mechanics Science of the action of forces on material bodies. It forms a central part of all physical science and engineering. Beginning with Newton's laws of motion in the 17th century, the theory has since been modified and expanded by the theories of quantum mechanics and relativ¬ ity. Newton’s theory of mechanics, known as classical mechanics, accu¬ rately represented the effects of forces under all conditions known in his time. It can be divided into statics, the study of equilibrium, and dynamics, the study of motion caused by forces. Though classical mechanics fails on the scale of atoms and molecules, it remains the framework for much of modem science and technology.
mechanism In mechanical construction, the means of transmitting and modifying motion in a machine or an assembly of mechanical parts. The chief characteristic of the mechanism of a machine is that all members have constrained motion; that is, the parts can move only in certain ways in relation to each other. Despite its complexity, the mechanism of a machine can always be analyzed as a group of simple basic mechanisms, each of which contains members that transmit motion from one moving link to another. In general, motion is transmitted in one of three ways: by a wrapping connector such as a chain drive or belt drive, by direct contact as in a cam or gear, or by a pin-connected linkage.
mechanism Form of materialism that holds that all natural processes can be explained in terms of laws of matter in motion. Upholders of mecha¬ nism were mainly concerned with eliminating from science all occult entities, such as substantial form, that could not be empirically observed or mathematically treated. It thus opposed the use of teleological assump¬ tions as explanatory principles in natural science (see teleology). See also ATOMISM.
mechanization Use of machines, either wholly or in part, to replace human or animal labour. Unlike automation, which may not depend at all on a human operator, mechanization requires human participation to pro¬ vide information or instruction. Mechanization began with human- operated machines to replace the handwork of craftspeople; today computers are frequently used to control mechanized processes.
mechanoreception X.me-ko-no-ri-'sep-shonN Ability to detect and respond to mechanical stimuli in one’s environment. A slight deformation of a mechanoreceptive neuron causes an electric charge at its surface, activating a response. Mechanoreceptors in “pain spots” (pressure points) in the skin (probably clusters of nerve endings) vary in sensitivity. They respond to a wide range of stimuli, sometimes with a reflex (e.g., a pricked finger pulled away before the brain registers pain). The structures that respond to sound (see ear), sense orientation with respect to gravity (see inner ear), or detect the position and movement of limbs (see propriocep¬ tion) are mechanoreceptors. Some animals have mechanoreceptors that detect water motion or air currents. See also sense.
Mecherino See Domenico Beccafumi
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Meckel, Johann Friedrich (b. Oct. 17, 1781, Halle, Prussia—d. Oct. 31, 1833, Halle) German anatomist. He was the first to describe the embryonic cartilage (Meckel cartilage) that becomes part of the lower jaw in fishes, amphibians, and birds and also described a congenital pouch (Meckel diverticulum) of the small intestine. He wrote a treatise on patho¬ logical anatomy and an atlas of human abnormalities.
Med fly See Mediterranean fruit fly
medal Piece of metal struck with a design to commemorate a person, place, or event. Medals can be of various sizes and shapes, ranging from large medallions to small plaques, or plaquettes. Most medals are made of gold, silver, bronze, or lead, the precious metals being used for the finer productions. The art of the medalist began in the mid-15th century with bronze medals of Ital¬ ian Renaissance rulers and human¬ ists. Some of the most beautiful were made by Benvenuto Cellini.
Medan \ma-'dan\ City (pop., 1995 est.: 1,843,919), northeastern
Sumatra. After tobacco plantations were introduced in 1873, Medan became the commercial centre of an agricultural region where cash crops, including tobacco and rubber, were raised for export. It was made a city by the Dutch in 1886. It was occu¬ pied by the Japanese during World War II. The sultan of Deli’s palace dates to the 19th century. Medan is the seat of the University of North Sumatra and the Islamic University of North Sumatra.
Medawar Vme-do-worV Sir Peter B(rian) (b. Feb. 28, 1915, Rio de Janeiro, Braz.—d. Oct. 2, 1987, London, Eng.) Brazilian-born British zoologist. Educated at Oxford, he began transplant research in 1949. His finding (1953) that adult animals injected with foreign cells early in life accept skin grafts from the original donor or its twin lent support to Mac- farlane Burnet’s hypothesis that cells learn, during and just after birth, to distinguish “own” from “foreign.” He found that nonidentical cattle twins accept skin grafts from each other, proving that antigens “leak” between the embryos’ yolk sacs, and showed with mice that each cell contains genetic antigens important to immunity. His work deflected immunology from dealing with the fully developed immunity mechanism to attempt¬ ing to alter the mechanism itself (e.g., suppression of transplant rejection). He and Burnet shared a 1960 Nobel Prize.
Medea Nmo-'de-oX In Greek mythology, the daughter of King Aeetes of Colchis. After helping Jason, leader of the Argonauts, to obtain the Golden Fleece from her father, the two were married and she returned with him to Iolcos, where she killed the king who had deprived Jason of his inheritance. Forced into exile, the couple settled in Corinth. In Eurip¬ ides' tragedy Medea, Jason later deserts her for the daughter of King Creon, and Medea takes revenge by killing Creon, his daughter, and her own two children by Jason before fleeing to Athens.
Medecins Sans Frontieres See Doctors Without Borders
Medellin \,ma-tha-'yen\ City (pop., 2003: 1,955,753), northwestern Colombia. It is one of the country’s largest cities and is heavily indus¬ trialized. Founded in 1675 as a mining town, it grew rapidly after the completion of the Panama Canal and the arrival of the railroad in 1914. It is now noted for its textile mills, clothing factories, and steel mills. It is one of Colombia’s largest trading centres for coffee. It also became a centre for the illegal international distribution of narcotics (mainly cocaine) in the late 20th century.
Media Vme-de-oV Ancient country, Middle East. It was situated in present-day northwestern Iran and was home to the Medes, an Iranian people. In 625 bc Cyaxares united the area’s tribes into a kingdom. In 614 bc he captured Ashur and later defeated the Assyrian empire and seized territory in Iran, northern Assyria, and Armenia. In 550 bc it became part of the new Persian Achaemenian dynasty under Cyrus II. Alexander the Great occupied it in 330 bc. In the partition of his empire, southern Media was
given to the Macedonians and then to the Seleucids; northern Media became the kingdom of Atropatene, which passed to Parthia, Armenia, and Rome. In 226 bc the whole of Media passed to the Sasanians, another Persian dynasty.
median See mean, median, and mode
mediation In law, a nonbinding intervention between parties to pro¬ mote resolution of a grievance, reconciliation, settlement, or compromise. It is used especially in labour disputes. In many industrialized countries, the government provides mediation services in order to protect the pub¬ lic interest. In the U.S., the National Mediation Board functions in this capacity. Mediation is also commonly used in international conflicts. See also ARBITRATION.
medical examiner See coroner medical imaging See diagnostic imaging
medical jurisprudence or legal medicine Science of applying medical facts to legal problems. Routine tasks include filling out birth and death certificates, deciding insurance eligibility, and reporting infectious disease. Perhaps more significant is medical testimony in court. When merely relating observations, doctors are ordinary witnesses; interpreting facts based on medical knowledge makes them expert witnesses, required to present their opinions without bias toward the side that called them. Conflicts between medicine and law can occur, usually over medical con¬ fidentiality. See also forensic medicine.