Machine, heart-lung: A machine that does the work both of the heart (pump blood) and the lungs (oxygenate the blood). Used, for example, in open heart surgery. Blood returning to the heart is diverted through the machine before returning it to the arterial circulation. Also called a pump-oxygenator.

Macro-: Prefix from the Greek makros meaning large or long. The opposite of micro-.

Macrobiota: The living organisms (or flora and fauna) of a region that are large enough to be seen with the naked eye. From the Greek macro-, large + bios, life.

Macrobiotic: Macrobiotic refers to the macrobiota, a region’s living organisms (or flora and fauna) large enough to be seen with the naked eye. However, macro- comes from the Greek "makros" meaning not only "large" but also "long". So macrobiotic can also be taken to mean "long life." Thus, the idea with a macrobiotic diet is that it is for a long life, that is that the diet will lengthen life.

Macrocephaly: An abnormally large head.

Macrocytic: Enlarged red blood cells (RBCs). Folic acid deficiency is one cause of macrocytic anemia.

Macroglossia: Enlarged tongue.

Macrophage: Type of white blood that takes in (ingests) foreign material. Macrophages are key players in the immune response to foreign invaders of the body, such as infectious microorganisms.

Macroscopic: Large enough to be seen with naked eye. As opposed to microscopic. A big tumor may well be macroscopic while a tiny tumor is microcopic (cannot be seen without the aid of microscope).

Macrosomia: Overly large body. A child with macrosomia has significant overgrowth.

Macula: A small spot. A macula on the skin is a small flat spot while the macula in the eye is a small spot where vision is keenest in the retina.

Magnesia: Named after a town in presentday Turkey where an ore containing magnesium carbonate was mined. Milk of Magnesia, the laxative, is magnesium hydroxide.

Magnesium: A mineral involved in many processes in the body including nerve signaling, the building of healthy bones, and normal muscle contraction. Magnesium is contained in all unprocessed foods. High concentrations of magnesium are found in nuts, unmilled grains and legumes such as peas and beans. Magnesium deficiency can occur due to inadequate intake or impaired intestinal absorption of magnesium. It is often associated with low calcium (hypocalcemia) and low potassium (hypokalemia). Deficiency of magnesium causes increased irritability of the nervous system with tetany (spasms of the hands and feet, muscular twitching and cramps, spasm of the larynx, etc.). According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water. Persons with impaired kidney function should be especially careful about their magnesium intake because they can accumulate magnesium, a dangerous situation.

Magnesium deficiency: Can occur due to inadequate intake or impaired intestinal absorption of magnesium. Low magnesium (hypomagnesemia) is often associated with low calcium (hypocalcemia) and low potassium (hypokalemia). Deficiency of magnesium causes increased irritability of the nervous system with tetany (spasms of the hands and feet, muscular twitching and cramps, spasm of the larynx, etc.). According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water.

Magnesium excess: Persons with impaired kidney function should be especially careful about their magnesium intake because they can accumulate magnesium, a dangerous situation. According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water.

Magnetic resonance imaging (MRI): A procedure using a magnet linked to a computer to create pictures of areas inside the body.

Maimonides' prayer: A prayer written by the 12th-century physician-philosopher Maimondes, like the famous oath of Hippocrates, is often recited by new medical graduates.

Maintenance therapy: Chemotherapy that is given to leukemia patients in remission to prevent a relapse.

Major histocompatabilty complex (MHC): A cluster of genes on chromosome 6 concerned with antigen production and critical to transplantation. The MHC includes the human leukocyte antigen (HLA) genes.

Malabsorption: Poor intestinal absorption of nutrients.

Malacia: Means softening. Osteomalacia is thus softening of bone (due to deficiency of calcium and vitamin D).

Malady: From the French maladie for illness.

Malaise: A vague feeling of discomfort, one that cannot be pinned down but is often sensed as "just not right." Malaise comes straight from the French who compounded it from "mal" (bad or ill) + "aise" (ease) = ill at ease.

Malar: Referring to the cheek.

Malaria: Infectious disease involving many million of people, caused by the protozoan parasite Plasmodium transmitted by the sting of the Anopheles mosquito or by a contaminated needle or transfusion. The name comes from the Italian mal'aria for bad air; the disease was thought due to bad air wafting from the swamps. Among the many names for malaria: are ague, jungle fever, marsh or swamp fever, and paludism.

Malaria, falciparum: The most dangerous type of malaria. Persons carrying the sickle cell gene have some protection against malaria. Persons with a gene for hemoglobin C (another abnormal hemoglobin like sickle hemoglobin), thalassemia trait or deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD) are thought also to have partial protection against malaria.

Male: The traditional definition of male was "an individual of the sex that produces sperm" (or some such). However, things are not so simple today. Male can be defined by physical appearance, by chromosome constitution (see Male chromosome complement), or by gender identification.

Male chromosome complement: The large majority of males have a 46, XY chromosome complement (46 chromosomes including an X and a Y chromosome). A minority of males have other chromosome constitutions such as 47,XXY (47 chromosomes including two X chromosomes and a Y chromosome) and 47,XYY (47 chromosomes including an X and two Y chromosomes).

Malignancy: A tumor that is malignant.

Malignant: Malignant means to resistant to treatment, or severe (As in "malignant hypertension"). When referring to an abnormal growth it implies a tendency to metastasize. The word malignant comes the Latin combination of mal meaning bad and nascor meaning to be born; malignant literally means born to be bad.

Malignant giant cell tumor: A type of bone tumor.

Malignant melanoma: See melanoma

Malleolus: Bony prominence on either side of the ankle.

Malleus: Tiny bone truly shaped like a minute mallet in the middle ear.

Malrotated ear: An ear that is slanted more than usual. Technically, an ear is slanted when the angle of the slope of the auricle is more than 15 degrees from the perpendicular. Slanted ears are considered a minor anomaly. The presence of 2 or more minor anomalies in a child increases the probability that the child has a major malformation.

