well-field system

Communal land organization supposedly in effect early in the Zhou dynasty (1046–256 BC). It is mentioned in the writings of Mencius, who advocated it. In the well-field system, one unit of land was divided among eight peasant families. A shared field was surrounded by eight fields, each worked by an individual family. The field in the centre was worked jointly by the families for their lord. Later reformers referred to the concept to justify their land-redistribution systems or to criticize government land practices.

Learn more about well-field system with a free trial on

Any integrated system for the control and operation of a specific type of weaponry. Weapons are usually divided into two categories, strategic and tactical. Strategic weapons strike at the seat of an enemy's military, economic, and political power, targeting cities, factories, military bases, transportation and communications networks, and seats of government. Most nuclear weapons are part of strategic weapons systems. Tactical weapons are designed instead for offensive or defensive use at relatively short range—for example, guided missiles intended as antiaircraft and antitank weapons, or other weapons used in aerial and naval combat.

Learn more about weapons system with a free trial on

Facilities for the collection, treatment, storage, and distribution of water. Ancient systems included wells, storage reservoirs, canals and aqueducts, and water-distribution systems. Highly advanced systems appeared circa 2500 BC and reached their peak in the Roman aqueduct system. In the Middle Ages, water supplies were largely neglected and epidemics caused by waterborne organisms were common. In the 17th–18th century, distribution systems utilizing cast-iron pipes, aqueducts, and pumps began to be installed. The link between polluted water and disease came to be understood in the 19th century, and treatment methods such as slow sand filtration and disinfection with chlorine were introduced. Modern reservoirs are formed usually by constructing dams near the collection point of mountain-water runoff or across rivers. After the water reaches collection points, it is treated to improve its quality; it is then pumped either directly into a city or town's distribution system or to an elevated storage location, such as a water tank. Seealso plumbing.

Learn more about water-supply system with a free trial on

Vessel that carries blood to the heart. Except for the pulmonary veins, veins bear deoxygenated blood from capillaries, which converge into threadlike venules and then veins, finally emptying into the venae cavae (see cardiovascular system; vena cava). Blood moves through veins by contraction of the surrounding muscles. Backflow is prevented by valves in most veins' inner layer (tunica intima), which lacks the elastic membrane lining of arteries. The thin middle layer (tunica media) is mostly collagen fibres, and the thick outer layer (tunica adventitia) is mostly connective tissue. Seealso circulation; varicose vein.

Learn more about vein with a free trial on

or varix

Twisted vein distended with blood. Varix also covers arteries and lymphatic vessels (see lymphatic system). Varicose veins occur mostly in the legs, when malfunctioning valves let blood pool in veins near the skin. Causes include hereditary valve and vein wall weakness and internal or external pressure on veins. Varices are common in pregnancy, suggesting that hormone abnormalities play a role. Symptoms include a heavy feeling, with leg cramps and swelling after standing a long time. Complications include skin ulcers and thrombosis. Treatment involves strong support hose, injection therapy, or surgery. Varices in the esophagus, which often occur in liver disease, can ulcerate and bleed. Seealso hemorrhoid.

Learn more about varicose vein with a free trial on

Pathways of the autonomic nervous system. Nerve impulses begin in motor neurons in the brain or elipsis

Part of the nervous system that is not under conscious control and that regulates the internal organs. It includes the sympathetic, parasympathetic, and enteric nervous systems. The first, which connects the internal organs to the brain via spinal nerves, responds to stress by increasing heart rate and blood flow to the muscles and decreasing blood flow to the skin. The second comprises the cranial nerves and the lower spinal nerves, which increase digestive secretions and slow the heartbeat. Both have sensory fibres that send feedback on the condition of internal organs to the central nervous system, information that helps maintain homeostasis. The third division, embedded in the walls of the stomach and intestines, controls digestive movement and secretions.

Learn more about autonomic nervous system with a free trial on

or renal system

System that produces and discharges urine to rid the body of waste products. It consists of the kidneys, which balance electrolytes in blood, retaining and adding needed ones and removing unneeded or dangerous ones for excretion; the ureters, two thin muscular tubes 10–12 in. (25–30 cm) long that move the urine by peristalsis; the hollow, muscular bladder, which receives and stores it; and the urethra, through which it leaves the body. In women the urethra is 1.5 in. (4 cm) long. In men it is longer (since it passes through the penis), about 8 in. (20 cm), and carries semen from the prostate gland as well as urine. Urinary disorders, which can lead to dehydration or edema and to a dangerous buildup of waste and toxic substances, include kidney failure, tumours, and bladder and kidney stones.

Learn more about urinary system with a free trial on

or patronage system

In U.S. politics, the practice by political parties of rewarding partisans and workers after winning an election. Proponents claim it helps maintain an active party organization by offering supporters jobs and contracts. Critics charge that it awards appointments to the unqualified and is inefficient because even jobs unrelated to public policy change hands after an election. In the U.S., the Pendleton Civil Service Act (1883) was the first step in introducing the merit system in the hiring of government workers. The merit system has almost completely replaced the spoils system. Seealso civil service.

Learn more about spoils system with a free trial on

The Sun, its eight major planets, the dwarf planets and small bodies, and interplanetary dust and gas under the Sun's gravitational control. Another component of the solar system is the solar wind. The Sun contains more than 99percnt of the mass of the solar system; most of the rest is distributed among the planets, with Jupiter containing about 70percnt. According to the prevailing theory, the solar system originated from the solar nebula. Seealso asteroid; Centaur object; Ceres; comet; Earth; Eris; Jupiter; Kuiper belt; Mars; Mercury; meteorite; Neptune; Oort cloud; Pluto; Saturn; Uranus; Venus.

