreted as waste products in the urine.

These waste materials then pass from the nephrons into a funnel-shaped area called the renal pelvis. From the renal pelvis, waste trickles out of the kidney into the ureter. The ureter empties into a hollow, muscular sac called the urinary bladder. A valvelike flap of tissue at the point of entry into the bladder prevents urine from flowing backward into the ureter. The urinary bladder is able to expand and contract according to how much urine it contains. As it fills with urine, the walls of the bladder stretch and become thinner, with the bladder itself lengthening to 12.5 cm (5 in) or more and holding up to about 0.5 liter (1 pt) of urine. A ringlike sphincter muscle surrounds the bladder’s outlet and prevents spontaneous emptying.

As the bladder becomes full, stretch-sensitive receptors in its walls are stimulated, and the person becomes aware of the fullness. When the person is ready to urinate, the sphincter relaxes and urine flows from the bladder to the outside through the urethra. In females, the urethra is about 3.8 cm (1.5 in) long and is strictly a urinary passage. In males, the urethra is about 20 cm (8 in) long; it passes through the penis and also serves to convey semen during sexual intercourse.

In addition to their vital role in ridding the body of wastes through the production of urine, kidneys play important regulatory roles. They maintain water balance, ensuring that the amount of water in body tissues remains at a constant level. The kidneys also control calcium levels in the blood to maintain healthy bones. They aid in regulating the acid-base balance of the blood and body fluids so that all body processes can proceed smoothly. By controlling salt levels, the kidneys help regulate blood pressure. Finally, they stimulate the body to make red blood cells, the primary component of healthy blood. Properly functioning kidneys are so vital to health that if they cease to function, death follows within days.

Lymph Nodes

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Along the course of the lymphatic vessels are situated the lymph nodes, more commonly called the lymph glands. These nodes are bean-shaped organs containing large numbers of leukocytes, embedded in a network of connective tissue. All the lymph being returned along the lymphatics to the bloodstream must pass through several of these nodes, which filter out infectious and toxic material and destroy it. The nodes serve as a center for the production of phagocytes, which engulf bacteria and poisonous substances (see Immune System). During the course of any infection, the nodes become enlarged because of the large number of phagocytes being produced; these nodes are often painful and inflamed. The swollen glands most often observed are located on the neck, in the armpit, and in the groin. Certain malignant tumors tend to “travel” along the lymphatics; surgical removal of all nodes that are suspected of being involved in the spread of such malignancies is an accepted therapeutic procedure.

OTHER ORGANS

In addition to the lymph nodes that occur in the lymphatic vessels, several organs, composed of similar tissue, are included in the lymphatic system. The largest and most important of these organs is the spleen.

Embryologically, the lymphatic vessels arise as outbuddings from several veins, especially from the internal jugular and iliac veins. The buds spread throughout the body and separate from the venous system at many points.

Lymphatic Capillaries

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These vessels are found in all body tissues except the central nervous system, which has a circulatory system known as the cerebrospinal system. The lymphatic capillaries run together to form larger ducts that intertwine about the arteries and veins. The lymph in these larger ducts, which are similar to thin, dilated veins, is moved along by the muscular movements of the body as a whole; it is prevented from moving back through the ducts by valves located along them at intervals. The ducts from the lower limbs and abdomen come together at the dorsal left side of the body to form a channel, known as the cisterna chyli, that gives rise to the chief lymphatic vessel of the body, the thoracic duct. This vessel receives lymph from the left side of the thorax, the left arm, and the left side of the head and neck; it empties into the junction of the left jugular and left subclavian veins. Another, smaller vessel, known as the right lymphatic duct, receives lymph from the right side of the thorax, the right arm, and the right side of the head and neck and empties its contents into the right subclavian vein.

Lymphatic System

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Lymphatic System, common name for the circulatory vessels or ducts in which the fluid bathing the tissue cells of vertebrates is collected and carried to join the bloodstream proper (see Lymph). The lymphatic system is of primary importance in transporting digested fat from the intestine to the bloodstream; in removing and destroying toxic substances; and in resisting the spread of disease throughout the body.

