The body as a physical system may be described at several different levels of organization, beginning with its basic chemical composition and ending with the nature of the human body as a member of the animal kingdom. CHEMICAL COMPOSITION The human body is made up of several different chemical elements, but its main components are oxygen, hydrogen, carbon, and nitrogen. These elements, which are derived directly or indirectly from the atmosphere or from water, together account for nearly 95% of the total body weight. The remaining elements include the minerals calcium and phosphorus, which as constituents of bone account for about another 3% of body weight, and--in order of decreasing amounts--potassium, sodium, magnesium, iron, zinc, copper, and several trace elements such as vanadium, chromium, silicon, and selenium. MOLECULAR COMPOSITION At the molecular level, the body's chemicals are organized into two major categories: inorganic and organic compounds. The first consists of water, which constitutes roughly 60% of the total body weight and which is essential for almost all chemical reactions within the body. The second category consists of the carbon-containing compounds, the main types of which include the \Tproteins\t, \Llipid\ls, \Lcarbohydrate\ls, and \Lnucleic acid\ls. Proteins, the main building blocks of \Lcell\ls, are large, complex molecules (macromolecules) made up of polypeptides (see \Tpeptide\t), which in turn are made up of the nitrogen-containing \Lamino acid\ls. Proteins, which constitute about 15% of the body by weight, form much of the body's structure and serve important chemical functions, particularly in the form of \Lenzyme\ls. Lipids, which make up most of the rest of the body's weight, serve as stores of food energy in the form of \Tfat\t. More complex lipids such as phospholipids are basic constituents of cell membranes, and \Lsteroid\ls--including many different \Lhormone\ls--perform other vital chemical functions. Carbohydrates include simple sugars (see \Tsugar\t) and giant polymers of such sugars; sugars serve as a source of cellular energy, as building blocks for certain complex molecules, and--in the form of \Tglycogen\t--as a way to store energy. The nucleic acid \TDNA\t, present in cell nuclei, is the repository of hereditary information passed on through \Treproduction\t; the nucleic acid \TRNA\t, contained in the cell cytoplasm, is concerned with \Tprotein\t \Lsynthe\lsis. CELLS AND TISSUES The molecular constituents of the body are combined to form cells and extracellular materials. The adult human body contains something on the order of 100 trillion cells, which are organized and differentiated to form a number of different kinds of tissues with specific functions (see \Ttissue, animal\t). Four main types of tissue may be described: epithelial, muscle, nerve, and connective. Epithelial tissue, which covers the body's surface and lines its tubes, protects these surfaces and is the site of various absorption and excretion processes between the body and its environment. Also included in epithelial tissue are the outgrowths and ingrowths and form the surfaces of sensory organs, glands, hair and nails, and other structures. Muscle tissue provides movement to the body through its ability to contract and relax. Nerve tissue, made up of neurons, conducts information signals and processes data. \Tconnective tissue\t, which contains large amounts of extracellular matter, provides support for the body in the form of tendons, ligaments, cartilage, bone, and fat deposits; bond also replaces some other connective tissues. Finally, \Tblood\t and lymph are fluids that are sometimes also considered as tissues; together they convey nutrients, waste products, and the specialized defensive cells of the immune system ( see \Timmunity\t) among the different parts of the body. BODY SYSTEMS As indicated above, the various body tissues form organs and other structures that can be grouped into a number of major body systems. These systems have well-defined functions but are also integrated with one another in essential ways; they may be described as follows: The musculoskeletal system (see \Tskeleton, human\t) provides support and movement for the body; its main components are the voluntary \Lmuscle\ls, \Tbone\t, and \Tligament\t. The \Tcirculatory system\t delivers blood to and from the different regions of the body, conveying essential nutrients; its components are the \Theart\t, ARTERIES, \Lvein\ls, and \Lcapillari\les. Closely associated with and often considered part of the circulatory system is the \Tlymphatic system\t, which includes the \Tspleen\t, \Tthymus\t, other specialized organs and tissues, as well as a network of capillaries. The \Trespiratory system\t, whose main elements are the \Tlung\t and \Tdiaphragm\t, provides for the oxygenation of blood and the elimination of carbon dioxide form the body. The \Tdigestive system\t includes major organs such as the \Tstomach\t and \Lintestine\ls, which provide for the digestion and absorption of nutrients other than oxygen; the \Tliver\t, which has important synthetic, storage, and excretory functions; and the \Tpancreas\t, which secretes certain digestive enzymes and is the source of several hormones, including \Tinsulin\t. The \Texcretory system\t has as its major components the \Tkidney\t and \Tbladder\t (see also \Turine\t). In addition, the \Tskin\t--actually the human body's largest organ--also excretes wastes, by means of the sweat glands. In the human male, the \Turethra\t of the excretory system also serves as the terminal part of the reproductive system (see \Treproductive system, human\t). By means of the structures of this system, human beings transmit their hereditary materials to form new generations (see \Tgenetics\t). INTEGRATING SYSTEMS Certain body systems provide overall control over other body functions, integrating the actions of body systems in general. One of these is the \Tnervous system\t, which includes the central \Tbrain\t and the \Tspinal cord\t as well as the sense organs, such as the \Tear\t, \Teye\t, and so on. Through its complex network of nerve fibers that extends throughout the body, the nervous system that receives and transmits electrochemical signals that enable the body to react to its external environment. The other system providing overall control is the \Tendocrine system\t, which consists of a number of glands such as the \Tadrenal gland\t, the \Tpituitary gland\t, and the \Tthyroid gland\t, among several others. Because these glands are widely separated in the body, they depend on the circulatory system to convey the hormones by which the interior functions of the separate body systems are integrated. The nervous system and endocrine system, together with other regulatory mechanisms that function within cells and coordinate body \Tmetabolism\t, make possible the condition known as \Thomeostasis\t. This condition, which is defined as the maintenance of a steady state within a biological system by means of self-regulating mechanisms, is typical of any healthy organism. Homeostasis is one of the fundamental concepts of modern biology. THE BODY AS A WHOLE In terms of the theory of \Tevolution\t, the human body can be assigned a definite place within the \Tanimal\t kingdom because of its structures and modes of function (see \Tclassification, biological\t). Thus, because the human body possesses a spinal column, it is classified as belonging in the phylum Chordata, subphylum Vertebrata. Beyond that, humans are placed within the class Mammalia (see \Tmammal\t) because their bodies exhibit a number of diagnostic features typifying this class, including warm-bloodedness (see body \Ttemperature\t) and the bearing of living young that are nourished by milk from mammary glands. Because human beings possess grasping hands that bear nails instead of claws, in addition to other defining structures such as collarbones, they are classified in the order Primates (see \Tprimate\t). This order includes the monkeys and their relatives. The great apes of the superfamily Hominoidea, in particular, are considered the closest living relatives of modern humans, which are classified as species Homo sapiens. The search for the fossil record that would establish the immediate family relationships of human beings with primate ancestors is still ongoing, although it is aided by genetic studies (see \Tprehistoric humans\t). Certain anatomical features set off modern humans from all other primates. One of these is the upright posture made possible by changes in pelvic structure, combined with the development of strong rump muscles for propelling the body forward and balancing it alternately on one leg and then the other in the course of walking. The human leg is also longer and more powerful relative to the rest of the body than in other primates. Upright posture is also aided by the curve in the lower spine, although this development has also contributed to the back problems suffered by many people. The outstanding feature of human beings, however, is the huge brain they possess, which is twice as large as that of any other primate. The brain contains more than 100 billion neurons and an enormously greater number of synapses between these neurons. It is this structure that has made possible the development of language and of human culture. Reviewed by \TAaron\t D. FREEDMAN, M. D. HUMAN BODY -- SYSTEMS AND STRUCTURES The human body is a marvelously complex array of cells and fluids combined into tissues and systems that function together as a single organism. The Skeletal System The skeleton, a lightweight frame and lever system operating in conjunction with the muscular system, enables the body to move and maintain its posture. The adult skeleton consists of 206 bones, 22 of them in the skull alone, together with associated cartilages. The skull and rib cage also serve to protect the organs lying within the areas they encompass; in