Learn more about solar system with a free trial on

Collection of pipes and mains, treatment works, and discharge lines (sewers) for the wastewater of a community. Early civilizations often built drainage systems in urban areas to handle storm runoff. The Romans constructed elaborate systems that also drained wastewater from the public baths. In the Middle Ages these systems fell into disrepair. As the populations of cities grew, disastrous epidemics of cholera and typhoid fever broke out, the result of ineffective separation of sewage and drinking water. In the mid-19th century the first steps were taken to treat wastewater. The concentration of population and the addition to sewage of manufacturing waste that occurred during the Industrial Revolution increased the need for effective sewage treatment. Sewer pipe is laid following street patterns, and access holes with metal covers allow periodic inspection and cleaning. Catch basins at street corners and along street gutters collect surface runoff of storm water and direct it to the storm sewers. Civil engineers determine the volume of sewage likely, the route of the system, and the slope of the pipe to ensure an even flow by gravity that will not leave solids behind. In flat regions, pumping stations are sometimes needed. Modern sewage systems include domestic and industrial sewers and storm sewers. Sewage treatment plants remove organic matter from waste water through a series of steps. As sewage enters the plant, large objects (such as wood and gravel) are screened out; grit and sand are then removed by settling or screening with finer mesh. The remaining sewage passes into primary sedimentation tanks where suspended solids (sludge) settle out. The remaining sewage is aerated and mixed with microorganisms to decompose organic matter. A secondary sedimentation tank allows any remaining solids to settle out; the remaining liquid effluent is discharged into a body of water. Sludge from the sedimentation tanks may be disposed of in landfills, dumped at sea, used as fertilizer, or decomposed further in heated tanks (digestion tanks) to produce methane gas to power the treatment plant.

Learn more about sewage system with a free trial on

or sensory reception or sense perception

Mechanism by which information is received about one's external or internal environment. Stimuli received by nerves, in some cases through specialized organs with receptor cells sensitive to one type of stimulus, are converted into impulses that travel to specialized areas of the brain, where they are analyzed. In addition to the “five senses”—sight, hearing, smell, taste, and touch—humans have senses of motion (kinesthetic sense), heat, cold, pressure, pain, and balance. Temperature, pressure, and pain are cutaneous (skin) senses; different points on the skin are particularly sensitive to each. Seealso chemoreception, ear, eye, inner ear, mechanoreception, nose, photoreception, proprioception, taste, thermoreception, tongue.

Learn more about sense with a free trial on

or seignorialism

Political, economic, and social system by which the peasants of medieval Europe were tied to their land and their lord through serfdom. The basic unit was the manor, a self-sufficient landed estate, or fief, under the control of a lord. Free tenants paid rent or provided military service in exchange for the use of the land. Peasants farmed small plots of land and owed rent and labor to their lord, and most were not free to leave the estate. The manorial system was flourishing in Western Europe by the 8th century and had begun to decline by the 13th century, while in Eastern Europe it achieved its greatest strength after the 15th century.

Learn more about manorialism with a free trial on

Structure of a root. The apical meristem is an area of actively dividing cells that forms all the elipsis

In botany, the underground anchoring part of a plant. It grows downward in response to gravity, absorbs water and dissolved minerals, and stores reserve food. Primary root systems have a deep sturdy taproot (in gymnosperms and dicots; see cotyledon) plus secondary or lateral smaller roots, and root hairs. Grasses and other monocots produce a shallow diffuse mass of fibrous secondary roots. Additional support (e.g., in corn and orchids) comes from stem offshoots called adventitious, or prop, roots. Fleshy roots that store food may be modified taproots (e.g., carrots, turnips, and beets) or modified adventitious roots (e.g. cassava). Tubers such as the potato are modified, fleshy, underground stems, or rhizomes. Aerial roots arise from the stem and either pass for some distance through the air before reaching the soil or remain hanging in the air.

Learn more about root with a free trial on

(born Feb. 15, 1845, Clinton, N.Y., U.S.—died Feb. 7, 1937, New York, N.Y.) U.S. lawyer and diplomat. He became a U.S. attorney for the southern district of New York in 1883. He served as secretary of war from 1899 to 1904. After the Spanish-American War, he set up a civil government in Puerto Rico and organized U.S. control of the Philippines. As secretary of state (1905–09) under Theodore Roosevelt, he concluded treaties with Japan and persuaded Latin American states to participate in the second Hague conference in 1907 (see Hague Conventions). He was awarded the Nobel Prize for Peace in 1912. He served in the U.S. Senate from 1909 to 1915. A supporter of the League of Nations, he helped frame the statute that established the International Court of Justice.

Learn more about Root, Elihu with a free trial on

(born Feb. 15, 1845, Clinton, N.Y., U.S.—died Feb. 7, 1937, New York, N.Y.) U.S. lawyer and diplomat. He became a U.S. attorney for the southern district of New York in 1883. He served as secretary of war from 1899 to 1904. After the Spanish-American War, he set up a civil government in Puerto Rico and organized U.S. control of the Philippines. As secretary of state (1905–09) under Theodore Roosevelt, he concluded treaties with Japan and persuaded Latin American states to participate in the second Hague conference in 1907 (see Hague Conventions). He was awarded the Nobel Prize for Peace in 1912. He served in the U.S. Senate from 1909 to 1915. A supporter of the League of Nations, he helped frame the statute that established the International Court of Justice.

Learn more about Root, Elihu with a free trial on

or macrophage system or mononuclear phagocyte system

Part of the body's defenses, consisting of a class of cells widely distributed in the body. Reticuloendothelial cells filter out and destroy bacteria, viruses, and foreign substances and destroy worn-out or abnormal cells and tissues. Precursor cells in bone marrow develop into monocytes (see leukocyte), which are released into the bloodstream. Most enter body tissues, developing into much larger cells called macrophages, with different appearances in various locations. Some roam through the circulation and between cells and can coalesce into a single cell around a foreign object to engulf it. Reticuloendothelial cells also interact with lymphocytes in immune reactions. Cells in the spleen destroy old red blood cells and recycle their hemoglobin; uncontrolled, this process causes anemia. Tumours of the reticuloendothelial system can be localized or widespread throughout the body. Seealso lymphatic system.

