Glossary

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Magnesium - a mineral that is essential for many body functions, including nerve impulse transmission, formation of bones and teeth, and muscle contraction

Magnetic resonance imaging (MRI) - a technique that uses magnetic fields and radio waves to create high-quality cross-sectional images of the body without using radiation

Malabsorption - an impaired ability of the lining of the small intestine to absorb nutrients from food

Malaria - a parasitic disease spread by mosquitos that causes chills and fever; potentially fatal complications in the liver, kidneys, blood, and brain are possible

Malformation - abnormal development of an organ or tissue

Malignant - a word used to describe a condition that is characterized by uncontrolled growth and/or that can be fatal, such as a cancerous tumor

Malignant hyperthermia - a reaction to certain anesthesia gases involving intense muscle contractions and a high fever

Malignant melanoma - the most serious type of skin cancer, in which a mole changes shape, darkens, becomes painful, and/or bleeds easily

Mallory-Weiss syndrome - a condition associated with alcoholism in which the lower end of the esophagus tears, causing vomiting of blood

Mammography - an X-ray procedure done to detect breast cancer

Mammoplasty - a general term for a cosmetic operation on the breasts; includes breast reduction, enlargement, and reconstruction after a mastectomy

Mania - a mental disorder characterized by extreme excitement, happiness, overactivity, and agitation; usually refers to the high of the highs and lows experienced in manic-depressive disorder

Manic-depressive disorder - a mental disorder characterized by extreme mood swings, including either mania, depression, or a continuing shift between the two extremes

MAO inhibitor - see Monoamine oxidase inhibitor antidepressant

Marfan's syndrome - a rare genetic disorder that affects connective tissue, leading to abnormalities of joints, bones, tendons, ligaments, arteries, and/or the heart

Mast cell - a type of cell present in most body tissues that releases substances in response to an allergen, which causes symptoms such as inflammation

Mastectomy - a surgical procedure in which all or part of the breast is removed to prevent the spread of cancer

Mastitis - inflammation of the breast, which is usually caused by a bacterial infection

Measles - an illness caused by a viral infection, causing a characteristic rash and a fever; primarily affects children

Meconium - thick, sticky, greenish-brown stool passed by a postmature fetus, or one experiencing fetal distress, into the amniotic fluid, or by an infant during the first couple of days after birth

Medial - a term used to describe something situated on or near the midline of the body or a body structure

Median nerve - a nerve running down the arm to the hand; controls muscle movement in the forearm and hand and conveys sensation from part of the hand

Megacolon - a severely swollen large intestine, causing severe constipation and abdominal bloating; may be present at birth or develops later

Megaloblastic anemia - a type of anemia in which a lack of the vitamin B12 or folic acid interferes with red blood cells and causes them to be enlarged and deformed, resulting in tiredness and weight loss

Meiosis - the type of cell division that occurs only in the ovaries and testicles, producing cells with half the genes of the original cell; these cells then form eggs and sperm

Melanin - the pigment that gives skin, hair, and eyes their coloring

Melanocytes - cells that produce the pigment melanin

Melanocyte-stimulating hormone - a hormone that coordinates pigmentation of the skin, eyes, and hair

Melanoma - a skin tumor composed of cells called melanocytes

Menarche - the beginning of menstruation

Meninges - the three membranes that surround and protect the spinal cord and brain

Meningioma - a rare noncancerous tumor developing in the protective membranes covering the brain called the meninges; can cause headaches and problems with vision and mental function

Meningitis - inflammation of the meninges; usually caused by infection by a microorganism (meningitis caused by bacteria is life-threatening; viral meningitis is milder)

Meningocele - a protrusion of the meninges through an opening in the skull or spinal cord due to a genetic defect

Menopause - the period in a woman's life when menstruation stops, resulting in a reduced production of estrogen and cessation of egg production

Menorrhagia - excessive loss of blood during menstruation, which can be caused by disorders of the uterus

Menstrual cycle - the periodic discharge of blood and mucosal tissue from the uterus, occurring from puberty to menopause in a woman who is not pregnant

Menstruation - the shedding of the lining of the uterus during the menstrual cycle

Mesothelium - a tissue layer that lines the heart, abdomen, chest cavity, and lungs

Metabolic rate - the speed at which the body uses energy

Metabolism - a general term for all of the chemical processes that occur in the body

Metabolite - any substance that takes part in a chemical reaction in the body

Metastasis - the spreading of a cancerous tumor to another part of the body through lymph, blood, or across a cavity; also sometimes refers to a tumor that has been produced in this way

Microbe - another term for a microorganism, especially one that causes disease

Microbiology - the study of microorganisms

Microcephaly - an abnormally small head

Microorganism - any tiny, single-celled organism (such as a bacterium, virus, or fungus)

Microsurgery - a surgical technique that uses a special binocular microscope to operate on tiny, delicate, or hard-to-reach tissues

Micturition syncope - fainting or feeling weak while standing at the toilet; caused by an abnormal heartbeat or a drop in blood pressure