addition, the circulatory system's red blood cells are produced within the marrow of some of the larger bones. The skeleton is divided into two major parts: the axial skeleton, which includes the skull, spinal column (vertebrae), ribs, and breastbone (sternum); and the appendicular skeleton, which includes the bones of the arms and legs, including the shoulder and pelvic bones. The Muscular System The body contains about 700 skeletal, or voluntary, muscles, which transform chemical into mechanical energy for moving and supporting the body. Among the many different kinds of muscle action are flexion and extension (decreasing and increasing, respectively, the angle between two bones); and abduction and adduction (moving a limb or digit away from or toward, respectively, the axis of the trunk or limb). Many muscles have more than one action, and most act together with other muscles. Bone and Muscle Structures Bones are dense, hard, slightly elastic organs surrounded by a membrane called the periosteum and composed largely of skeletal tissue. Skeletal tissue, in turn, is composed of living cells embedded in a matrix of calcium minerals bonded by the protein collagen and other organic substances. Two major types of bone exist. One type, compact tissue, is made up of rodlike structures, each constructed of layers of bone tissue surrounding a central blood vessel; the living cells occur in irregular spaces among the layers. The other type of bone, spongy bone, takes the form of a loose network of rigid bone; its spaces contain the blood-forming tissue, marrow, along with fat cells. Muscle tissue, consisting of fibers supported by connective tissue, is highly elastic. It functions by contraction of the fibers, which can shorten to two-thirds of their resting length. In terms of structure, muscles are either smooth or striated. Smooth muscle is found in the walls of the hollow organs and tubes of the body, such as the intestines and blood vessels. Lacking striations, they react slowly to stimuli from the autonomic nervous system by means of circular and longitudinal fiber layers. Striated muscle is capable of fast contractions. The specialized striated muscle of the wall of the heart has branching, connecting fibers; skeletal muscle contains many bundles of long, multinucleate fibers. Joints, or unions between adjacent bones, range from immovable to freely movable. The immovable, ligament-connected sutures between the bones of the skull are examples of what are called fibrous joints; the slightly movable unions between the bodies of the vertebrae in the spinal column are examples of so-called cartilaginous joints. Freely movable joints, such as the knee, have bone ends also covered by cartilage but separated by a cavity and enclosed by a capsule with a delicate inner layer, the synovial membrane; they are called synovial joints. The knee is the largest and most complicated joint in the human body. The femur and tibia are joined by the capsular ligament, which is continuous with the periosteum, or outer membrane, of the bones. In front, the patellar ligament and tendon of the quadriceps muscle attach to the patella, or kneecap. The bursae and articular cavity, filled with synovial, or lubricating, fluid, increase joint mobility, as does the cartilage. The menisci and ligaments lend stability to the knee. The biceps muscle controls flexion and lateral rotation of the knee. The gastrocnemius muscle controls ankle flexion. The Hand The hand contains a total of 27 bones and is the most flexible part of the human skeleton. Like other higher primates, humans possess nails instead of claws and have a thumb that can be rotated to oppose the other digits, enabling them to manipulate objects delicately and precisely. Higher primates other than humans, however, also use hands for locomotion; the development of an upright posture and consequent use of the hands for manipulation alone probably took place concurrently with the increase in brain size in humans. The Foot The human foot differs from the feet of other primates in that it is used solely for locomotion, so that the big toe is no longer opposable to the other digits as it is in the great apes. Instead the bones of the feet have evolved in a way that enables humans to stride, and toes other than the big toe begin to show signs of degeneration. The heel bone bears most of the weight of the body and helps to form the longitudinal arch of the foot, along with the transverse arch formed by the metatarsal bones. If the ligaments between the bones weaken, the result is flatfood, in which all instead of only part of the sole rests on the ground. The Circulatory System By means of a network of vessels, the fluid called blood transports nutrients throughout hte body and helps to rid cells of their waste products. This circulatory system has two main divisions: the pulmonary and the systemic. The pulmonary system consists of pulmonary arteries that circulate oxygen-poor blood to the lungs, where oxygen is absorbed and carbon dioxide is released; and pulmonary veins that carry the oxygen-enriched blood back to the heart. In the systemic system, blood is carried to and from all other parts of the body. The blood leaves the heart by way of the aortic arch and travels through smaller and smaller arteries into capillaries, from which oxygen and nutrients pass to the cells of body tissues. Other capillaries then pick up cellular waste products and carbon dioxide. The blood flows into larger and larger veins, merging in the vena cava, which carries the blood back to the heart. Blood vessels also absorb nutrients from the stomach and the intestines; the liver and the spleen act as blood reservoirs and as filtering systems of the blood (portal vein). The kidneys maintain the salt and water balance in the body and filter toxic wastes from the blood (renal artery). The heart, brain, and lungs receive a large blood supply to maintain their vital functions. The four chambers of the heart are the right and left atria and the right and left ventricles. The right atrium receives unoxygenated blood from the body by way of the superior and inferior vena cava. The tricuspid valve regulates blood flow between the right atrium and the right ventricle. Blood then passes through a semilunar valve into the pulmonary artery and from there to the lungs. Oxygenated blood returns to the heart by way of two left and two right pulmonary veins, flowing into the left atrium through the mitral valve to the left ventricle. The blood is then pumped to all parts of the body by way of the aorta. In the directly supporting system of coronary blood vessels, the right and left coronary arteries, which originate at the aorta, are the two main blood vessels that supply the heart with oxygen and nutrients. The left coronary artery divides into anterior and posterior branches, Cardiac veins carry blood from the heart to a central coronary sinus, which leads to the right atrium. The coronary arteries are unique in the circulatory system in that they relax in response to stimulation from the sympathetic nervous system and contract in response to parasympathetic action; these responses are opposite to the behavior of systemic blood vessels. With regard to arteries and veins, they are the vessels in which blood is circulated throughout the body. Although both kinds of vessels have similar structures, arteries have thicker walls, because they have to withstand the pressure of blood being pumped from the heart. The inner layer of a blood vessel consists of a lining of epithelial cells. This membrane is backed by a layer of connective tissue and of elastic tissue. The middle layer is composed of smooth muscle. Smaller arteries contain less elastic tissue and more muscle tissue than do larger arteries. The outer layer is made of connective tissue and is supplied with blood by capillaries. The minute blood vessels called capillaries form a network that interacts with tissues throughout the body. Their walls have a single layer of cells, enabling them to deliver oxygen and nutrients and to remove carbon dioxide and other waste products. The difference in pressure at the arterial and the venous ends of the capillaries results in the movement of fluid into the tissue spaces at the arterial end, and back into the capillaries at the venous end. The Nervous System The nervous system receives, interprets, and responds to information from the body's external and internal environments. It has two main divisions: the central nervous system, including the brain and spinal cord (connected by the medulla), which coordinates the activity of the entire system; and the peripheral nervous system, which consists of all the remaining nervous tissues. The peripheral nervous system includes 12 pairs of cranial nerves that radiate from the brain, and 31 pairs of nerves that arise from the spinal cord. Of these spinal pairs, 8 are cervical (neck), 12 thoracic (chest), 6 lumbar (loin), 5 sacral (sacrum), and 1 coccygeal (coccyx). The ventral root of each spinal nerve carries impulses away from the spinal cord; the dorsal roots, whose cell bodies accumulate in ganglia, transmit signals toward the spinal cord. The somatic nerves which coordinate voluntary actions, spread through the skeletal muscle, joints, and the skin. The autonomic nerves, which coordinate functions such as secretion, heartbeat, and peristalsis--mainly involuntary functions--have two main divisions. The sympathetic system includes a chain of ganglia (the sympathetic trunk) on each side of the spinal cord, attached to the thoracic and upper lumbar spinal nerves; the parasympathetic system runs through certain cranial and sacral spinal nerves. The human brain, containing about 10 billion nerve cells, is notable for the evolutionary increase in the size and complexity of the two cerebral hemispheres of the forebrain, relative to other brain structures. The two hemispheres are connected by a