The portions of the lymphatic system that collect the tissue fluids are known as lymphatic capillaries and are similar in structure to ordinary capillaries. The lymphatic capillaries that pick up digested fat in the villi of the intestine are known as lacteals. The lymphatic capillaries are more permeable than ordinary capillaries and allow passage of larger particles than would ordinarily pass through capillary walls; large-molecule proteins, produced as a result of tissue breakdown, pass into the lymphatics for transport away from the tissues.

Among the abnormal conditions affecting the lymphatic system are inflammation of the lymphatics or of the lymph nodes, seen in infections; tuberculosis of the lymph nodes; malignancies in the lymphatic system (see Cancer; Hodgkin's Disease); and elephantiasis.

Topics:

Sexual Dysfunctions

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Sexual dysfunctions are problems with sexual response that cause distress. Erectile dysfunction (impotence) refers to the inability of a man to have or maintain an erection. Premature ejaculation occurs when a man is not able to postpone or control his ejaculation. Inhibited male orgasm, or retarded ejaculation, occurs when a man cannot have an orgasm despite being highly aroused.

Female orgasmic dysfunction (anorgasmia, or inhibited female orgasm) refers to the inability of a woman to have an orgasm. Orgasmic dysfunction may be primary, meaning that the woman has never experienced an orgasm; secondary, meaning that the woman has had orgasms in the past but cannot have them now; or situational, meaning that she has orgasms in some situations but not in others. Vaginismus refers to a spastic contraction of the outer third of the vagina, a condition that can close the entrance of the vagina, preventing intercourse.

Dyspareunia refers to painful intercourse in either women or men. Low sexual desire is a lack of interest in sexual activity. Discrepant sexual desire refers to a condition in which partners have considerably different levels of sexual interest. These dysfunctions may be caused by physical problems such as fatigue or illness; the use of prescription medications, other drugs, or alcohol; or psychological factors, including learned inhibition of sexual response, anxiety, interfering thoughts, spectatoring (observing and judging one's own sexual performance), lack of communication between partners, insufficient or ineffective sexual stimulation, and relationship conflicts. In such cases, a qualified sex therapist can work with a physician, if necessary, to determine the cause and best treatment options.

Sexual Risks

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There are a number of pressing sexually related public health and social policy issues facing countries around the world today. According to the United States Centers for Disease Control and Prevention, in the United States a teen becomes pregnant every 30 seconds, and every 13 seconds a teen contracts a sexually transmitted disease (STD). For most people in the United States, engaging in heterosexual intercourse without the use of a condom is the behavior that puts them at greatest risk for infection with human immunodeficiency virus (HIV), which can lead to acquired immunodeficiency syndrome (AIDS) and is often ultimately fatal. Although there is currently no cure for AIDS, there are medications that can help delay the onset of symptoms. Another serious sexually transmitted disease is syphilis, which if left untreated for many years, can lead to paralysis, psychiatric illness, and death. Gonorrhea and chlamydia may produce no obvious symptoms in a woman, but they can lead to sterility if she is not treated. STDs should be diagnosed and treated by qualified medical practitioners, and all sexual partners must be treated in order to avoid reinfection.

Individuals can reduce their exposure to such sexual risks by practicing abstinence, using appropriate methods of contraception to avoid unwanted pregnancies, and using of safer sex practices. Such practices include using condoms to avoid exchanging bodily fluids, limiting the number of sexual partners, and restricting sexual behaviors to those with less risk, such as manual stimulation and massage.

Orgasm Stage

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Orgasm, or climax, is an intense and usually pleasurable sensation that occurs at the peak of sexual arousal and is followed by a drop in sexual tension. Not all sexual arousal leads to orgasm, and individuals require different conditions and different types and amounts of stimulation in order to have an orgasm. Orgasm consists of a series of rhythmic contractions in the genital region and pelvic organs. Breathing rate, pulse rate, and blood pressure increase dramatically during orgasm. General muscle contraction may lead to facial contortions and contractions of muscles in the extremities, back, and buttocks.

In men, orgasm occurs in two stages. First, the vas deferens, seminal vesicles, and prostate contract, sending seminal fluid to the bulb at the base of the urethra, and the man feels a sensation of ejaculatory inevitability—a feeling that ejaculation is just about to happen and cannot be stopped. Second, the urethral bulb and penis contract rhythmically, expelling the semen—a process called ejaculation. For most adult men, orgasm and ejaculation are closely linked, but some men experience orgasm separately from ejaculation.