Malrotation of the intestine: Failure for the intestine to rotate normally during embryonic development.

Mammary gland: Breast (male or female).

Mammogram: An x-ray of the breast.

Mandible: The mandible is the the bone of the lower jaw. The joint where the mandible meets the upper jaw at the temporal bone is called the temporomandibular joint.

Mania, symptoms: Symptoms of mania include *Inappropriate elation. *Inappropriate irritability. *Severe insomnia. *Grandiose notions. *Increased talking speed and/or volume. *Disconnected and racing thoughts. *Increased sexual desire. *Markedly increased energy. *Poor judgment. *Inappropriate social behavior.

Manic: Refers to a mood disorder in which a person seems "high", euphoric, expansive, sometimes agitated, hyperexcitable, with flights of ideas and speech.

Manic-depression: Alternating moods of abnormal highs (mania) and lows (depression). Called bipolar disease because of the swings between these opposing poles in mood.

Manic-depressive disease: See manic-depression.

Map, contig: A map depicting the relative order of a linked library of small overlapping clones representing a complete chromosome segment.

Map, linkage: A map of the genes on a chromosome based on linkage analysis. A linkage map does not show the physical distances between genes but rather their relative positions, as determined by how often two gene loci are inherited together. The closer two genes are (the more tightly they are linked), the more often they will be inherited together. Linkage distance is measured in centimorgans (cM).

Map, physical: A map of the locations of identifiable landmarks on chromosomes. Physical distance is measured in base pairs. The physical map differs from the genetic map which is based purely on genetic linkage data. In the human genome, the lowest-resolution physical map is the banding patterns of the 24 different chromosomes. The highest-resolution physical map is the complete nucleotide sequence of all chromosomes, a future goal.

Maple syrup urine disease (MSUD): Hereditary disease due to deficiency of an enzyme involved in amino acid metabolism, characterized by urine that smells like maple syrup.

Mapping, gene: Charting the positions of genes on chromosome and learning the distance, in linkage units or physical units, between genes.

Marasmus: Wasting away, as occurs with children who have kwashiorkor. Also called cachexia, is usually a result of protein and calorie deficiency.

Mapping: Charting the location of genes on chromosomes.

Marfan syndrome: Inherited disorder with long fingers and toes, dislocation of the lens, and aortic wall weakness and aneurysm. (It has been suggested that Abraham Lincoln had Marfan syndrome.)

Marker: An identifiable heritable spot on a chromosome. A marker can be an expressed region of DNA (a gene) or a segment of DNA with no known coding function. All that matters is that the marker can be monitored.

Marker chromosome: An abnormal chromosome that is distinctive in appearance but not fully identified. For example, the fragile X chromosome was once called the marker X.

Marriage, cousin: A form of consanguinity. Everyone carries recessive alleles, genes that are generally innocuous in the heterozygous state but that in the company of another gene of the same type are capable of causing disease. We are all genetic reservoirs for genetic disease. Since first cousins share a set of grandparents, for any particular allele (gene) in the father, the chance that the mother inherited the same allele from the same source is 1/8. And for any gene the father passes to his child, the chance is 1/8 that the mother has the same gene and ½ that she transmits it to the child, so 1/8 X ½ = 1/16. A first-cousin marriage therefore has a coefficient of inbreeding of 1/16. The added risks for first cousins depend not only upon this coefficient of inbreeding but also upon their genetic family histories and, in some cases, upon test results (for example, for the risk of beta thalassemia in first cousins of Greek or Italian descent). There are always added risks from the mating of closely related persons.

Marrow: The bone marrow.

Marsh fever: See malaria.

Masochism: Pleasure from one's own pain. Named after the 19th-century Austrian writer Leopold von Sacher-Masoch (masoch-ism).

Masseter: The muscle that raises the lower jaw.

Mast cell: A connective tissue cell whose normal function is unknown, the mast cell is frequently injured during allergic reactions, releasing strong chemicals including histamine into the tissues and blood that are very irritating and cause itching, swelling, and fluid leaking from cells. These allergic chemicals can also cause muscle spasm and lead to lung and throat tightening as is found in asthma and loss of voice.

Mastectomy: Mastectomy is a general term for removal of the breast. A modified radical mastectomy involves removal of the breast and the axillary lymph nodes. A simple mastectomy removes the breast, but not the lymph nodes.

Masticate: To chew.

Mastitis: Inflammation of the breast.

Mastoid: The rounded protrusion of bone just behind the ear once thought to look like the breast. The word comes from the Greek mastos meaning breast + -oid= breast-like.

Mastoiditis: Inflammation of the mastoid, often secondary to ear infection.

Maternal mortality rate: The number of maternal deaths related to childbearing divided by the number of live births (or by the number of live births + fetal deaths) in that year. The maternal mortality rate in the United States in 1993 (and 1994) was 0.1 per 1,000 live births, or 1 mother dying per 10,000 live births.

Maternal serum alpha-fetoprotein: A plasma protein, alpha-fetoprotein (AFP) is normally produced by the fetus. The level of AFP in the blood serum of pregnant women provides a screening test for open neural tube defects (anencephaly and spina bifida) and for Down syndrome (and other chromosome abnormalities). The maternal serum AFP (MSAFP) tends to be unusally high with open neural tube defects and unsually low with Down syndrome.

Matter, gray: The cortex of the brain which contains nerve cell bodies. The gray matter is as opposed to the white matter, the part of the brain that contains myelinated nerve fibers. The gray matter is so named because it in fact appears gray. In "The Mysterious Affair at Styles" (1920), Agatha Christie first quoted the fictional Belgian detective Hercule Poirot in regard to his gray matter: "’This affair must be unravelled from within.’ He tapped his forehead. ‘These little grey cells. It is "up to them"—as you say over here.’"

Matter, white: The part of the brain that contains myelinated nerve fibers. The white matter is white because it is the color of myelin, the insulation covering the nerve fibers. The white matter is as opposed to the gray matter (the cortex of the brain which contains nerve cell bodies).