Learn more about reticuloendothelial system with a free trial on

As air enters the nasal cavity through the nostrils, it is warmed and moistened by mucous membranes elipsis

Organ system involved in respiration. In humans, the diaphragm and, to a lesser extent, the muscles between the ribs generate a pumping action, moving air in and out of the lungs through a system of pipes (conducting airways), divided into upper and lower airway systems. The upper airway system comprises the nasal cavity (see nose), sinuses, and pharynx; the lower airway system consists of the larynx, trachea, bronchi, bronchioles, and alveolar ducts (see pulmonary alveolus). The blood and cardiovascular system can be considered elements of a working respiratory system. Seealso thoracic cavity.

Learn more about respiratory system with a free trial on

Organs of the human reproductive system. In a male, the scrotum, a pouch of skin, is divided into elipsis

Organ system by which humans reproduce. In females, the ovaries sit near the openings of the fallopian tubes, which carry eggs from the ovaries to the uterus. The cervix extends from the lower end of the uterus into the vagina, whose opening, as well as that of the urethra (see urinary system), is covered by four folds of skin (the labia); the clitoris, a small erectile organ, is located where the labia join in front. The activity of the ovaries and uterus goes through a monthly cycle of changes (see menstruation) throughout the reproductive years except during pregnancy and nursing. In males, the testes lie in a sac of skin (the scrotum). A long duct (the vas deferens) leads from each testis and carries sperm to the ejaculatory ducts in the prostate gland; these join the urethra, which continues through the penis. In the urethra, sperm mixes with secretions from the seminal vesicles, prostate gland, and Cowper gland to form semen. In early embryos, the reproductive systems are undetermined. By birth the organs appropriate to each sex have typically developed but are not functioning. They continue to grow, and at puberty their activity increases and maturation occurs, enabling sexual reproduction.

Learn more about reproductive system, human with a free trial on

or putting-out system

Production system widespread in 17th-century Europe in which merchant-employers “put out” materials to rural home workers, who then returned finished products to the employers for payment. The domestic system differed from the handicraft system of home production in that the workers neither bought materials nor sold products. It undermined the urban guilds and brought the first widespread industrial employment of women and children. The system was generally superseded by employment in factories but was retained in the 20th century in some industries, notably watchmaking in Switzerland, toy manufacturing in Germany, and many industries in India and China.

Learn more about domestic system with a free trial on

System that allows persons to send letters, parcels, or packages to addressees in the same country or abroad. Postal systems are usually government-run and paid for by a combination of user charges and government subsidies. There are early references to postal services in Egypt circa 2000 BC and in Zhou-dynasty China circa 1000 BC. The Roman Empire developed various centralized methods of relaying messages. In the Middle Ages there were no centralized postal services. Private postal systems developed with the rise of nation-states during the Renaissance; subsequently, postal services became government monopolies. Charges based on weight rather than distance and the use of prepaid stamps were first proposed in 1837. The General Postal Union (1875; later Universal Postal Union) improved international mail delivery by allowing member countries to retain the postage they collected on outgoing international mail and requiring them to treat incoming international mail as they did domestic mail. Airmail and automated mail handling were developed in the 20th century. Seealso e-mail.

Learn more about postal system with a free trial on

In building construction, a system in which two upright members, the posts, hold up a third member, the beam, laid horizontally across their top surfaces. In Britain it is called post-and-lintel system, but in the U.S. “lintel” is usually reserved for a short beam that spans a window or door opening. The post and beam formed the basis of architecture from prehistoric to Roman times, and is illustrated by such ancient structures as Stonehenge. All structural openings evolved from this system, which is seen in pure form only in colonnades and in framed structures, the posts of doors, windows, ceilings, and roofs usually being hidden in walls. The beam must bear loads that rest on it as well as its own load without deforming or breaking. Post-and-beam construction has largely been supplanted by the modern steel frame.

Learn more about post-and-beam system with a free trial on

Electoral process in which the candidate who polls more votes than any other candidate is elected. It is distinguished from the majority system, in which, to win, a candidate must receive more votes than all other candidates combined. It is the most common method of selecting candidates for public office. Its chief advantage is that it avoids the need for runoffs to produce a winner. Its chief disadvantage is that it may result in a winner who has received a minority of the votes cast. It operates best in a two-party system, where the small vote for any third party will rarely result in an outcome seriously at odds with the voters' will.

Learn more about plurality system with a free trial on

Group of persons organized to acquire and exercise political power. Formal political parties originated in their modern form in Europe and the U.S. in the 19th century. Whereas mass-based parties appeal for support to the whole electorate, cadre parties aim at attracting only an active elite; most parties have features of both types. All parties develop a political program that defines their ideology and sets out the agenda they would pursue should they win elective office or gain power through extraparliamentary means. Most countries have single-party, two-party, or multiparty systems (see party system). In the U.S., party candidates are usually selected through primary elections at the state level.

Learn more about political party with a free trial on

Software that controls the operation of a computer, directs the input and output of data, keeps track of files, and controls the processing of computer programs. Its roles include managing the functioning of the computer hardware, running the applications programs, serving as an interface between the computer and the user, and allocating computer resources to various functions. When several jobs reside in the computer simultaneously and share resources (multitasking), the OS allocates fixed amounts of CPU time and memory in turn or allows one job to read data while another writes to a printer and still another performs computations. Through a process called time-sharing, a large computer can handle interaction with hundreds of users simultaneously, giving each the perception of being the sole user. Modern computer operating systems are becoming increasingly machine-independent, capable of running on any hardware platform; a widely used platform-independent operating system in use today on mainframe computers is UNIX. Most personal computers run on Microsoft's Windows operating system, which grew out of and eventually replaced MS-DOS. Seealso Linux.

Learn more about operating system (OS) with a free trial on

System of specialized cells (neurons, or nerve cells) that conduct stimuli from a sensory receptor through a neuron network to the site (e.g., a gland or muscle) where the response occurs. In humans, it consists of the central and peripheral nervous systems, the former consisting of the brain and spinal cord and the latter of the nerves, which carry impulses to and from the central nervous system. The cranial nerves handle head and neck sensory and motor activities, except the vagus nerve, which conducts signals to visceral organs. Each spinal nerve is attached to the spinal cord by a sensory and a motor root. These exit between the vertebrae and merge to form a large mixed nerve, which branches to supply a defined area of the body. Disorders include amyotrophic lateral sclerosis, chorea, epilepsy, myasthenia gravis, neural tube defect, parkinsonism, and poliomyelitis. Effects of disorders range from transient tics and minor personality changes to major personality disruptions, seizures, paralysis, and death.

