labyrinth and separated from it by the perilymph; made up of the
semicircular ducts, the saccule and utricle, and the cochlear duct.
Menarche (me-NAR-ke) The first menses (menstrual flow) and begin-
ning of ovarian and uterine cycles.
Meninges (me-NIN-jez) Three membranes covering the brain and spi-
nal cord, called the dura mater, arachnoid mater, and pia mater.
S in g u la r
is meninx (MEN-inks).
Menopause (MEN-o-pawz) The termination of the menstrual cycles.
Menstruation (men'-stroo-A-shun) Periodic discharge of blood, tis-
sue fluid, mucus, and epithelial cells that usually lasts for 5 days;
caused by a sudden reduction in estrogens and progesterone.
Also called the menstrual phase or menses.
Merkel disc (MER-kel) Modified epidermal cell in the stratum basale
of hairless skin that functions as a cutaneous receptor for touch.
Also called a tactile disc.
Mesenchyme (MEZ-en-kTm) An embryonic connective tissue from
which almost all other connective tissues arise.
Mesentery (MEZ-en-ter'-e) A fold of peritoneum that attaches the
small intestine to the posterior abdominal wall.
Mesoderm (MEZ-o-derm) The middle primary germ layer that gives
rise to connective tissues, blood and blood vessels, and muscles.
Metabolism (me-TAB-o-lizm) All the biochemical reactions that occur
within an organism, including the synthetic (anabolic) reactions
and decomposition (catabolic) reactions.
Metacarpus (met'-a-KAR-pus) A collective term for the five bones
that make up the palm.
Metaphase (MET-a-phaz) The second stage of mitosis, in which chro-
matid pairs line up on the metaphase plate of the cell.
Metaphysis (me-TAF-i-sis) Region of a long bone between the diaphy-
sis and epiphysis that contains the epiphyseal plate in a growing
Metastasis (me-TAS-ta-sis) The spread of cancer to surrounding tis-
sues (local) or to other body sites (distant).
Metatarsus (met'-a-TAR-sus) A collective term for the five bones
located in the foot between the tarsals and the phalanges.
Micelle (mT-SEL) A spherical aggregate of bile salts that dissolves
fatty acids and monoglycerides so that they can be absorbed into
small intestinal epithelial cells.
Microglia (mT-kro-GLE-a) Neuroglial cells that carry on phagocytosis.
Microvilli (mT'-kro-VIL-e) Microscopic, fingerlike projections of the
plasma membranes of cells that increase surface area for absorp-
tion, especially in the small intestine and proximal convoluted
tubules of the kidneys.
Micturition (mik'-choo-RISH-un) The act of expelling urine from the
urinary bladder. Also called urination (u-ri-NA-shun).
Midbrain The part of the brain between the pons and the diencepha-
lon. Also called the mesencephalon (mes'-en-SEF-a-lon).
Middle ear A small, epithelial-lined cavity hollowed out of the
temporal bone, separated from the external ear by the eardrum
and from the internal ear by a thin bony partition containing
the oval and round windows; extending across the middle ear
are the three auditory ossicles. Also called the tympanic cavity
Midline An imaginary vertical line that divides the body into equal
left and right sides.
Midsagittal plane A vertical plane through the midline of the body
that divides the body or organs into
e q u a l
right and left sides.
Also called a median plane.
Milk ejection reflex Contraction of alveolar cells to force milk into
ducts of mammary glands, stimulated by oxytocin (TO), which
is released from the posterior pituitary in response to suckling
action. Also called the milk letdown reflex.
Mineral Inorganic, homogeneous solid substance that may perform
a function vital to life; examples include calcium and phosphorus.
Mineralocorticoids (min'-er-al-o-KOR-ti-koyds) A group of hormones
of the adrenal cortex that help regulate sodium and potassium
Minute ventilation (MV) Total volume of air inhaled and exhaled per
minute; about 6000 mL at rest.
Mitochondrion (mT'-to-KON-dre-on) A double-membraned organelle
that plays a central role in the production of ATP; known as the
"powerhouse" of the cell.
Mitosis (mT-TO-sis) The orderly division of the nucleus of a cell that
ensures that each new nucleus has the same number and kind
of chromosomes as the original parent nucleus. The process
includes the replication of chromosomes and the distribution of
the two sets of chromosomes into two separate and equal nuclei.
Mitotic spindle (mT-TOT-ik) Collective term for a football-shaped
assembly of microtubules that is responsible for the movement of
chromosomes during cell division.
Modality (mo-DAL-i-te) Any of the specific sensory entities, such as
vision, smell, taste, or touch.
Molecule (MOL-e-kul) The chemical combination of two or more
atoms covalently bonded together.
Monocyte (MON-o-sTt') The largest type of white blood cell, charac-
terized by agranular cytoplasm.
Monounsaturated fat A fatty acid that contains one double covalent
bond between its carbon atoms; it is not completely saturated with
hydrogen atoms. Plentiful in triglycerides of olive and peanut oils.
Mons pubis (MONZ P0-bis) The rounded, fatty prominence over the
pubic symphysis, covered by coarse pubic hair.
Morula (MOR-u-la) A solid sphere of cells produced by successive
cleavages of a fertilized ovum about four days after fertilization.
Motor end plate Region of the sarcolemma of a muscle fiber (cell)
that includes acetylcholine (ACh) receptors, which bind ACh
released by synaptic end bulbs of somatic motor neurons.
Motor neurons (NOO-ronz) Neurons that conduct impulses from
the brain toward the spinal cord or out of the brain and spinal
cord into cranial or spinal nerves to effectors that may be either
muscles or glands. Also called efferent neurons (EF-er-ent).
Motor unit A motor neuron together with the muscle fibers (cells) it
Mucin (M0-sin) A protein found in mucus.
Mucous cell (M0-kus) A unicellular gland that secretes mucus. Two
types are mucous neck cells and surface mucous cells in the