Maxilla: The maxilla is the major bone of the upper jaw

Measles: Rubeola or the hard (or 10-day) measles. The name measles comes from the Middle English maselen meaning many little spots referring, of course, to the rash.

Measles immunization: The standard MMR vaccine is given to prevent measles, mumps and rubella (German measles). The MMR vaccine is now given in two dosages. The first should be given at12-15 months of age. The second vaccination should be given at 4-6 years (or, alternatively, 11-12 years) of age. Most colleges require proof of a second measles or MMR vaccination prior to entrance. Most children should receive MMR vaccinations. Exceptions may include children born with an inability to fight off infection, some children with cancer, on treatment with radiation or drugs for cancer, on long term steroids (cortisone). People with severe allergic reactions to eggs or the drug neomycin should probably avoid the MMR vaccine. Pregnant women should wait until after delivery before being immunized with MMR. People with HIV or AIDS should normally receive MMR vaccine. Measles, mumps, and rubella vaccines may be administered as individual shots, if necessary, or as a measles-rubella combination.

Measly: The word measly can refer to measles, and, thence, to spotty and, thence, to something that is of little value. In medicine, the measly tapeworn is the pork tapeworm (T. solium) which can be contracted through eating measly pork (pork infected with the larval form of T. solium).

Measly tapeworm: The pork tapeworm, formally known as Taenia solium. Contracted from undercooked or measly pork (pork infected with the larval forms of the tapeworm). Can grow to be 3-6 feet (0.9-1.8 m) long in the human intestine. Also known as the armed tapeworm.

Meatus: A passageway.

Meconium: Dark sticky material normally present in the intestine at birth and passed in the feces after birth. The passage of meconium before birth can be a sign of fetal distress.

Meckel's diverticulum: An out-pouching of the small bowel (ileum). Present in about 2% of people and usually occurs about 2 feet before the junction with the colon. Can be lined by stomach-type mucosa and ulcerate, perforate, or cause small bowel obstruction.

Medial: The side of the body or bdy part that is nearer to the middle or center (median) of the body. For example, when referring to the knee, medial would mean the side of the knee that is closest to the other knee The opposite of medial is lateral.

Median: The middle. Like the median strip in a highway.

Mediastinoscopy: A procedure in which the doctor inserts a tube into the chest to view the organs in the mediastinum. The tube is inserted through an incision above the breastbone.

Mediastinotomy: A procedure in which the doctor inserts a tube into the chest to view the organs in the mediastinum. The tube is inserted through an incision next to the breastbone.

Mediastinum: The area between the lungs. The organs in this area include the heart and its large veins and arteries, the trachea, the esophagus, the bronchi, and lymph nodes.

Medical directives, advance: Advance directives preserve the person’s right to accept or reject a course of medical treatment even after that person becomes mentally or physically incapacitated to the point of being unable to communicate those wishes. There ared two basic types of advance directives: (1) a living will, in which the person outlines specific treatment guidelines that are to be followed by health care providers; (2) a health care proxy (also called a power of attorney for health-care decision-making) in which the person designates a trusted individual to make medical decisions in the event that he or she becomes too incapacitated to make such decisions. Advance directive requirements vary greatly from one jurisdiction to another and should therefore be drawn up in consultation with an attorney who is familiar with the laws of the particular jurisdiction. (This entry is based upon material from the National MS Society).

Medical Research Council (MRC): Key government agency for medical research in the U.K..

Medication, ACE-inhibitor: Agents that inhibit ACE (angiotensin converting enzyme), thereby acting as vasodilators (really as anti-vasoconstrictors), lightening the stress load on the heart.

Medication, anti-coagulant: Blood thinners. Drugs, like heparin and warfarin, used as "blood-thinners" to prevent blood clots and to maintain open blood vessels.

Medication, anti-platelet: Platelet-blocking drugs. Drugs that, like aspirin, reduce the tendency of platelets in the blood to clump and clot.

Medication, beta-blocker: Drugs that antagonize the action of adrenaline (a beta adrenergic substance) and relieve stress to the heart muscle. Beta-blockers are often used to slow the heart rate or lower the blood pressure.

Medication, clot-dissolving: Drugs used to dissolve blood clots. Agents such as plasminogen-activator (t-PA) and streptokinase that are effective in dissolving clots and re-opening arteries. Used, for example, in the treatment of heart attacks. Clot-dissolvers are also called thrombolytic agents.

Medication, vasodilator: Drugs that act as blood vessel dilator (vasodilators) and open vessels by relaxing their muscular walls). For example, nitroglycerin is a vasodilator. So are the ACE (angiotensin converting enzyme) inhibitors.

Mediterranean anemia: Better known today as thalassemia (or as beta thalassemia or thalassemia major) .The clinical picture of this important type of anemia was first described in 1925 by the pediatrician Thomas Benton Cooley. The name thalassemia was coined by the Nobel Prize winning pathologist George Whipple and the professor of pediatrics Wm Bradford at U. of Rochester because thalassa in Greek means the sea (like the Mediterrranean Sea) + -emia means in the blood so thalassemia means sea in the blood. Thalassemia is not just one disease. It is a complex contingent of genetic (inherited) disorders all of which involve underproduction of hemoglobin, the indispensable molecule in red blood cells that carries oxygen. The globin part of normal adult hemoglobin is made up of 2 alpha and 2 beta polypeptide chains. In beta thalassemia, there is a mutation (change) in both beta globin chains leading to underproduction (or absence) of beta chains, underproduction of hemoglobin, and profound anemia. The gene for beta thalassemia is relatively frequent in people of Mediterranean origin (for example, from Italy and Greece). Children with this disease inherit one gene for it from each parent. The parents are carriers (heterozygotes) with just one thalassemia gene, are said to have thalassemia minor, and are essentially normal. Their children affected with beta thalassemia seem entirely normal at birth because at birth we still have predominantly fetal hemoglobin which does not contain beta chains. The anemia surfaces in the first few months after birth and becomes progressively more severe leading to pallor and easy fatiguability, failure to thrive (grow), bouts of fever (due to infections) and diarrhea. Treatment based on blood transfusions is helpful but not curative. Gene therapy will, it is hoped, be applicable to this disease.