Learn more about nervous system with a free trial on

Major muscles of the human body. (1) frontalis, (2) occipitalis, (3) temporalis, (4) orbicularis of elipsis

Contractile tissue that produces motion for functions, including body movements, digestion, focusing, circulation, and body warmth. It can be classified as striated, cardiac, and smooth or as phasic and tonic (responding quickly or gradually to stimulation, respectively). Striated muscle, whose fibres appear striped under a microscope, is responsible for voluntary movement. Most of these muscles are phasic. They are attached to the skeleton and move the body by contracting in response to signals from the central nervous system; contraction is achieved by the sliding of thin filaments (of actin) between thick ones (of myosin); stretch receptors in the tissue provide feedback, allowing smooth motion and fine motor control. The branched fibres of cardiac muscle give it a netlike structure; contraction originates in the heart's muscle tissue itself with a signal from the natural pacemaker; vagus and sympathetic nerves control heart rate. Smooth muscle, the muscle of internal organs and blood vessels, is generally involuntary and tonic; its cells can operate either collectively or individually (in response to separate nerve endings) and have different shapes. Disorders of voluntary muscle cause weakening, atrophy, pain, and twitching. Some systemic diseases (e.g., dermatomyositis, polymyositis) can cause muscle inflammation. Seealso abdominal muscle; muscle tumour; muscular dystrophy; myasthenia gravis.

Learn more about muscle with a free trial on

Any of the muscles of the front and side walls of the abdominal cavity. Three flat layers—the external oblique, internal oblique, and transverse abdominis muscles—extend from each side of the spine between the lower ribs and the hipbone. The abdominal muscles attach to aponeuroses, connective tissue sheaths that merge toward the midline, sheathing the rectus abdominis muscle on each side of the midline. The abdominal muscles support and protect the internal organs and take part in exhaling, coughing, urinating, defecating, childbirth, and motion of the trunk, groin, and lower limbs.

Learn more about abdominal muscle with a free trial on

Former rapids, Tennessee River, northwestern Alabama, U.S. At about 37 mi (60 km) long, it was a navigation hazard but is now submerged under at least 9 ft (3 m) of water by the Wilson, Wheeler, and Pickwick Landing dams, which completely eliminated the rapids. Manufacturing plants and hydroelectric power facilities are administered by the Tennessee Valley Authority (TVA). The city of Muscle Shoals (pop., 2000: 11,924) developed from the TVA complex in the Wilson Dam area.

Learn more about Muscle Shoals with a free trial on

Miniature devices formed by combining mechanical parts and electronic circuits, typically on a semiconductor chip, with dimensions from tens to a few hundred micrometres (millionths of a metre). Common applications for MEMS include sensors, actuators, and process-control units.

Learn more about microelectromechanical system (MEMS) with a free trial on

International decimal system of weights and measures, based on the metre (m) for length and the kilogram (kg) for mass, originally adopted in France in 1795. All other metric units were derived from the metre, including the gram (g) for weight (1 cc of water at its maximum density) and the litre (l, or L) for capacity (0.001 cu m). In the 20th century, the metric system became the basis for the International System of Units, which is now used officially almost worldwide.

Learn more about metric system with a free trial on

System of lymph nodes, vessels, and nodules and lymphoid tissue, including the thymus, spleen, tonsils, and bone marrow, through which lymph circulates and is filtered. Its primary function is to return proteins, waste products, and fluids to the blood; molecules too big to enter the capillaries pass through the more permeable walls of lymphatic vessels. Valves keep lymph flowing in one direction, more slowly than blood and at a lower pressure. The lymphatic system also has a role in the immune system. Nodes filter bacteria and foreign matter from lymph. Smaller nodules, which often produce lymphocytes, form in areas more exposed to such materials. They can merge and become permanent, as in the tonsils. Blockage of a lymph vessel may cause fluid to collect in the tissues, producing lymphedema (tissue swelling). Other lymphatic system disorders include lymphocytic leukemias and lymphoma. Seealso reticuloendothelial system.

Learn more about lymphatic system with a free trial on

Branch of government in which judicial power is vested. The principal work of any judiciary is the adjudication of disputes or controversies. Regulations govern what parties are allowed before a judicial assembly, or court, what evidence will be admitted, what trial procedure will be followed, and what types of judgments may be rendered. Typically present in court are the presiding judge, the parties to the matter (sometimes called litigants), the lawyers representing the parties, and other individuals including witnesses, clerks, bailiffs, and jurors when the proceeding involves a jury. Though the courts' stated function is to administer justice according to rules enacted by the legislative branch, courts also unavoidably make law. In deciding, for example, how legislative provisions are to be applied to specific cases, the courts in effect make law by laying down rules for future cases; this is known as the doctrine of precedent. In some jurisdictions, courts have the power of judicial review, enabling them to declare unconstitutional legislation or acts of the executive.

Learn more about judiciary with a free trial on

Form of land tenancy introduced in India by the early sultans of Delhi in the early 13th century. Under the system, land, its revenues, and the power to govern it was assigned to an official of the state. The land reverted to the government on the official's death, but heirs could renew the land assignment by paying a fee. Feudalistic in character, the jagirdar system tended to enfeeble the central government by setting up quasi-independent baronies. Periodically abolished, it was always renewed. After Indian independence, measures were taken to abolish absentee landownership.

Learn more about jagirdar system with a free trial on

Covering of the body, which protects it from the outside world and from drying out. In humans and other mammals it consists of the skin (including outer epidermis and inner dermis) and its related structures, including hair, nails, and sebaceous and sweat glands.

