594 Introduction to Anatomy and Physiology
membranes thin sheets or layers of pliable tissue
membranous labyrinth (MEHM-bray-nus)
membrane-covered tubes inside the bony labyrinth
memory cells B lymphocytes and T lymphocytes in
lymphatic tissues that can respond if a previously
encountered antigen invades the body again
memory cytotoxic T cell one of the daughter cells
produced by clonal selection of cytotoxic T lymphocytes
memory helper B cell one of the daughter cells
produced by clonal selection of helper B lymphocytes
memory helper T cell one of the daughter cells created
by clonal selection of helper T lymphocytes
menarche (MEHN-ar-kee) the fi rst menstrual bleeding
Ménière’s disease (men-YAIRZ) a condition
characterized by chronic infl ammation of the semicircular
canals of the inner ear
meninges (meh-NIN-jeez) three protective membranes
that surround the brain and spinal cord
meningitis (MEHN-in-JIGH-tis) an infection-induced
infl ammation of the meninges surrounding the brain and
spinal cord
menstrual discharge the blood and tissue lost in the
menstrual phase
menstrual phase the phase of the uterine cycle during
which the endometrium breaks down
Merkel cells touch receptors in the skin
mesentery (MEHS-ehn-TER-ee) a double layer of
peritoneum attached to the abdominal wall
messenger RNA single-strand RNA molecule whose
base sequence carries the information needed by a
ribosome to make a protein
metabolic rate the speed at which the body consumes
energy
metabolism (meh-TAB-oh-lizm) the sum of the chemical
and physical reactions that occur in an organism to keep
it alive
metacarpals (meht-a-KAR-pals) the fi ve interior bones
of the hand, connecting the carpals in the wrist to the
phalanges in the fi ngers
metaphase the second stage of cell division, in which
chromosomes align along the center of the cell
metastasis (meh-TAS-ta-sis) the spreading of cancerous
cells from their original location to other areas of the body
metatarsals (meht-a-TAR-salz) small bones of the ankle
metric system international system of measurement that
is used in all fi elds of science
microfi lament a type of fi ber in the cytoskeleton; made
of actin
microglia (migh-KROHG-lee-a) glial cells that absorb
and dispose of dead cells and bacteria
micronutrients vitamins and minerals that are essential
to the body in small amounts
microscopic anatomy the study of the tiny parts of the
body seen only with a microscope
microtubule (MIGH-kroh-TOO-byool) a type of fi ber in
the cytoskeleton; made of tubulin
microvilli (MIGH-kroh-VIL-ee) fi nger-like extensions that
increase the surface area of a cell
micturition (mik-choo-RISH-un) urination
midbrain relay station for sensory and motor impulses;
located on the superior end of the brain stem
middle ear cavities openings in the skull that serve as
chambers for transmitting and amplifying sound
mineralocorticoids (min-eh-ra-loh-KORT-i-koids) a group
of steroid hormones produced by the adrenal cortex that
regulate the body’s sodium and fl uid levels
minerals elements that the body needs in relatively small
amounts
mitochondria (MIGH-toh-KAHN-dree-a) organelles in the
cytoplasm that make ATP
mitosis the division of a cell nucleus and chromosomes
into two nuclei, each with its own set of identical
chromosomes
mitral valve (migh-tral) the valve that closes the orifi ce
between the left atrium and left ventricle of the heart;
bicuspid valve
mitral valve prolapse an incomplete closing of the mitral
valve, causing blood to fl ow backward into the left atrium
when the left ventricle contracts
modeling process through which new bone is created
during growth
molecules chemical combinations of two or more atoms
monocytes (MAHN-oh-sight) leukocytes that develop into
phagocytizing macrophages when they migrate out of
lymphatic circulation into surrounding tissue
monounsaturated fats one category of unsaturated fatty
acids; sources include canola oil and olive oil
mons pubis the region of skin and underlying fat in front
of the vaginal opening on which pubic hair grows during
puberty
motor neuron a nerve that stimulates skeletal muscle
tissue
motor unit a single motor neuron and all of the muscle
fi bers that it stimulates
mucosa the mucous membrane of the alimentary canal;
the innermost layer
mucosa-associated lymphatic tissue (MALT) lymphatic
tissue found in mucous membranes that line
passageways open to the outside world; these include
the respiratory, gastrointestinal, urinary, and reproductive
tracts
mucous layer a viscous coating that surrounds the
olfactory hairs
mucous membranes thin sheets of tissue lining the
body cavities that open to the outside world
multiple myeloma (MIGH-eh-LOH-ma) a cancer of the
plasma cells in blood marrow
multiple sclerosis a chronic, slowly progressive disease
of the central nervous system that destroys the myelin
sheath of nerve cell axons
multipolar neurons nerve cells that have one axon and
multiple dendrites
muscle cramps moderate to severe muscle spasms that
cause pain
muscle fi ber an individual skeletal muscle cell
muscle physiology the study of how muscles function
internally
muscle strain an injury that occurs when a muscle is
stretched beyond the limits to which it is accustomed
muscle tissue a type of tissue that generates force and
allows the body to move
muscular dystrophy (MD) a group of similar, inherited
disorders characterized by progressively worsening
muscle weakness and loss of muscle tissue
muscularis externa outer layer of smooth muscle in the
wall of the esophagus, stomach, small intestine, and
large intestine
myelin sheath (MIGH-eh-lin) the fatty bands of insulation
surrounding axon fi bers
myocardial infarction tissue death that occurs in a
segment of heart muscle from blockage of a coronary
artery; heart attack