Mediterranean Fever: See Familial Mediterranean Fever.

Medulla: The innermost part. The spinal medulla, for example, is that part of the spinal cord which is lodged within the vertebral canal.

Medulloblastoma: A type of brain tumor.

Mega-: Prefix meaning big, abnormally large.

Megacolon: An abnormally enlarged colon.

Megakaryocyte: A giant cell in the bone marrow that is the ancestor of blood platelets.

Meibomian cyst: An inflammation of the oil gland of the eyelid. Also called a chalazian or a tarsal cyst.

Meiosis: What chromosomes do during germ cell formation to halve the chromosome number from 46 to 23.

Meiotic: Pertaining to meiosis.

Meiotic nondisjunction: Failure of two memberrs of a chromosome pair to separate (disjoin) during meiosis so that both go to one daughter cell and none to the other. This mechanism is responsible for the extra chromosome 21 in trisomy 21 (Down syndrome) and for extra and missing chromosomes causing other birth defects and many spontaneous abortions (miscarriages).

Melan-: Prefix meaning dark or black.

Melancholia: Old term for depression.

Melanin: A skin pigment (substance that gives the skin its color). Dark-skinned people have more melanin than light-skinned people.

Melanocytes: Cells in the skin that produce and contain the pigment called melanin.

Melanoma: Cancer of the cells that produce pigment in the skin. Melanoma usually begins in a mole.

Melena: Stools or vomit stained black by blood pigment or dark blood products.

Membrane: A very thin layer of tissue that covers a surface.

Menarche: The time in a girl's life that menstruation first begins. Therefore, the opposite of the menopause.

Mendelian: Referring to the Austrian monk Gregor Mendel (1822-84) who formulated laws forming the foundation of classical genetics.

Meninges: The three membranes that cover the brain and spinal cord.

Meningioma: A type of brain tumor.

Meningocele (MM): : Protrusion of the membranes that cover the spine and part of the spinal cord through a bone defect in the vertebral column. MM is due to failure of closure during embryonic life of bottom end of the neural tube, the structure which gives rise to the central nervous system (the brain and spinal cord). The term spina bifida refers specifically to the bony defect in the vertebral column through which the meningeal membrane and cord may protrude (spina bifida cystica) or may not protrude so that the defect remains hidden, covered by skin (spina bifida occulta). However, through usage the term spina bifida is gradually becoming synonymous with MM. The risk of MM (and all neural tube defects) can be decreased by the mother eating ample folic acid during pregnancy.

Meningomyelocele: Protrusion of the membranes that cover the spine but some of the spinal cord itself through a defect in the bony encasement of the vertebral column. The bony defect is spina bifida.

Menometorrhagia: Excessive uterine bleeding both at the usual time of menstrial periods and at other irregular intervals.

Menopause: The time of a woman's life when menstrual periods permanently stop; also called "change of life." Menopause is the opposite of the menarche.

Menorrhagia: Excessive uterine bleeding at the regular menstrual times lasting longer than usual.

Menstrual cycle: The hormone changes that lead up to a period (menstruation). For most women, one cycle takes up to 28 days.

Menstruation: The periodic blood that flows as a discharge from the uterus. Also called menorrhea, the time during which menstruation occurs is referred to as menses. The menses occurs at approximately 4 week intervals to compose the menstrual cycle.

Mesentery: A fold of tissue which attaches organs to the body wall. Unqualified, usually refers to the small bowel mesentery which anchors the small intestines to the back of the abdominal wall. Blood vessels, nerves, and lymphatics branch through the mesentery to supply the intestine. Other mesenteries exist to support the sigmoid colon, appendix, transverse colon, and portions of the ascending and descending colon.

Messenger RNA (mRNA): An RNA that acts as a messenger, an intermediary, between DNA and protein. The DNA of the gene is transcribed into mRNA which then is translated into the sequence of amino acids that make up protein.

Metabolic rate, basal: A measure of the rate of metabolism. For example, someone with an overly active thyroid will have an elevated basal metabolic rate.

Metabolism: The whole range of biochemical processes that occur within us (or any living organism). Metabolism consists both of anabolism and catabolism (the buildup and breakdown of substances, respectively).

Metacarpals: Five cylindrical bones extending from the wrist to the fingers.

Metacentric: A chromosome with arms of equal length.

Metaphase: Stage in the cell when the chromosomes are most condensed and easiest to study.

Metastasis: The spread of cancer from one part of the body to another. Cells that have metastasized are like those in the original (primary) tumor.

Metastasize: The spread from one part of the body to another. When cancer cells metastasize and cause secondary tumors, the cells in the metastatic tumor are like those in the original cancer.

Metatarsals: Five cylindrical bones extending from the heel to the toes.

Methemoglobin: Hemoglobin in a form incapable of carrying oxygen.

Metorrhagia: Uterine bleeding at irregular intervals.

Meuse fever: The area around the Meuse River was one of the great battlegrounds of World War I during which this louse-borne disease was first recognized in the trenches (called trench fever), again a major problem in the military in World War II, seen endemically in Mexico, N. Africa, E, Europe, and elsewhere. The cause, Rochalimaea quintana, is an unusual rickettsia that multiplies in the gut of the body louse. Transmission to people can occur by rubbing infected louse feces into abraded (scuffed) skin or conjunctiva (whites of the eyes). Onset of symptoms is sudden, with high fever, headache, back and leg pain and a fleeting rash. Recovery takes a month or more. Relapses are common. Also called Wolhynia fever, shin bone fever, quintan fever, five-day fever, His’ disease, His-Werner disease, Werner-His disease.

MHC: See major histocompatability complex.

MI: Stands for myocardial infarction, a heart attack.