In women, orgasm is characterized by a series of rhythmic muscular contractions of the orgasmic platform and uterus. These contractions can range in number and intensity. The sensation is very intense—more intense than the tingling or pleasure that accompany strong sexual arousal.

Resolution Stage

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During resolution, the processes of the excitement and plateau stages reverse, and the bodies of both women and men return to the unaroused state. The muscle contractions that occurred during orgasm lead to a reduction in muscular tension and release of blood from the engorged tissues.

The woman's breasts return to normal size during resolution. As they do, the nipples may appear erect as they stand out more than the surrounding breast tissue. Sex flush may disappear soon after orgasm. The clitoris quickly returns to its normal position and more gradually begins to shrink to its normal size, and the orgasmic platform relaxes and starts to shrink. The ballooning of the vagina subsides and the uterus returns to its normal size. Resolution generally takes from 15 to 30 minutes, but it may take longer, especially if orgasm has not occurred.

In men, erection subsides rapidly and the penis returns to its normal size. The scrotum and testes shrink and return to their unaroused position. Men typically enter a refractory period, during which they are incapable of erection and orgasm. The length of the refractory period depends on the individual. It may last for only a few minutes or for as long as 24 hours, and the length generally increases with age. Women do not appear to have a refractory period and, because of this, women can have multiple orgasms within a short period of time. Some men also experience multiple orgasms. This is sometimes related to the ability to have some orgasms without ejaculation.

Plateau Stage

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During the plateau stage, vasocongestion peaks and the processes begun in the excitement stage continue until sufficient tension is built up for orgasm to occur. Breathing rate, pulse rate, and blood pressure increase. The man's penis becomes completely erect and the glans swells. Fluid secreted from the Cowper's gland (located near the urethra, below the prostate) may appear at the tip of the penis. This fluid, which nourishes the sperm, may contain active sperm capable of impregnating a woman. In women, the breasts continue to swell, the lower third of the vagina swells, creating what is called the orgasmic platform, the clitoris retracts into the body, and the uterus enlarges. As the woman approaches orgasm, the labia majora darken.

Excitement Stage

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The excitement stage of sexual arousal is characterized by increased blood flow to blood vessels (vasocongestion), which causes tissues to swell. In men, the tissues in the penis become engorged with blood, causing the penis to become larger and erect. The skin of the scrotum thickens, tension increases in the scrotal sac, and the scrotum is pulled up closer to the body. Men may also experience nipple erection.

In women, vasocongestion occurs in the tissue surrounding the vagina, causing fluids to seep through the vaginal walls to produce vaginal lubrication. In a process similar to male erection, the glans of the clitoris becomes larger and harder than usual. Muscular contraction around the nipples causes them to become erect. However, as the excitement phase continues, vasocongestion causes the breasts to enlarge slightly so that sometimes the nipples may not appear erect. Vasocongestion also causes the labia majora to flatten and spread apart somewhat and the labia minora to swell and open. The upper two-thirds of the vagina expands in a “ballooning” response in which the cervix and the uterus pull up, helping to accommodate the penis during sexual intercourse.

Both women and men may develop “sex flush” during this or later stages of the sexual response cycle, although this reaction appears to be more common among women. Sex flush usually starts on the upper abdomen and spreads to the chest, resembling measles. In addition, pulse rate and blood pressure increase during the excitement phase.

Physiology of Sex

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Understanding the processes and underlying mechanisms of sexual arousal and orgasm is important to help people become more familiar with their bodies and their sexual responses and to assist in the diagnosis and treatment of sexual dysfunctions. Nevertheless, it was not until the work of American gynecologist William H. Masters and American psychologist Virginia Johnson that detailed laboratory studies were conducted on the physiological aspects of sexual arousal and orgasm in a large number of men and women. Based on data from 312 men and 382 women and observations from more than 10,000 cycles of sexual arousal and orgasm, Masters and Johnson described the human sexual response cycle in four stages: excitement, plateau, orgasm, and resolution.

In men who are unaroused, the penis is relaxed, or flaccid. In unaroused women, the labia majora lie close to each other, the labia minora are usually folded over the vaginal opening, and the walls of the vagina lie against each other like an uninflated balloon.