band of fibers called the corpus callosum. The limbic system is associated with memory and emotion; the cerebellum controls involuntary body functions. The thalamus, hypothalamus, and pituitary gland are all parts of the forebrain. The brain stem includes the pons and medulla oblongata and connects the rest of the brain with the spinal cord. Nerve cells, or neurons, are the most complex cells in the body. The cell's control center is the cell body, containing the nucleus. The processes extending from the center are of two types: the shorter dendrites and the stalklike axon. Axons of the cells in the spinal cord that convey impulses to the feet may be up to 100 cm (40 in) long, but in the brain an entire neuron is usually less than 0.1 cm (0.04 in) long. Dendrites are the pathways for receiving impulses from other cells, whereas the axon is the pathway for the impulses transmitted by the cell body. The tiny gap between neurons is called a synapse. The axon of the cell seen here is sheathed in an insulating membrane called myelin. Such cells make up the so-called gray matter of the brain, as opposed to unmyelinated white matter. In sheathed axons, impulses travel from one node of Ranvier to the next (areas where the sheath is interrupted); in unsheathed axons, the impulses flow continuously. The mitochondria are the cell's energy sources. Chemical substances such as the neurotransmitters that carry messages to other neurons are manufactured in the rough endoplasmic reticulum and carried along the axon by the smooth endoplasmic reticulum to the nerve ending, where they are packaged in vescicles for transmission. Such vescicles can contain more than one kind of neurotransmitter. The Major Sense Organs Sense organs provide specific kinds of information about the body's environment. There are five major organs for sensing the outer world; other such organs include the pain, pressure, and temperature receptors of the skin. Interior sense receptors, located mainly in the visceral organs, provide information on such sensations as pain, hunger, and fatigue. Sense organs called proprioceptors, which are located in connective and muscle tissues and in the organs of balance in the ear, provide information about body orientation and movement. The ear is the sense organ for detecting sound waves. Waves received by the outer ear travel through the auditory canal to the eardrum, a membrane that transmits them to the middle ear. This chamber, filled with air coming from the mouth by way of the eustachian tube, contains three tiny bones--the hammer, anvil, and stirrup--that amplify the waves and transmit them to the inner ear. In the fluid-filled cochlea, the waves are converted into nerve impulses that are related to the brain by the auditory nerve. The semicircular canals play little part in hearing, but they provide information about the orientation of the head in space. The eye, the light-detecting sense organ, has three membrane layers. The outer, fibrous tunic contains the sclera, to which the muscles that move the eyeball are attached; and the transparent cornea, protected by the thin conjunctiva that covers the exposed eye surface and lines the inner eyelid. The middle, vascular membrane consists of the choroid layer, which carries the blood vessels; the pigmented iris, with muscles that act like a camera diaphragm by changing the size of the iris's circular opening, the pupil; and the ciliary body, which holds the transparent lens and adjusts its curvature. The inner membrane, the retina, receives the light and relays information to the brain by way of the optic nerve. Vision is most acute at the fovea, or focal point; at the blind spot, light-receiving cells are absent. The aqueous humor and vitreous humor fill the eye's cavities. The Respiratory System Humans and other animals (except for certain types of bacteria and yeasts) require oxygen to support the chemical reactions by which the body produces its energy. The act of obtaining oxygen from the environment and releasing carbon dioxide, a waste product, is called respiration. In humans and other higher vertebrates, the organs of respiration are a pair of elastic chambers, called lungs, that are made up of light, spongy tissue. The thin linings of these chambers are the site at which oxygen and carbon dioxide are exchanged between the bloodstream and the air drawn into and exhaled from the lungs. During inhalation, the diaphragm and rib cage expand the thoracic cavity in which the lungs are located, and the lungs inflate with air; the diaphragm and rib-cage muscles relax. The right lung is divided into three lobes and the left lung into two lobes. When air is drawn into the body, it passes through the trachea, or windpipe, and travels through the bronchi into the lungs. In the lungs the bronchi branch repeatedly into bronchilose, which segment into alveolar ducts and finally into clusters of alveoli, or air sacs. These thin-walled air sacs, surrounded by networks