Learn more about integument with a free trial on

An electronic system that continuously monitors the position, velocity, and acceleration of a vehicle, usually a submarine, missile, or airplane, and thus provides navigational data or control. The basic components of an inertial guidance system are gyroscopes, accelerometers, and a computer.

Learn more about inertial guidance system with a free trial on

Cells, cell products, organs, and structures of the body involved in the detection and destruction of foreign invaders, such as bacteria, viruses, and cancer cells. Immunity is based on the system's ability to launch a defense against such invaders. For the system to function properly, it must be able to distinguish between the material of its own body (self) and material that originates outside of it (nonself). Failure to make this distinction can result in autoimmune diseases. An exaggerated or inappropriate response by the immune system to nonharmful substances (e.g., pollen, animal dander) can result in allergies. The system's principal cells include lymphocytes that recognize antigens and related accessory cells (such as phagocytic macrophages, which engulf and destroy foreign material). Lymphocytes arise in the bone marrow from stem cells, with T lymphocytes (T cells) migrating to the thymus to mature and B lymphocytes (B cells) maturing in the bone marrow. Mature lymphocytes enter the bloodstream, and many become lodged, along with accessory cells, in various body tissues, including the spleen, lymph nodes, tonsils, and intestinal lining. Organs or tissues containing such concentrations are termed lymphoid. Within these organs and tissues the lymphocytes are confined within a delicate network of connective tissue that channels them so they come into contact with antigens. T cells and B cells can mature and multiply further in lymphoid tissue when suitably stimulated. Fluid (lymph) draining from lymphoid tissues is conveyed to the blood through lymphatic vessels. Lymph nodes distributed along these vessels filter the lymph, exposing macrophages and lymphocytes contained within to any antigen present. The spleen plays a similar role, sampling the blood for the presence of antigens. The capability of lymphocytes to pass between lymphoid tissue, the blood, and lymph is an important element in the system's functioning. Seealso immunodeficiency; immunology.

Learn more about immune system with a free trial on

In a gasoline engine, the means used for producing an electric spark to ignite the fuel-air mixture in the cylinders to produce the motive force. The ignition system consists of a storage battery recharged by a generator, an induction coil, a device to produce timed high-voltage discharges from the induction coil, a distributor, and a set of spark plugs. The battery provides an electric current of low voltage, usually 12 volts, that is converted by the system to some 40,000 volts. The distributor routes the successive bursts of high-voltage current to each spark plug in the proper firing order.

Learn more about ignition system with a free trial on

Major glands of the human endocrine system. The hypothalamus stimulates the pituitary gland and elipsis

Group of ductless glands that secrete hormones necessary for normal growth and development, reproduction, and homeostasis. The major endocrine glands are the hypothalamus, pituitary, pineal, thyroid, parathyroids, adrenals, islets of Langerhans in the pancreas, ovaries, and testes. Secretion is regulated either by regulators in a gland that detect high or low levels of a chemical and inhibit or stimulate secretion or by a complex mechanism involving the hypothalamus and the pituitary. Tumours that produce hormones can throw off this balance. Diseases of the endocrine system result from over- or underproduction of a hormone or an abnormal response to a hormone.

Learn more about endocrine system with a free trial on

Computerized system that relates and displays data collected from a geographic entity in the form of a map. The ability of GIS to overlay existing data with new information and display it in colour on a computer screen is used primarily to conduct analyses and make decisions related to geology, ecology, land use, demographics, transportation, and other domains, most of which relate to the human use of the physical environment. Through the process of geocoding, geographic data from a database is converted into images in the form of maps.

Learn more about geographic information system (GIS) with a free trial on

In logic, a formal language together with a deductive apparatus by which some well-formed formulas can be derived from others. Each formal system has a formal language composed of primitive symbols that figure in certain rules of formation (statements concerning the expressions allowable in the system) and a set of theorems developed by inference from a set of axioms. In an axiomatic system, the primitive symbols are undefined and all other symbols are defined in terms of them. In Euclidean geometry, for example, such concepts as “point,” “line,” and “lies on” are usually posited as primitive terms. From the primitive symbols, certain formulas are defined as well formed, some of which are listed as axioms; and rules are stated for inferring one formula as a conclusion from one or more other formulas taken as premises. A theorem within such a system is a formula capable of proof through a finite sequence of well-formed formulas, each of which either is an axiom or is validly inferred from earlier formulas.

Learn more about formal system with a free trial on

Term that emerged in the 17th century that has been used to describe economic, legal, political, social, and economic relationships in the European Middle Ages. Derived from the Latin word feudum (fief) but unknown to people of the Middle Ages, the term “feudalism” has been used most broadly to refer to medieval society as a whole, and in this way may be understood as a socio-economic system that is often called manorialism. It has been used most narrowly to describe relations between lords and vassals that involve the exchange of land for military service. Feudalism in this sense is thought to have emerged in a time of political disorder in the 11th century as a means to restore order, and it was later a key element in the establishment of strong monarchies. “Feudalism” also has been applied, often inappropriately, to non-Western societies where institutions similar to those of medieval Europe are thought to have existed. The many ways “feudalism” has been used have drained it of specific meaning, however, and caused some scholars to reject it as a useful concept for understanding medieval society.

Learn more about feudalism with a free trial on

Structure in which work is organized to meet the need for production on a large scale usually with power-driven machinery. In the 17th–18th century, the domestic system of work in Europe began giving way to larger units of production, and capital became available for investment in industrial enterprises. The movement of population from country to city also contributed to change in work methods. Mass production, which transformed the organization of work, came about by the development of the machine-tool industry. With precision equipment, large numbers of identical parts could be produced at low cost and with a small workforce. The assembly line was first widely used in the U.S. meat-packing industry; Henry Ford designed an automobile assembly line in 1913. By mid-1914, chassis assembly time had fallen from 1212 man-hours to 93 man-minutes. Some countries, particularly in Asia and South America, began industrializing in the 1970s and later. Seealso American System of manufacture.