Micro-: Prefix from the Greek mikros meaning small. The opposite of macro-.

Microbe: A minute organism including bacteria, fungi, and protozoan parasites best visualized with a microscope.

Microcephaly: An abnormally small head. Often associated with developmental delay and mental retardation.

Microdeletion: Loss of a piece from a chromosome that is too small to be seen through a microscope. Microdeletions require high-resolution chromosome banding, molecular chromosome analysis (with FISH), or DNA analysis for detection. Disorders caused by microdeletions include Angelman, DiGeorge, Prader-Willi, and Williams syndromes.

Microscope: An optical instrument that augments the power of the eye to see small objects. The name microscope was coined by Johannes Faber (1574-1629) who in 1628 borrowed from the Greek to combined micro-, small with skopein, to view. Although the first microscopes were simple microscopes, most (if not all) optical microscopes today are compound microscopes.

Microscope, compound: A microscope that consists of two microscopes in series, the first serving as the ocular lens (close to the eye) and the second serving as the objective lens (close to the object to be viewed). Credit for creating the compound microscope goes usually to the Dutch spectaclemakers Hans and Zacharias Janssen who in 1590 invented an instrument that could be used as either a microscope or telescope. The compound microscope evolved into the dominant type of optical microscope today.

Microscope, electron (EM): A microscope in which an electron beam replaces light to form the image. EM has its pluses (greater magnification and resolution than optical microscopes) and minuses (you are not really "seeing" objects, but rather their electron densities, and artefacts may abound). EM has greatly extended the powers of the microscope, although EM also has its own set of limitations.

Microscope, fluorescent: A microscope equipped to examine material that fluoresces under ultraviolet (UV) light.

Microscope, simple: A microscope that has a single converging lens (or a combination of lenses that function optically as a single converging lens). Anton van Leeuwenhoek (1632-1723) made good use of the simple microscope to look at the life within a drop of water, and such. The magnifying properties of lenses had been well known in ancient times (for example to the Greeks and Romans) but it was not until about 1600 that it became possible to make small lenses with the precision needed to make a microscope.

Microscopic: So small it cannot be seen without the aid of microscope. As opposed to macroscopic (large enough to be seen with naked eye). A tiny tumor is microscopic while a big tumor is macrocopic.

Microsomia: Too small a body. A child with microsomia has significant undergrowth.

Micturition: To urinate.

Midwife: A person who assists a woman during childbirth. Historically, a midwife could be a man or woman and be an obstetrician. Today, a midwife is a nurse-midwife.

Migraine: Periodic attacks of headaches usually on one side of the head that may be accompanied by nausea, vomiting, increased sensitivity of the eyes to light and other symptoms.

Migraine, classic: Migraine with aura. Accounts for no more than most 20% of migraines. See Migraine.

Migraine, common: Migraine without aura. The most frequent type, accounting for about 80-85% of migraines. See Migraine.

Migraine headache: The most common type of vascular headache involving (it is thought). abnormal sensitivity of blood vessels (arteries) in the brain to various triggers resulting in rapid changes in the artery size due to spasm (constriction). Other arteries in the brain and scalp then open (dilate), and throbbing pain is perceived in the head. The tendency to migraine is inherited and appears to involve serotonin, a chemical in the brain involved in the transmission of nerve impulses that trigger the release of substances in the blood vessels that in turn cause the pain of the migraine. These nerve impulses cause the flashing lights and other sensory phenomena known as an aura that may accompany a migraine. Not all severe headaches are migraines and not all migraines are severe.

Milzbrand: Known also as anthrax, milzbrand is a serious bacterial infection. It is not primarily a human disease but rather an infection of animals. Cattle, sheep, horses, mules, and some wild animals are highly susceptible. Humans (and swine) are generally resistant to anthrax. Anthrax can take different forms. With the lung form of the disease. People inhale the anthrax spores and, if untreated, are likely to die. An intestinal form is caused by eating meat contaminated with anthrax. But most human anthrax comes from skin contact with animal products. Cutaneous (skin) anthrax was once well known among people who handled infected animals, like farmers, woolsorters, tanners, brushmakers and carpetmakers in the days when the brushes and carpets were animal products. The hallmark of skin anthrax is a carbuncle, a cluster of boils, that ulcerates in an ugly way. Typically this lesion has a hard black center surrounded by bright red inflammation. This accounts for its name, "anthrax", the Greek word for "coal". "Milzbrand" means "anthrax" in German.

Mineralocorticoids: A group of hormones (the most important being aldosterone) that regulate the balance of water and electrolytes (ions such as sodium and potassium) in the body. The mineralocorticoid hormones act on the kidney (and specifically on the tubules of the kidney).

Monoarticular: Involving just one joint. As opposed to polyarticular (affecting many joints). From the Latin "articulus," meaning a joint.

Minor salivary gland: A small gland which produces saliva. There are numerous minor salivary glands distributed within the mouth and palate.

Miosis: Contraction of the pupil. The opposite of mydriasis.

Miscarriage: Inadvertant loss of a pregnancy before the fetus is viable. A considerable proportion of pregnancies end in miscarriage. Also called a spontaneous abortion.

Miscarriages, multiple, chromosomes in: Couples who have had more than one miscarriage have about a 5% chance that one member of the couple is carrying a chromsome translocation responsible for the miscarriages.

Missense mutation: A genetic change that results in the substitution of one amino acid in protein for another. A missense mutation is responsible for sickle hemoglobin, the molecular basis of sickle cell trait and sickle cell anemia.

Mite-borne typhus: Scrub: typhus, a mite-borne infectious disease caused by a microorganism, Rickettsia tsutsugamushi, characteristically with fever, headache, a raised (macular) rash, swollen glands (lymphadenopathy) and a dark crusted ulcer (called an eschar or tache noire) at the site of the chigger (mite larva) bite. This disease occurs in the area bounded by Japan, India, and Australia. Known also as Tsutsugamushi disease and tropical typhus.