of capillaries, are the sites of gas exchange by means of diffusion, or passive transfer, of molecules of oxygen and carbon dioxide throughout the walls of collagenous connective tissue, which are no more than 0.7 microns thick. A pair of lungs may contain 700 million alveoli, with a combined respiratory surface of roughly 70 sq m (750 sq ft). The heart pumps oxygen-depleted blood through pulmonary arteries to the lungs. Inhaled oxygen diffuses from the alveoli to the capillaries, and the oxygenated blood returns to the heart through the pulmonary veins. Carbon dioxide diffuses into the alveoli and is exhaled. The Digestive System The digestive system provides body energy by breaking down food into simpler products and absorbing them so that they can be circulated to body cells. Digestion begins as salivary juices in the mouth act on food. The food enters the esophagus and passes into the stomach for preliminary digestion before being acted on by juices from the liver and pancreas in the small intestine, where the nutrients are then absorbed into the circulatory system. The large intestine condenses unused matter and expels it by way of the rectum and the anus. The stomach kneads and moves food along with its muscular walls. The walls in its fundic region secrete hydrochloric acid and a material that becomes the protein-digesting enzyme pepsin. Mucus is secreted in the fundic, cardiac, and pyloric regions to protect the walls from acid. Peristaltic waves force food into the small intestine through the pyloric sphincter. The small intestine is the main site of digestion and absorption of food nutrients. The portion lying immediately beyond the stomach is the duodenum; this is followed by the jejunum and the ileum, the latter terminating at the ileocecal valve, a sphincter muscle that guards the entrance to the large intestine. The walls of the small intestine consist of a mucosal layer, a circular and a longitudinal muscle layer for moving food along, and a layer of connective tissue. Pancreatic juices and liver bile enter the intestine from the hepatopancreatic duct to digest the food, which is then absorbed by fingerlike villi that carpet the inner surface of the intestine. Each villus contains a lacteal gland with blood and lymph vessels for transporting nutrients. Other Major Organs The kidneys, the most important organs of the excretory system, are located in the upper abdomen. Each kidney contains about one million nephrons, which purify the blood of the waste products that form urine. Urine travels from the nephron to the renal pelvis and into a ureter. The ureter of each kidney moves urine toward the bladder by peristalsis, or wavelike contractions of muscle. The bladder, an elastic tissue, contracts when it is full, pushing urine through the urethra and out of the body. The liver is part of the digestive system. Situated in the upper abdomen below the diaphragm on the right side of the body, it lies next to the stomach and spleen and above the duodenum and pancreas. It performs at least 500 functions, more than any other organ. Oxygenated blood enters through the portal vein. After processing by the liver, the blood exits through the hepatic vein. Bile, stored in the gallbladder, is released into the duodenum through the bile duct. The pancreas lies within the curve of the duodenum. The exocrine tissue of the pancreas consists of lobules, each of which contains glandular alveoli, or acini. Alveoli secrete digestive enzymes into ducts that lead into the main pancreatic duct, which opens into the duodenum. The endocrine portion includes the islets of Langerhans, which secrete hormones into the blood through capillaries; they contain A cells, the source of glucagon, and B cells, the source of insulin. The skin, the body's largest organ, consists of two strata, the epidermis and dermis, supported by a layer of fat. The outer epidermis contains dead cells that are continuously shed. Below, granular cells produce a protein, keratin, that forms hair and nails. The next layer consists of flattened polygonal cells; below are columnar cells and melanocytes--cells that produce the pigment melanin. The dermis consists of the fibrous proteins collagen and elastin, in which are embedded nerve fibers, nerve endings, capillaries, lymph vessels, sweat glands, and hair follicles. Each follicle bears a hair shaft in a sheath, an oil gland for lubrication, and a small muscle that tightens in response to fear or chills. The Endocrine System The endocrine system consists of a number of specialized glands that secrete a complex array of chemicals, called hormones, directly into the bloodstream. These chemicals interact with body organs, systems, and one another so as to adjust body activities to varying demands of the external and internal environment. The endocrine glands include the pituitary, which is located in the brain and secretes growth hormone. The thyroid and parathyroid glands produce a number of hormones, such as