A. Excitement
B. Plateau
C. Orgasm
D. Resolution

Adulthood

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In adulthood, more permanent relationships, in the form of marriage or cohabitation, become prevalent. The frequency of sexual activity is different for different individuals. People in monogamous relationships often engage in sexual activity more frequently than those who have several partners. It is not unusual for some new couples to have sexual intercourse almost every day, but in general, among married or cohabiting couples, the frequency of sexual intercourse tends to decline the longer the two people are together.

As people age, they may experience physical changes, illnesses, or emotional upheavals, such as the loss of a partner, that can lead to a decline in sexual interest and behavior. In women, there is a gradual decline in the function of the ovaries and in the production of estrogen. The average age at which menopause (the end of the menstrual cycle) occurs is about 50. Decreased estrogen leads to thinning of the vaginal walls, shrinking of the vagina and labia majora, and decreased vaginal lubrication. These conditions can be severe enough to cause the woman pain during intercourse. Women who were sexually active either through intercourse or through masturbation before menopause and who continue sexual activity after menopause are less likely to experience vaginal problems. Women can use hormone-replacement therapy or hormone-containing creams to help maintain vaginal health. In men, testosterone production declines over the years, and the testes become smaller. The volume and force of ejaculation decrease and sperm count is reduced, but viable sperm may still be produced in elderly men. Erection takes longer to attain, and the time after orgasm during which erection cannot occur (the refractory period) increases. Medications and vascular disease, diabetes, and other medical conditions can cause erectile dysfunction.

Adolescence

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Whereas the term puberty refers to the period of physical maturation, the term adolescence typically refers to the socially defined period during which a person adjusts to the physical, emotional, and social changes associated with the transition from childhood to adulthood. Adolescence, which occurs from about the age of 12 to the age of 17 or older, is a period marked by increased sexual behavior. By the end of adolescence, two-thirds of young women and almost all young men have masturbated to orgasm. In recent decades, surveys indicate that more adolescents have begun engaging in intercourse at a younger age. However, studies of college students often find that 20 to 30 percent of these students have not had sexual intercourse. Adolescence can be particularly difficult for teens who feel different from their peers. Sexually active adolescents may wonder if their peers are abstinent, while sexually inactive adolescents may believe that their peers are sexually active. Others may struggle with same-sex attractions.

Sexual orientation may become a question during puberty or adolescence. The term sexual orientation refers to a person's erotic, romantic, or affectional attraction to the other sex, the same sex, or both. A person who is attracted to the other sex is labeled heterosexual, or sometimes straight. A person attracted to the same sex is labeled homosexual. The word gay may be used to describe homosexuals and is most often applied to men, whereas the term lesbian is applied to homosexual women. A person who is attracted to both men and women is labeled bisexual. A transsexual is a person whose sense of self is not consistent with his or her anatomical sex—for example, a person whose sense of self is female but who has male genitals. Homosexuality is not synonymous with transsexuality. Homosexual men's sense of self is male and lesbian women's sense of self is female.

Puberty

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Puberty marks the second stage of physical sexual differentiation—the time when both primary and secondary sexual characteristics as well as adult reproductive capacity develop, and when sexual interest surges. Puberty typically begins in girls from 8 to 12 years of age, whereas boys start about two years later. The hypothalamus initiates pubertal changes by directing pituitary growth hormones and gonadotropins (hormones that control the ovaries and testes).

A girl's breasts grow, her pubic hair develops, and her body grows and takes on the rounded contours of an adult woman. This is followed by the first menstrual period (menarche) at about age 12 or 13 (although ages of onset range from 10 to 16.5), underarm-hair growth, and increased secretions from oil- and sweat-producing glands. It may take a year or two before menstruation and ovulation occur regularly. The hormones primarily responsible for these changes in young girls are the adrenal androgens, estrogens, progesterone, and growth hormone.

During puberty, a boy's testes and scrotal sac grow, his pubic hair develops, his body grows and develops, his penis grows, his voice deepens, facial and underarm hair appear, and secretions from his oil- and sweat-producing glands increase. Penile erections increase in frequency, and first ejaculation (thorarche) typically occurs sometime from the age of 11 to the age of 15. For a boy who has not masturbated, a nocturnal emission, or so-called wet dream, may be his first ejaculation. The ability to produce sperm may take another year or two and typically begins at about age 14. Growth hormone and androgens, particularly testosterone, are responsible for these pubertal changes in boys.