Mitochondria: Normal structures in cells. Mitochondria are located in the cell's cytoplasm outside the nucleus (which is home to the normal human complement of 46 chromosomes). Each mitochondrion has a chromosome made of DNA but, otherwise, the mitochondrial chromosome is quite different in size and shape from other chromosomes: The mitochondrial chromosome is much smaller, it is round (whereas the chromosomes in the nucleus are shaped like rods) and there are many copies of the mitochondrial chromosome per cell. No matter whether we are male or female, we all inherit our mitochondrial chromosome from our mother so we all owe our mitochondrial chromosomes to Eve (rather than to Adam).

Mitochondrial: Referring to mitochondria.

Mitochondrial disease: Mutations (changes) in the mitochondrial chromosome are responsible for a number of disorders including an eye disease (Leber's hereditary optic atrophy), a type of epilepsy (called MERRF which stands for Myoclonus Epilepsy with Ragged Red Fibers), and a cause of dementia (called MELAS for Mitochondrial Encephalopathy, Lactic Acidosis and Stroke-like episodes). All mitochondrial diseases were entirely enigmatic before it was discovered that they were due to mutations not in regular chromosomes but the mitochondrial chromosome..

Mitochondrial genome: All of the DNA in the mitochondrial chromosome.

Mitochondrial inheritance: The inheritance of a trait encoded in the mitochondrial genome. Because of the oddities of mitochondria, mitochondrial inheritance does not obey the classic rules of genetics. Persons with a mitochondrial disease may be male or female but they are always related in the maternal line and no male with the disease can transmit it to his children.

Mitochondrion: Singular of mitochondria. (See mitochondria).

Mitosis: Ordinary division of a body cell to form two daughter cells each with the same chromosome complement as the parent cell.

Mitotic: Pertaining to mitosis.

Mitotic nondisjunction: Failure of the two members of a chromosome pair to separate (disjoin) during mitosis so that both go to one daughter cell and none to the other.

Mitral insufficiency: Malfunction of the mitral valve. Mitral insufficiency allows the backflow of blood (regurgitation) from the left ventricle into the left atrium.

Mitral prolapse: Drooping down or abnormal bulging of the mitral valve cusps during the contraction of the heart.

Mitral regurgitation: Backflow of blood from the left ventricle to the left atrium due to mitral valve insufficiency (malfunction).

Mitral valve: Heart valve with two cusps situated between the left atrium and ventricle. Called mitral because it looks like a bishop's miter or headdress.

Mittelschmerz: Pain in between the menstrual periods. From the German mittel for middle and schmerz for pain.

MM: Meningomyelocele.

MMR: Measles, mumps, rubella vaccine.

Molar: In dentistry, a molar is one of the posterior teeth well adapted to grinding, in keeping with its origin from the Latin mola meaning millstone.

Molars: Molars are the large teeth at the back of the mouth.

Mold: A large group of fungi (like Penicillium) that cause mold (as on bread or cheese). A common trigger for allergies.

Mole: 1. A pigmented spot on the skin (nevus). 2. A mass within the uterus (womb) formed by partly developed products of conception.

Molecule: The smallest unit of a substance that can exist alone and retain the character of that substance.

Molecules, recombinant DNA: A combination of DNA molecules of different origin that are joined using recombinant DNA technology.

Mongolism: See Down syndrome.

Monilia: A yeast-like fungus now called Candida.

Monitor, Holter: A technique for long-term, continuous cardiac surveillance. A cassette tape is worn by the patient continuously while carrying out his/her usual activities. The patient simultaneously keeps a diary of palpitations or other symptoms during the recording period. Symptoms of palpitations can later be correlated with the presence or absence of arrhythmias (abnormal heart rhythms) on the Holter tape. The recordings can be analyzed by a doctor at a later date. Named for the late American biophysicist Norman Holter.

Mono: Popular name for infectious mononucleosis.

Monoclonal: Derived from a single cell and cells identical to that cell.

Monoclonal antibodies: Identical antibodies that are made in large amounts in the laboratory. Doctors are studying ways of using monoclonal antibodies to treat leukemia.

Monocyte: A white blood cell that has a single nucleus and can take in (ingest) foreign material.

Mononucleosis: See infectious mononucleosis.

Monosomy: Missing one chromosome from a pair. A female with 45 chromosomes including just one X chromosome (X monosomy) resulting in Turner syndrome.

Monozygous twins: Identical twins. Called monozygous because they originate from a single fertilized egg (a zygote).

Morbidity: Illness, disease.

Morgue: A place where bodies of the dead are kept before funeral ceremonials. The first Morgue was in Paris. In the 1880s the word morgue entered English to mean a mortuary.

Morphology: Literally, the study of form (structure). It is also the form itself.

Mortality rate, fetal: The ratio of fetal deaths to the sum of the births (the live births + the fetal deaths) in that year. In the United States, the fetal mortality rate plummeted from 19.2 per 1,000 births in 1950 to 9.2 per 1,000 births in 1980.

Mortality rate, infant: The number of children dying under a year of age divided by the number of live births that year. The infant mortality rate in the United States, which was 12.5 per 1,000 live births in 1980, fell to 9.2 per 1,000 live births in 1990.

Mortality rate, maternal: The number of maternal deaths related to childbearing divided by the number of live births (or by the number of live births + fetal deaths) in that year. The maternal mortality rate in the United States in 1993 (and 1994) was 0.1 per 1,000 live births, or 1 mother dying per 10,000 live births.

Mortality rate, neonatal: The number of children dying under 28 days of age divided by the number of live births that year. The neonatal mortality rate in the United States, which was 8.4 per 1,000 live births in 1980, declined to 5.8 per 1,000 live births in 1990.

Mosaic: An individual or tissue containing two or more types of genetically different cells. All females are mosaics because of X-chromosome inactivation (lyonization).

Motion, range of: The range through which a joint can be moved, usually its range of flexion and extension. Due to an injury, the knee may for example lack 10 degrees of full extension.