Middle ear - the small cavity between the eardrum and inner ear; contains three tiny, linked bones that transmit sound to the inner ear

Middle ear effusion - the buildup of fluid in the middle ear, which can affect hearing

Midwifery - a profession concerned with providing care to a mother and baby during pregnancy and childbirth

Migraine - a severe headache, usually accompanied by vision problems and/or nausea and vomiting, and that typically recurs

Mineral - a substance that is a necessary part of a healthy diet (such as potassium, calcium, sodium, phosphorus, and magnesium)

Minipill - an oral contraceptive containing only the synthetic hormone progesterone (birth control pills contain estrogen and progesterone)

Miscarriage - expulsion of a fetus before it has developed sufficiently to survive on its own

Mitosis - the process by which most cells divide in order to reproduce

Mitral insufficiency - a problem with the ability of the mitral valve in the heart to close, which causes the heart to pump harder and reduces its efficiency

Mitral stenosis - a condition in which the mitral valve in the heart becomes narrowed, making the heart work harder to pump blood; can lead to symptoms such as shortness of breath

Mitral valve - the valve in the heart that allows blood to flow from the left atrium to the left ventricle, but prevents blood from flowing back in

Mitral valve prolapse - a common condition in which the mitral valve in the heart is deformed, causing blood to leak back across the valve; characterized by a heart murmur and sometimes chest pain and disturbed heart rhythm

Modified radical mastectomy - a treatment for breast cancer in which the entire breast, a section of the chest muscle, and lymph nodes in the chest and underarm are removed

Mole - a brown to dark-brown spot on the skin that can be flat or raised

Molecule - the smallest unit of a substance that possesses its characteristics

Molluscum contagiosum - a viral infection that causes white bumps on the skin; usually clears up in a few months

Mongolian spot - a brown to blue-black spot on the lower back and buttocks at birth, caused by a concentration of pigment-producing cells; usually disappears by the age of 3 or 4 years

Monoamine oxidase inhibitor antidepressant - a substance that works by stopping an enzyme that breaks down stimulating chemicals in the brain; used to treat depression

Mononucleosis - an infection caused by a virus that invades a type of white blood cell called a monocyte, causing fever, sore throat, and swollen lymph nodes

Monounsaturated fat - a type of fat that is thought to be beneficial in the prevention of coronary heart disease; found in foods such as olive oil and peanut oil

Morbidity - the state of being ill or having a disease

Morning sickness - nausea and vomiting experienced early in a pregnancy, affecting about half of all pregnant women

Mortality - the death rate, measured as the number of deaths per a certain population; may describe the population as a whole, or a specific group within a population (such as infant mortality)

MRI - see Magnetic resonance imaging

MS - see Multiple sclerosis

Mucous membrane - the soft, pink layer of cells that produce mucus in order to keep body structures lubricated; found in structures such as the eyelids, respiratory tract, and urinary tract

Mucus - a slippery fluid produced by mucous membranes that lubricates and protects the internal surfaces of the body

Multiple pregnancy - the presence of more than one fetus in the uterus, such as occurs with twins

Multiple sclerosis - a disease in which the protective coverings (myelin) of nerve fibers in the brain are gradually destroyed; symptoms vary from numbness to paralysis and loss of control of bodily function

Mumps - a viral infection that causes inflammation of salivary glands; primarily affects children

Murmur - a characteristic sound (heard through a stethoscope) of blood flowing irregularly through the heart; can be harmless or may be an indication of disease

Muscle fibers - specialized, contracting cells that are bundled together to form muscles

Muscle tone - the natural tension in resting muscles

Muscle wasting - the degeneration of a muscle (loss of bulk), caused by disease or starvation

Muscular dystrophy - a rare genetic disorder in which muscles degenerate gradually and strength is lost

Mutagen - anything that can increase the rate of abnormal change in cells, which can lead to cancer

Mutation - a change in the genetic information within a cell

Myalgia - the medical term for muscle pain

Myasthenia gravis - a disease in which the muscles, mainly those in the face, eyes, throat, and limbs, become weak and tire quickly; caused by the body's immune system attacking the receptors in the muscles that pick up nerve impulses

Mycoplasma - the smallest free-living microorganisms

Myelocele - protrusion of the spinal cord and its coverings out from the spine; one of the more severe forms of spina bifida

Myeloma - a cancer affecting cells in the bone marrow; sometimes used as an abbreviation for multiple myeloma

Myelosclerosis - buildup of fibrous connective tissue in the bone marrow, affecting the production of blood components

Myocardial infarction - the death of an area of heart muscle as a result of being deprived of its blood supply; characterized by severe pain in the chest; commonly called a heart attack

Myocarditis - inflammation of the heart muscle, which can be caused by a virus, certain drugs, or radiation therapy

Myocardium - the medical term for heart muscle

Myomectomy - the surgical removal of a noncancerous tumor from muscle

Myopathy - a muscle disease, usually one that results in the deterioration of muscle

Myopia - the medical term for nearsightedness

Myositis - muscle inflammation, causing pain and weakness

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