The fact that boys tend to develop more slowly than girls can cause some social awkwardness. Girls who have grown earlier may find themselves much taller than their dates, for example, and they may be more physically and psychologically mature than their male peers.

The first menstruation and first ejaculation are often considered the most important events of puberty, particularly for the individual. However, it is the development of the secondary sexual characteristics that serve as more apparent signals to others that the person is becoming a man or a woman. These signals lead to increasingly differential treatment of adolescent girls and boys by parents or other adults. The changes in hormone levels that occur during puberty may cause boys and girls to perceive the world in different ways, leading them to react differently to situations. Thus, puberty augments behavioral sex differences between young men and women. In some cultures and religions, puberty is recognized with rituals that mark the transition into adulthood.

Childhood

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After birth, the process of sex-role socialization begins immediately. There may be small, physiologically-based differences present at birth that lead girls and boys to perceive the world or behave in slightly different ways. There are also well-documented differences in the ways that boys and girls are treated from birth onward. The behavioral differences between the sexes, such as differences in toy and play preference and in the degree of aggressive behavior, are most likely the product of complex interactions between the way that the child perceives the world and the ways that parents, siblings, and others react to the child. The messages about appropriate behavior for girls and boys intensify differences between the sexes as the child grows older.

It is not uncommon for children to touch or play with their genitals or to play games, such as “doctor” or “house,” that include sexual exploration. Such experiences are usually not labeled sexual by the children. Adults will often discourage such behavior and respond negatively to it. Generally by the age of six or seven, children develop a sense of privacy and are aware of social restrictions on sexual expression.

As the first bodily changes of puberty begin, sometime from the age of 8 to the age of 12, the child may become self-conscious and more private. During this period, more children gain experience with masturbation (self-stimulation of genitals). Surveys indicate that about one-third of all girls and about half of all boys have masturbated to orgasm by the time they reach the age of 13, boys generally starting earlier than girls. Because preadolescents tend to play with others of their own sex, it is not at all uncommon that early sexual exploration and experience may happen with other members of the same sex.

Prenatal Sexual Development

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About six weeks after conception, if a Y chromosome is present in the embryo's cells (as it is in normal males), a gene on the chromosome directs the undifferentiated gonads to become testes. If the Y chromosome is not present (as in normal females), the undifferentiated gonads will become ovaries.

If the gonads become testes, they begin to produce androgens (male hormones, primarily testosterone) by about eight weeks after conception. These androgens stimulate development of the one set of the genital ducts into the epididymes, vas deferens, and ejaculatory duct. The presence of androgens also stimulates development of the penis and the scrotum. The testes later descend into the scrotum. Males also produce a substance that inhibits the development of the second set of ducts into female organs. In the absence of such hormonal stimulation, female structures develop.

Prenatal hormones also play a role in the sexual differentiation of the brain. For example, prenatal hormones direct the development of sex differences in some cells and the neural pathways in the hypothalamus (the part of the brain that controls the endocrine system). Beginning at puberty, based on prenatal sexual differentiation, the hypothalamus directs either the cyclic secretion of sex hormones that controls the female menstrual cycle or the relatively continuous production of male sex hormones. Other brain differences may be related to differences in sexual and aggressive behavior or in cognitive and perceptual characteristics. Most of the research on sexual differentiation of the brain has been performed with animals or with biased human samples, and there is much debate about the nature and behavioral relevance of these differences in humans.

Sexual Development

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There are two periods of marked sexual differentiation in human life. The first occurs prenatally and the second occurs at puberty. Although adult women and men may differ greatly in genital appearance and secondary sexual characteristics, they are almost identical during prenatal development. When an egg and a sperm unite during fertilization, they each bring to the new cell half the number of chromosomes (threadlike structures that contain genetic material) present in other cells. From fertilization through about the first six weeks of development, male and female embryos differ only in the pair of sex chromosomes they have in each cell—two X chromosomes (XX) in females and one X and one Y chromosome (XY) in males. At this stage, both male and female embryos have undifferentiated gonads (ovaries or testes), two sets of ducts (one set capable of developing into male internal organs and the other into female organs), and undifferentiated external genital folds and swellings. See Embryology.