Motor: Something that produces or refers to motion. For example, a motor neuron is a nerve cell that conveys an impulse to muscle for contraction, which moves a joint.

M.P.H.: Master of Public Health (master’s degree in this area of medicine).

MRC: The Medical Research Council (U.K.).

MRI: A procedure using a magnet linked to a computer to create pictures of areas inside the body. MRI stands for magnetic resonance imaging.

mRNA: See messenger RNA.

MS: Multiple sclerosis.

MSAFP: Abbreviation for maternal serum alpha-fetoprotein.

Mucosa: Having to do with a mucous membrane. For example, the oral mucosa.

Mucoviscidosis: An old name (but one that has prevailed in France and some other nations) for cystic fibrosis (CF), one of the most common and serious of all genetic (inherited) diseases. The CF gene is carried by 1/20 persons (in Caucasian populations) and 1 in 400 couples is at risk for having children with CF. CF is characterized by the production of abnormal secretions leading to mucous build-up. which can impair the pancreas (and, secondarily, the intestine). CF mucous build-up in lungs can impair respiration. Without treatment, CF results in death for 95% of children before age 5. Early diagnosis of CF is of great importance. Early and continuing treatment of CF is valuable.

Mucocutaneous lymph node syndrome: A syndrome of unknown origin, mainly affecting young children, causing fever, reddening of the eyes (conjunctivitis), lips and mucous membranes of the mouth, ulcerative gum disease (gingivitis), swollen glands in the neck (cervical lymphadenopathy), and a rash that is raised and bright red (maculoerythematous) in a glove-and-sock fashion over the skin of the hands and feet which becomes hard, swollen (edematous), and peels off. Also called Kawasaki’s disease.

Mucus: A thick fluid produced by the lining of some organs of the body.

Mucus colitis: A common gastrointestinal disorder characterized by abdominal pain, bloating, mucous in stools, and irregular bowel habits with alternating diarrhea and constipation, symptoms that tend to be chronic and wax and wane over the years. Although mucus colitis can cause chronic recurrent discomfort, it appears to be an abnormal condition of gut contractions (motility) and does not lead to any serious organ problems. Diagnosis usually involves excluding other illnesses. Treatment is directed toward relief of symptoms and includes high fiber diet, exercise, relaxation techniques, avoidance of caffeine, milk products and sweeteners, and medications. Alternative names include irritable bowel syndrome, spastic colitis and nervous colon syndrome.

Multifactorial: Referring to multiple factors.

Multifactorial inheritance: Type of hereditary pattern seen with a combination of genetic factors, sometimes with environmental influence. Skin color, for example, is multifactorially determined.

Multi-Infarct Dementia: Dementia brought on by a series of strokes.

Multipara: A woman who has had 2 or more pregnancies resulting in potentially viable offspring. A woman who is "para III" has had 3 such pregnancies. A woman who is "para VI" or more is called a grand multipara.

Multiple myeloma: A malignancy of plasma cells (a form of lymphocyte) that typically involves multiple sites within the bone morrow and secretes all or part of a monoclonal antibody . Also called plasma cell myeloma.

Multiple sclerosis (MS): The National Multiple Sclerosis Society says of MS that it is "a disease that randomly attacks your central nervous system, wearing away the control you have over your body. Symptoms may range from numbness to paralysis and blindness. The progress, severity and specific symptoms cannot be foreseen. You never know when attacks will occur, how long they will last, or how severe they will be. Most people are diagnosed with MS between the ages of 20 and 40...." In medical terms, MS involves demyelinization of the white matter sometimes extending into the gray matter. Demyelinization is loss of myelin, the coating of nerve fibers composed of lipids (fats) and protein that serves as insulation and permits efficient nerve fiber conduction. The "white matter" is the part of the brain which contains myelinated nerve fibers and appears white, whereas the gray matter is the cortex of the brain which contains nerve cell bodies and appears gray. When myelin is damaged in MS, nerve fiber conduction is faulty or absent. Impaired bodily functions or altered sensations associated with those demyelinated nerve fibers give rise to the symptoms of MS. Recent research (1998) has also identified nerve cell death as part of the nervous system injury in MS.

Mumps: An acute (sudden and shortlived) viral illness, mumps usually presents with inflammation of the salivary glands, particularly the parotid glands. A child with mumps often looks like a chipmunk with a full mouth due to the swelling of the child's parotids (salivary glands near the ears). Mumps can also cause inflammation of other tissues, most frequently the covering and substance of the central nervous system (meningoencephalitis), next the pancreas (pancreatitis) and, especially after adolescence, the ovary (oophoritis) and the testis (orchitis). The mature testis is particularly susceptible to damage from mumps which can lead to infertility. Together with the likes of measles and chickenpox, mumps was once considered one of the inevitable infectious diseases of childhood. Since a mumps vaccine became available in 1967, the incidence of mumps has declined in the U.S., but there are still many underimmunized populations (for example, more blacks than whites have not yet been immunized). The origin of the word mumps is not clear. It may have to do with the English usage, now obsolete, of "mump" to mean a grimace. More probably, mumps comes from a colder climate, Iceland, where mumpa meant to fill the mouth too full.

Mumps immunization: The standard MMR vaccine is given to prevent measles, mumps and rubella (German measles). The MMR vaccine is now given in two dosages. The first should be given at12-15 months of age. The second vaccination should be given at 4-6 years (or, alternatively, 11-12 years) of age. Most colleges require proof of a second measles or MMR vaccination prior to entrance. Most children should receive MMR vaccinations. Exceptions may include children born with an inability to fight off infection, some children with cancer, on treatment with radiation or drugs for cancer, on long term steroids (cortisone). People with severe allergic reactions to eggs or the drug neomycin should probably avoid the MMR vaccine. Pregnant women should wait until after delivery before being immunized with MMR. People with HIV or AIDS should normally receive MMR vaccine. Measles, mumps, and rubella vaccines may be administered as individual shots, if necessary, or as a measles-rubella combination.