Learn more about factory with a free trial on

Computer-based system designed to respond like a human expert in a given field. Expert systems are built on knowledge gathered from human experts, analogous to a database but containing rules that may be applied to solving a specific problem. An interface allows the user to specify symptoms and to clarify a problem by responding to questions posed by the system. Software tools exist to help designers build a special-purpose expert system with minimal effort. An outgrowth of work in artificial intelligence, expert systems show promise for an ever-widening range of applications. There are now widely used expert systems in the fields of medicine, personnel screening, and education.

Learn more about expert system with a free trial on

Chinese land-distribution system, AD 485–8th century. Borrowed by Japan in 646, it lasted about a century there. Under the system, all adults were assigned a fixed amount of land; a portion of its produce was paid as taxes. On a person's death, most of the land was returned to the government. Increases in population and a tendency for the land to come to be held permanently led to the system's collapse in China; tax-free status and additional allotments for nobles and monasteries resulted in its demise in Japan.

Learn more about equal-field system with a free trial on

Electoral device for choosing a party's candidates for public office. The formal primary system is peculiar to the U.S., where it came into widespread use in the early 20th century. Most U.S. states use it for elections to statewide offices and to the national presidency; in presidential elections, delegates are selected to attend a national convention, where they vote for the candidate to whom they are pledged. A closed-vote primary is restricted to party members; an open-vote primary is open to all voters in the district. Names can be placed on a ballot by an eligible citizen's declaration of candidacy, by nomination at a pre-primary convention, or by a petition signed by a required number of voters. In the late 20th and early 21st centuries political parties in some countries (e.g., the United Kingdom and Israel) adopted similar procedures for the election of the national party leader. Seealso electoral system; party system.

Learn more about primary election with a free trial on

Formal process by which voters make their political choices on public issues or candidates for public office. The use of elections in the modern era dates to the emergence of representative government in Europe and North America since the 17th century. Regular elections serve to hold leaders accountable for their performance and permit an exchange of influence between the governors and the governed. The availability of alternatives is a necessary condition. Votes may be secret or public. Seealso electoral system, party system, plebiscite, primary election, referendum and initiative.

Learn more about election with a free trial on

Set of principles and techniques by which a society decides and organizes the ownership and allocation of economic resources. At one extreme, usually called a free-enterprise system, all resources are privately owned. This system, following Adam Smith, is based on the belief that the common good is maximized when all members of society are allowed to pursue their rational self-interest. At the other extreme, usually called a pure-communist system, all resources are publicly owned. This system, following Karl Marx and Vladimir Ilich Lenin, is based on the belief that public ownership of the means of production and government control of every aspect of the economy are necessary to minimize inequalities of wealth and achieve other agreed-upon social objectives. No nation exemplifies either extreme. As one moves from capitalism through socialism to communism, a greater share of a nation's productive resources is publicly owned and a greater reliance is placed on economic planning. Fascism, more a political than an economic system, is a hybrid; privately owned resources are combined into syndicates and placed at the disposal of a centrally planned state.

Learn more about economic system with a free trial on

Apparatus used to keep the temperature of a structure or device from exceeding limits imposed by needs of safety and efficiency. In a mechanical transmission, the oil loses its lubricating capacity if overheated; in a hydraulic coupling or converter, the fluid leaks under the pressure created. In an electric motor, overheating causes deterioration of the insulation. In an overheated internal-combustion engine, the pistons may seize in the cylinders. The cooling agents customarily employed are air and a liquid (usually water), either alone or in combination. In some cases, direct contact with ambient air (free convection) may be sufficient, as in cooling towers; in other cases, it may be necessary to employ forced convection, created either by a fan or by the natural motion of the hot body. Cooling systems are used in automobiles, industrial plant machinery, nuclear reactors, and many other types of machinery. Seealso air conditioning, heat exchanger.

Learn more about cooling system with a free trial on

Means by which a set of variable quantities is held constant or caused to vary in a prescribed way. Control systems are intimately related to the concept of automation but have an ancient history. Roman engineers maintained water levels in aqueducts by means of floating valves that opened and closed at appropriate levels. James Watt's flyball governor (1769) regulated steam flow to a steam engine to maintain constant engine speed despite a changing load. In World War II, control-system theory was applied to anti-aircraft batteries and fire-control systems. The introduction of analog and digital computers opened the way for much greater complexity in automatic control theory. Seealso Jacquard loom, pneumatic device, servomechanism.

Learn more about control system with a free trial on

Use of a computer-generated system to represent the dynamic responses and behaviour of a real or proposed system. A mathematical description of a system is developed as a computer program that uses equations to represent the functional relationships within the system. When the program is run, the resulting mathematical dynamics form an analog, usually represented graphically, of the behaviour of the modeled system. Variables in the program can be adjusted to simulate varying conditions in the system. Computer simulations are used to study the behaviour of objects or systems that cannot be easily or safely tested in real life, such as weather patterns or a nuclear blast. Simpler simulations performed by personal computers are business models and geometric models. Seealso scientific visualization.

Learn more about simulation, computer with a free trial on

Set of ordered instructions that enable a computer to carry out a specific task. A program is prepared by first formulating the task and then expressing it in an appropriate programming language. Programmers may work in machine language or in assembly languages. But most applications programmers use one of the high-level languages (such as BASIC or C++) or fourth-generation languages that more closely resemble human communication. Other programs then translate the instructions into machine language for the computer to use. Programs are stored on permanent media (such as a hard disk), and loaded into RAM to be executed by the computer's processor, which executes each instruction in the program, one at a time. Programs are often divided into applications and system programs. Applications perform tasks such as word processing, database functions, or accessing the Internet. System programs control the functioning of the computer itself; an operating system is a very large program that controls the operations of the computer, the transfer of files, and the processing of other programs.