Sub-Topics:

Male Sexual Organs

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The external sex organs of men are the penis and the scrotum. The penis is a sensitive organ important to reproduction and urination and to sexual pleasure. At its tip is the glans, which contains the urethral opening, through which urine passes. The ridge that separates the glans from the body of the penis is called the corona (Latin for “crown”), or coronal ridge. The glans and the corona are the most sensitive parts of the penis. The glans is covered with a foreskin (prepuce) unless the man has been circumcised, in which case the foreskin has been surgically removed.

The penis contains three cylinders of tissue that run parallel to the urethra. During sexual arousal, these tissues become engorged with blood and expand, causing the penis to enlarge and become erect (erection or tumescence). Men do not have a penis bone or a muscle that causes erection, as do some other animals.

The scrotum is a pouch that hangs below the penis and contains the two testes, which produce sperm (the male sex cell responsible for fertilization) and are considered part of the internal genitalia. The testes also are the primary producers of testosterone (male sex hormone) in men. Inside the testes are about 1000 seminiferous tubules that manufacture and store the sperm. The scrotum can pull up closer to the body when the surrounding temperature is low and can drop farther away when the temperature is hot in order to keep the testes at an optimal, constant temperature somewhat lower than body temperature.

After sperm are produced, they move out of the testes and into the epididymes, a long tube coiled against the testes, where the sperm are stored and mature. The vas deferens transports the sperm from the epididymes through the prostate, after which the vas deferens becomes the ejaculatory duct. Here, fluids from the prostate and seminal vesicles (small sacs that hold semen) combine with the sperm to form semen, a thick, yellowish-white fluid. The average discharge of semen, called ejaculate, contains approximately 300 million sperm.

Female Sexual Organs

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Primary sexual characteristics of women include the external genitalia (vulva) and the internal organs that make it possible for a woman to produce ova (eggs) and become pregnant. The vulva includes the mons pubis, the most visible part of the woman's external genitalia, which is the pad of fatty tissue that covers the pubic bone and is commonly covered by pubic hair; the labia majora, the large outer lips; and the labia minora, the smaller, hairless inner lips that run along the edge of the vaginal opening and often fold over to cover it. The labia minora come together in front to form the clitoral hood, which covers the clitoris, a sensitive organ that is very important to the woman's sexual response. The opening of the urethra, the tubular vessel through which urine passes, is located midway between the clitoris and the vaginal opening. The area where the labia majora join behind the vagina is called the fourchette. The area of skin between the vaginal opening and the anus is the perineum. The hymen is a thin membrane that partially covers the vaginal opening. If the hymen is extensive and is still present at first intercourse, it may be broken or stretched as the penis enters the vagina and some bleeding and pain may occur, although more typically its presence is unnoticed. The presence or absence of a hymen is not a reliable indicator of virginity, although historically it was viewed as such.

The internal sex organs of the female consist of the vagina, uterus, fallopian tubes (or oviducts), and ovaries. The vagina is a flexible tube-shaped organ that is the passageway between the uterus and the opening in the vulva. Because during birth the baby travels from the uterus through the vagina, the vagina is also known as the birth canal. The woman's menstrual flow comes out of the uterus and through the vagina. When a man and a woman engage in vaginal intercourse, the penis is inserted into the vagina.

The cervix is located at the bottom of the uterus and includes the opening between the vagina and the uterus. The uterus is a muscular organ that has an inner lining (endometrium) richly supplied with blood vessels and glands. During pregnancy, the uterus holds and nourishes the developing fetus. Although the uterus is normally about the size of a fist, during pregnancy it is capable of stretching to accommodate a fully developed fetus, which is typically about 50 cm (about 20 in) long and weighs about 3.5 kg (about 7.5 lbs). The uterine muscles also produce the strong contractions of labor.