Mumps in pregnancy: It has been stated, we believe erroneously, that mumps is dangerous when contracted during pregnancy. For example, Vetter (Infect Med 14:730-733, 1997), citing a single 1980 article, writes: "Mumps infection during the first trimester of pregnancy can increase the rate of spontaneous abortion. Congenital anomalies associated with mumps infection during pregnancy include endocardial fibroelastosis; imperforate anus; spina bifida; and auditory, optic, and urogenital deformities." However, Shepard in his authorative Catalog of Teratogenic Agents (J Hopkins U Press, 8th edition, 1995) does not consider that mumps merits inclusion as a proven or even possible teratogenic agent, that is as an agent capable of causing a spontaneous abortion (a miscarriage) or causing congenital malformations (the baby is born with birth defects). Furthermore, mumps does not even warrant mention in Smith's Recognizable Patterns of Human Malformation (by KL Jones, WB Saunders Co., 5th edition, 1997), a key standard text on dysmorphology (the study of malformations). Based also on our review of the facts available at this time (Oct, 1997), we conclude that there is insufficient evidence to label mumps as a particular hazard in pregnancy.

Munchhausen syndrome: Recurrent feigning of catastrophic illnesses. Named for the fictitious Baron who told tales that were whopping lies.

Murine typhus: An acute infectious disease with fever, headache, and rash, all quite similar to, but milder than, epidemic typhus, caused by a related microoganism, Rickettsia typhi (mooseri), transmitted to humans by rat fleas (Xenopsylla cheopis). The animal reservoir includes rats, mice and other rodents. Murine typhus occurs sporadically worldwide but is more prevalent in congested rat-infested urban areas. Also known as endemic typhus, rat-flea typhus; urban typhus of Malaya).

Murmur: A Murmur is an abnormal "whooshing" sound created by blood flow through heart valves, as well as flow through chamber narrowings or unusual connections seen with congenital heart disease. It is usually heard by the doctor while listening to the chest with a stethoscope.

Muscle: Muscle is the tissue of the body which primarily functions as a source of power. There are three types of muscle in the body. Muscle which is responsible for moving extremities and external areas of the body is called "skeletal muscle." Heart muscle is called "cardiac muscle." Muscle that is in the walls of arteries and bowel is called "smooth muscle."

Muscle, adductor: Any muscle that pulls inward toward the midline of the body. For example, the adductor muscles of the leg serve to pull the legs together. The opposite of "adductor" is "abductor." To keep these similar sounding terms straight, medical students learn to speak of "A B ductors" versus "A D ductors."

Muscle, central core disease of (CCD): One of the conditions that produces ‘floppy baby’ syndrome. CCD causes hypotonia (inadequately toned muscles characterized by floppiness) in the newborn baby, slowly progressive muscle weakness, and muscle cramps after exercise. Muscle biopsy shows a key diagnostic finding (absent mitochondria in the center of many type I muscle fibers). CCD is inherited as a dominant trait. The CCD gene is on chromosome 19 (and involves ryanodine receptor-1).

Muscular: Having to do with the muscles. Also, endowed with above average muscle development. Muscular system refers to all of the muscles of the body collectively.

Mutagen: Something capable of causing a gene-change. Among the known mutagens are radiation, certain chemicals and some viruses.

Mutant: An individual with a mutant (changed) gene.

Mutation: A gene-change.

Myalgia: Pain in muscles. The Greek "algos" means "pain."

Myasthenia gravis: A nerve-muscle (neuromuscular) disorder with fatigue and exhaustion of muscles.

Mycoplasma: A group of bacteria. A common cause of pneumonia in persons with HIV.

Mycosis fungoides: A type of non-Hodgkin's lymphoma that first appears on the skin. Also called cutaneous T-cell lymphoma.

Mydriasis: Dilation of the pupils induced by eyedrops. The opposite of miosis.

Myelin: The fatty substance that covers and protects nerves. Myelin is a layered tissue surrounding the axons or nerve fibers. The sheath around the nerve fibers which acts electrically as a conduit.

Myelitis: Inflammation of the spinal cord.

Myelodysplastic syndrome: A condition in which the bone marrow does not function normally. It does not produce enough blood cells. This condition may progress and become acute leukemia. Myelodysplastic syndrome also is called preleukemia or smoldering leukemia.

Myelofibrosis: Fibrosis (spontaneous scarring) of the bone marrow. This can be associated with a variety of diseases, primarily myeloproliferative (pre-leukemic) disorders. Sometimes used interchangeably with agnogenic myeloid metaplasia. Acute myelofibrosis: a distinct disorder characterized by acute inadequate blood cell production (pancytopenia), marrow fibrosis, but no enlargement of the spleen or liver.

Myelogenous: Referring to myelocytes, a type of white blood cell. Also called myeloid.

Myelogram: An x-ray of the spinal cord and the bones of the spine.

Myeloid: Referring to myelocytes, a type of white blood cells. Also called myelogenous.

Myeloma: A tumor of cells that are normally found in the bone marrow.

Myeloproliferative disorders: Tumors of certain bone marrow cells including those that give rise to red cells, granulocytes, and platelets. As opposed to the lymphoproliferative disorders.

Myocarditis: Inflammation of the heart muscle.

Myocardium: The heart muscle.

Myoclonus: Shock-like contraction of muscle.

Myoglobin: The pigment in muscle that carries oxygen.

Myoma: A tumor of muscle. Can specifically refer to a benign tumor of uterine muscle, also called a leiomyoma or a fibroid.

Myometrium: The muscular outer layer of the uterus.

Myopathy: Any and all disease of muscle.

Myopia: Nearsightedness.

Myotonic dystrophy: Inherited disease with myotonia (irritability and prolonged contraction of muscles), mask-like face, premature balding, cataracts, and cardiac disease. Due to a trinucleotide repeat (a stuttering sequence of three bases) in the DNA.

Myringotomy: Draining of fluid by making an opening in the middle ear, for example, in which to put ear tubes.