Learn more about program, computer with a free trial on

Microcomputer designed for use by one person at a time. A typical PC assemblage comprises a CPU; internal memory consisting of RAM and ROM; data storage devices (including a hard disc, a floppy disc, or CD-ROM); and input/output devices (including a display screen, keyboard, mouse, and printer). The PC industry began in 1977 when Apple Computer, Inc. (now Apple Inc.), introduced the Apple II. Radio Shack and Commodore Business Machines also introduced PCs that year. IBM entered the PC market in 1981. The IBM PC, with increased memory capacity and backed by IBM's large sales organization, quickly became the industry standard. Apple's Macintosh (1984) was particularly useful for desktop publishing. Microsoft Corp. introduced MS Windows (1985), a graphical user interface that gave PCs many of the capabilities of the Macintosh, initially as an overlay of MS-DOS. Windows went on to replace MS-DOS as the dominant operating system for personal computers. Uses of PCs multiplied as the machines became more powerful and application software proliferated. Today, PCs are used for word processing, Internet access, and many other daily tasks.

Learn more about personal computer (PC) with a free trial on

Two or more computers and peripheral equipment (e.g., printers) that are connected with one another for the purpose of exchanging data electronically. Two basic network types are local area networks (LANs) and wide-area networks. Wide-area networks connect computers and smaller networks to larger networks over greater geographical areas, including different continents. Communications may occur over cables, fibre optics, or satellites, but most computer users access the network with a modem, using telephone lines. The largest wide-area network is the Internet. In the 1990s the World Wide Web was introduced and became the most popular way to access other Internet sites.

Learn more about network, computer with a free trial on

Computer capable of solving problems by processing information expressed in discrete form. By manipulating combinations of binary digits (see binary code), it can perform mathematical calculations, organize and analyze data, control industrial and other processes, and simulate dynamic systems such as global weather patterns. Seealso analog computer.

Learn more about digital computer with a free trial on

Use of instructional material presented by a computer. Since the advent of microcomputers in the 1970s, computer use in schools has become widespread, from primary schools through the university level and in some preschool programs. Instructional computers either present information or fill a tutorial role, testing the student for comprehension. By providing one-on-one interaction and producing immediate responses to input answers, computers allow students to demonstrate mastery and learn new material at their own pace. A disadvantage is that computerized instruction cannot extend the lesson beyond the limits of the programming.

Learn more about computer-assisted instruction with a free trial on

Computer program designed to copy itself into other programs, with the intention of causing mischief or damage. A virus will usually execute when it is loaded into a computer's memory. On execution, it instructs its host program to copy the viral code into any number of other programs and files stored in the computer. The corrupted programs may continue to perform their intended functions while also executing the virus's instructions, thus further propagating it. The infection may transfer itself to other computers through storage devices, computer networks, and on-line systems. A harmless virus may simply cause a cryptic message to appear when the computer is turned on; a more damaging virus can destroy valuable data. Antivirus software may be used to detect and remove viruses from a computer, but the software must be updated frequently for protection against new viruses.

Learn more about computer virus with a free trial on

Study of computers, their design (see computer architecture), and their uses for computation, data processing, and systems control, including design and development of computer hardware and software, and programming. The field encompasses theory, mathematical activities such as design and analysis of algorithms, performance studies of systems and their components, and estimation of reliability and availability of systems by probabilistic techniques. Because computer systems are often too large and complicated for failure or success of a design to be predicted without testing, experimentation is built into the development cycle.

Learn more about computer science with a free trial on

Use of computers to produce visual images, or the images so produced. Creating computer graphics requires a digital computer to store and manipulate images, a display screen, input/output devices, and specialized software that enables the computer to draw, colour, and manipulate images held in memory. Common computer graphic formats include GIF and JPEG, for single images, and MPEG and Quicktime, for multiframe images. The field has widespread use in business, scientific research, and entertainment. Monitors attached to CAD/CAM systems have replaced drafting boards. Computer simulation using graphically displayed quantities permits scientific study and testing of such phenomena as nuclear and chemical reactions, gravitational interactions, and physiological systems. Seealso computer animation; computer art.

Learn more about computer graphics with a free trial on

Programmable machine that can store, retrieve, and process data. Today's computers have at least one CPU that performs most calculations and includes a main memory, a control unit, and an arithmetic logic unit. Increasingly, personal computers contain specialized graphic processors, with dedicated memory, for handling the computations needed to display complex graphics, such as for three-dimensional simulations and games. Auxiliary data storage is usually provided by an internal hard disk and may be supplemented by other media such as floppy disks or CD-ROMs. Peripheral equipment includes input devices (e.g., keyboard, mouse) and output devices (e.g., monitor, printer), as well as the circuitry and cabling that connect all the components. Generations of computers are characterized by their technology. First-generation digital computers, developed mostly in the U.S. after World War II, used vacuum tubes and were enormous. The second generation, introduced circa 1960, used transistors and were the first successful commercial computers. Third-generation computers (late 1960s and 1970s) were characterized by miniaturization of components and use of integrated circuits. The microprocessor chip, introduced in 1974, defines fourth-generation computers.

Learn more about computer with a free trial on

Computer in which continuously variable physical quantities, such as electrical potential, fluid pressure, or mechanical motion, are used to represent (analogously) the quantities in the problem to be solved. The analog system is set up according to initial conditions and then allowed to change freely. Answers to the problem are obtained by measuring the variables in the analog model. Analog computers are especially well suited to simulating dynamic systems; such simulations may be conducted in real time or at greatly accelerated rates, allowing experimentation by performing many runs with different variables. They have been widely used in simulating the operation of aircraft, nuclear power plants, and industrial chemical processes. Seealso digital computer.

Learn more about analog computer with a free trial on

In physiology, a complex system of at least 20 proteins (complement components) in normal blood serum. The binding of one component to an antigen-antibody complex begins a chemical chain reaction important in many immunological processes, including breakdown of foreign and infected cells, ingestion of foreign particles and cell debris, and inflammation of surrounding tissue. Complement components and antibodies are the substances in human serum responsible for killing bacteria.

Learn more about complement with a free trial on

In biology, a subset of individuals within a colony of social animals (chiefly ants, bees, termites, and wasps) that has a specialized function and is distinguished from other subsets by morphological and anatomical differences. Typical insect castes are the queen (the female responsible for reproduction), workers (the usually sterile female caretakers of the queen, eggs, and larvae), soldiers (defenders of the colony; also sterile females), and sometimes drones (short-lived males). The differentiation of larvae into various castes is often determined by diet, though hormonal and environmental factors can also play a role.