At the top of the uterus are the pair of fallopian tubes that lead to the ovaries. The two ovaries produce eggs, or ova (the female sex cells that can become fertilized), and female sex hormones, primarily estrogens and progesterone. The fallopian tubes have fingerlike projections at the ends near the ovaries that sweep the egg into the fallopian tube after it is released from the ovaries. If sperm are present in the fallopian tube, fertilization (conception) may occur and the fertilized egg will be swept into the uterus by cilia (hairlike projections inside the fallopian tube).

Human Sexual Characteristics

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Sexual characteristics are divided into two types. Primary sexual characteristics are directly related to reproduction and include the sex organs (genitalia). Secondary sexual characteristics are attributes other than the sex organs that generally distinguish one sex from the other but are not essential to reproduction, such as the larger breasts characteristic of women and the facial hair and deeper voices characteristic of men.

Sub-topics:

A. Female Sexual Organs
B. Male Sexual Organs

Gonads

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Structure of Human Gonads
Gonads—in the male, the testes (singular, testis), and in the female, the ovaries—are the organs that produce gametes and sex hormones. The male gamete is the spermatozoan, produced by cell division in the seminiferous tubules of the adult testes. Typically, several hundred million sperm reach maturity in the epididymis and are stored in the vas deferens each day. Whatever is not released in ejaculation is reabsorbed, part of a continuous cycle. In the female, the ovaries produce eggs, or ova. At birth, about 2 million oocytes, or immature eggs, are present in the ovaries. Once the female reaches puberty, one egg matures approximately every 28 days inside a saclike Graafian follicle. Ovulation occurs when the mature egg bursts from the follicle and the ovary, beginning its journey down the fallopian tube toward the uterus.

Organs that contain germ cells which later develop into male gametes or spermatozoa are known as testes or male gonads. Organs that contain germ cells which later develop into female gametes, eggs, or ova are known as ovaries.

In most higher mammals, including the human male, the testes are always enclosed in an external scrotum. During fetal life, the testes move through the muscles composing the posterior, ventral portion of the trunk and carry with them the portion of the peritoneum and skin surrounding these muscles. The channel in the muscles through which the testis moves is known as the inguinal canal; it usually closes after birth, but sometimes remains open and is then often the site of herniation (see Hernia). The portion of the peritoneum that the testis carries with it forms a double wall of membrane between the scrotum and testis and is known as the tunica vaginalis. Occasionally, the testes in the human male do not descend into the scrotal sac; this condition of nondescent, which is known as cryptorchidism, may result in sterility if not corrected by surgery or the administration of hormones. Retention of the testes within the body cavity subjects the germ cells to temperatures that are too high for their normal development; the descent of the testes into the scrotum in higher animals keeps the testes at optimum temperatures.

Unlike germ cells in the testis, female germ cells originate as single cells in the embryonic tissue that later develops into an ovary. At maturity, after the production of ova from the female germ cells, groups of ovary cells surrounding each ovum develop into “follicle cells” that secrete nutriment for the contained egg. As the ovum is prepared for release during the breeding season, the tissue surrounding the ovum hollows out and becomes filled with fluid and at the same time moves to the surface of the ovary; this mass of tissue, fluid, and ovum is known as a Graafian follicle. The ovary of the adult is merely a mass of glandular and connective tissue containing numerous Graafian follicles at various stages of maturity. When the Graafian follicle is completely mature, it bursts through the surface of the ovary, releasing the ovum, which is then ready for fertilization; the release of the ovum from the ovary is known as ovulation. The space formerly occupied by the Graafian follicle is filled by a blood clot known as the corpus hemorrhagicum; in four or five days this clot is replaced by a mass of yellow cells known as the corpus luteum, which secretes hormones playing an important part in preparation of the uterus for the reception of a fertilized ovum. If the ovum goes unfertilized, the corpus luteum is eventually replaced by scar tissue known as the corpus albicans. The ovary is located in the body cavity, attached to the peritoneum that lines this cavity.

The functioning of both male and female gonads is under the hormonal influence of the pituitary gland.

Reproductive System

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Reproductive System, term applied to the group of plant or animal organs that are necessary for or that are accessory to the reproductive processes (see Reproduction). The basic units of sexual reproduction are the male and female germ cells; this article deals with the organs within which the germ cells of animals mature and are stored, the organs through which they are transported in the process of producing a new individual, and accessory glandular organs. For the reproductive organs of plants, see Plant Propagation.

See: Gonads