thyroxine. The thymus yields the hormone thymosine during childhood. The adrenal glands, located on top of the kidneys, are composed of two different layers, each of which functions as a separate gland: the cortex, which secretes the hormones aldosterone and the corticosteroids; and the medulla, which makes epinephrine and norepinephrine. The pancreas secretes insulin and glucagon, among other substances; and sex hormones are secreted by ovaries in the female and testes in the male. Other glands and organs of the body also produce hormonelike substances. The Reproductive System The male reproductive system is designed to produce spermatozoa and to transmit them into the female reproductive system in order to fertilize an egg. Sperm and eggs are both gamete cells, containing chromosomes that join together to create new life. Millions of sperm are produced each day in the testicles, which hang outside the pelvic area in a pouch of skin, the scrotum, because internal body temperature is too high to permit sperm production. Mature sperm are stored in the epididymis. A testicle has a system of conical lobules, each containing coiled tubules in which sperm are made. Channels lead to the epididymis and thence to the vas deferens, or sperm ducts. During sexual stimulation, sperm pass into the urethra, where they mix with fluids from the seminal vescicles, prostate, and Cowper's gland to form semen. At the same time, spongelike structures in the penis become filled with blood, causing it to elongate and become erect. If stimulation continues, penis muscles contract rhythmically, expelling the semen. The penis and urethra also serve to eliminate urine from the bladder, but the bladder entrance is closed during sexual stimulation. The female reproductive system, during the years between menarche and menopause, functions cyclically in preparation for childbearing. Each month an ovary releases a mature egg. An egg follicle first develops at the bottom of the ovary and rises to the surface, releases a mature egg, becomes an estrogen-producing "yellow-body," and then disappears, leaving a temporary white scar. The egg is carried through the oviducts, or Fallopian tubes, to the womb, or uterus, a hollow, muscular organ. Its lower end, the cervix, protrudes into the vagina, the sheath that receives the penis during intercourse. In the first half of the above cycle, the inner lining of the uterus is enriched with blood and glandular fluids. If the egg is not fertilized, the lining is shed in a process known as menstruation; if fertilization occurs, the egg attaches itself to the lining and begins the growth process that results in childbirth. The bladder and urethra function separately from the reproductive system in the female. The clitoris is a sensory organ that fills with blood and rhythmically contracts during sexual stimulation. The Lymphatic System The lymphatic system provides the body with immune defenses and removes foreign substances and cell debris from the blood. Lymphatic fluid, or lymph, moves through a network of vessels. Pressure from blood in neighboring capillaries forces lymph into tissue spaces; it is collected by the vessels and returned to the blood through ducts near the collarbones. The vessels have valves to prevent lymph backflow. Concentrations of lymphocytes for attacking antigens occur in lymph nodes. The spleen, thymus, tonsils, and adenoids are all composed of lymphoid tissue. The spleen is made of sheaths of such tissue, or white pulp, and cords of cells and blood-filled venous sinuses, or red pulp. Lymphocytes are produced in the lymphatic follicles. Arterioles branch from the trabecular arteries to carry blood through these tissues, whence it flows into the trabecular veins. The Urinary System The urinary system rids the body of liquid wastes. filtered from the bloodstream by the kidneys, the wastes are combined there with excess water to form urine. A narrow tube, the ureter, transmits the urine to the bladder, moving the drops along by contractions of its muscular walls. Urine is then passed out of the body through the urethra, which in males also serves as the conduit for the reproductive system, receiving fluids from the seminal vescicles, prostate, and Cowper's gland, plus sperm from the testicles. The urinary and reproductive systems in females are separate; the bladder is located in front of the uterus, and the urethra ends in a small opening lying near the entrance to the vagina. In both sexes the outflow of urine is controlled by two sets of sphincter muscles. Bibliography: Lindqvist, Kjell, and Nordfeldt, Stig, The Body Victorious (1987); Mader, Sylvia, Human Biology (1987); Martins, Frederic, Fundamentals of Human Anatomy (1989); Memmler, Ruth and Weed, Dena, The Human Body in Health and Disease, 6th ed. (1986); Powis, R. L., The Human Body and Why It Works (1985); Rose, Kenneth, The Body in Time (1987); Winter, H. F., and Shourd, M. L., Review of Human Physiology, 3d ed. (1987).