Learn more about caste with a free trial on

System of vessels that convey blood to and from tissues throughout the body, bringing nutrients and oxygen and removing wastes and carbon dioxide. It is essentially a long, closed tube through which blood moves in a double circuit—one through the lungs (pulmonary circulation) and one through the rest of the body (systemic circulation). The heart pumps blood through the arteries, which branch into smaller arterioles, which feed into microscopic capillaries (see artery; capillary). These converge to form small venules, which join to become larger veins, generally following the same path as the arteries back to the heart. Cardiovascular diseases include atherosclerosis, congenital and rheumatic heart disease, and vascular inflammation.

Learn more about cardiovascular system with a free trial on

System of government in which the legislature comprises two houses. It originated in Britain (see Parliament), where eventually it served to represent the interests of both the common people and the elite and to ensure deliberation over legislation. In the U.S. the bicameral system is a compromise between the claims for equal representation among the states (each state is represented by two members of the Senate) and for equal representation among citizens (each member of the House of Representatives represents roughly the same number of people). Each house has powers not held by the other, and measures need the approval of both houses to become law. Many contemporary federal systems of government have bicameral legislatures. All U.S. states except Nebraska have bicameral legislatures. Seealso Canadian Parliament; Congress of the United States; Diet.

Learn more about bicameral system with a free trial on

In Anglo-American law, the principal method of offering evidence in court. It requires the opposing sides to present pertinent information and to introduce and cross-examine witnesses before a jury and/or a judge. Each side must conduct its own investigation. In criminal proceedings, the prosecution represents the government and has at its disposal the police department with its investigators and laboratories; the defense must arrange and pay for its own investigation. (Legal aid is available for the poor.) In civil (noncriminal) proceedings the adversary system works similarly, except that both sides engage private attorneys to prepare their cases. Skillful questioning often produces testimony that can be interpreted in various ways; in cross-examination, lawyers seek to alter the jury's initial perception of the testimony.

Learn more about adversary procedure with a free trial on

Ancient coastal city, Cyrenaica. Located in modern Libya, it received its name in the 3rd century BC from Ptolemy III, who united Cyrenaica with Egypt. Its economy was based on trade with the interior, and it flourished in Hellenistic times, in the early period of the Roman Empire, and again from the late 3rd century AD, when Diocletian made it the metropolis of the Roman province of Upper Libya.

Learn more about Ptolemais with a free trial on

or Système International d'Unités or SI system

International decimal system of weights and measures derived from and extending the metric system of units. Adopted by the 11th General Conference on Weights and Measures in 1960, it was developed to eliminate overlapping but different systems of units of measures fostered by rapid advances in science and technology in the 19th–20th centuries. Its fundamental units include the metre (m) for length, the kilogram (kg) for mass, and the second (sec) for time. Derived units include those for force (newton, N), energy (joule, J), and power (watt, W).

Learn more about International System of Units with a free trial on

Precise satellite-based navigation and location system originally developed for U.S. military use. GPS is a fleet of more than 24 communications satellites that transmit signals globally around the clock. With a GPS receiver, one can quickly and accurately determine the latitude, the longitude, and in most cases the altitude of a point on or above Earth's surface. A single GPS receiver can find its own position in seconds from GPS satellite signals to an accuracy of one metre; accuracy within one centimetre can be achieved with sophisticated military-specification receivers. This capability has reduced the cost of acquiring spatial data for making maps while increasing cartographic accuracy. Other applications include measuring the movement of polar ice sheets or even finding the best automobile route between given points.

Learn more about Global Positioning System (GPS) with a free trial on

U.S. central bank system consisting of 12 Federal Reserve districts with a Reserve bank in the principal commercial city of each district. The system is supervised by a board of governors in Washington, D.C., as well as by various advisory councils and committees. As a result of the Federal Reserve Act of 1913, all national banks are required to join the system; state banks may join if they meet membership qualifications. The Federal Reserve is responsible for monetary policy. The original act set fixed reserve requirements for the U.S. fractional reserve banking system. It allowed each district bank to determine its discount rate, the rate it charged on loans to member banks. The modern Federal Reserve resulted from the Federal Reserve Act of 1935, which allowed the board to determine reserve requirements within defined limits. It became responsible for approving the discount rates of the district banks. Most importantly, the act created the Federal Reserve Open Market Committee, which is responsible for conducting operations in financial markets that increase or decrease the amount of reserves in the system. If the Federal Reserve wants to ease monetary policy, it will use open market operations and increase the amount of reserves through the purchase of financial assets. Conversely, it can tighten monetary policy through the sale of financial assets.

Learn more about Federal Reserve System with a free trial on

or Copernican principle

Model of the solar system centred on the Sun, with Earth and other planets revolving around it, formulated by Nicolaus Copernicus in the mid 16th century. Having the Sun in this central position explained the apparent motion of planets relative to the fixed stars and was truer than the Earth-centred Ptolemaic system (see Ptolemy). Scientifically, the Copernican system led to belief in a much larger universe than before (because, if the Earth revolved around the Sun, the stars would have to be very distant not to appear to alter their position); more broadly, the Copernican principle is invoked to argue against any theory that would give the solar system a special place in the universe. Dethronement of Earth from the centre of the universe caused profound shock: the Copernican system challenged the entire system of ancient authority and required a complete change in the philosophical conception of the universe.

Learn more about Copernican system with a free trial on

In the Napoleonic Wars, the blockade designed by Napoleon to paralyze Britain through the destruction of British commerce. In the Decrees of Berlin (1806) and Milan (1807), France proclaimed that neutrals and French allies were not to trade with the British. The United Kingdom responded with a counterblockade, which led indirectly to the War of 1812. Because of Britain's naval superiority, the effort to enforce the system proved disastrous for Napoleon.

Learn more about Continental System with a free trial on

See L-system
Search another word or see systemon Dictionary | Thesaurus |Spanish
Copyright © 2015, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature