570 Glossarypathogens, dead or damaged cells, and old red blood
cells; also involved in recognition of foreign antigens and
activation of lymphocytes (Chapter 11).
Macula lutea(MAK-yoo-lah LOO-tee-ah) A spot in the
center of the retina; contains the fovea (Chapter 9).
Magnetic resonance imaging (MRI)(mag-NET-ik REZ-
ah-nanse IM-ah-jing) A diagnostic imaging technique
that uses a magnetic field and a computer to integrate the
images of internal organs (Chapter 1).
Malignant(muh-LIG-nunt) Tending to spread and become
worse; used especially with reference to cancer (Chapter
3).
Malleus(MAL-ee-us) The first of the three auditory bones
in the middle ear; transmits vibrations from the eardrum
to the incus (Chapter 9).
Maltase (MAWL-tays) A digestive enzyme that breaks
down maltose to glucose; secreted by the small intestine
(Chapter 16).
Maltose(MAWL-tohs) A disaccharide made of two glucose
molecules (Chapter 2).
Mammary glands(MAM-uh-ree) The glands of the female
breasts that secrete milk; secretion and release of milk are
under hormonal control (Chapter 20).
Mammography(mah-MOG-rah-fee) A diagnostic proce-
dure that uses radiography to detect breast cancer
(Chapter 20).
Mandible(MAN-di-buhl) The lower jaw bone (Chapter 6).
Manubrium(muh-NOO-bree-um) The upper part of the
sternum (Chapter 6).
Marrow canal(MA-roh ka-NAL) The cavity within the
diaphysis of a long bone; contains yellow bone marrow
(Chapter 4).
Mastoid sinus(MASS-toyd) An air cavity within the mas-
toid process of the temporal bone (Chapter 6).
Matrix(MAY-triks) 1. The non-living intercellular material
that is part of connective tissues. 2. The part of a hair
root in which mitosis takes place (Chapter 4).
Matter(MAT-ter) Anything that occupies space; may be
solid, liquid, or gas; may be living or non-living (Chapter
2).
Maxilla(mak-SILL-ah) The upper jaw bone (Chapter 6).
Mechanical digestion(muh-KAN-i-kuhl dye-JES-chun)
The physical breakdown of food into smaller pieces,
which increases the surface area for the action of diges-
tive enzymes (Chapter 16).
Medial(MEE-dee-uhl) Toward the midline, or in the mid-
dle (Chapter 1).
Mediastinum(ME-dee-ah-STYE-num) The area or space
between the lungs; contains the heart and great vessels
(Chapter 12).
Medulla(muh-DEW-lah) (muh-DULL-ah) 1. The part of
the brain superior to the spinal cord; regulates vital func-
tions such as heart rate, respiration, and blood pressure.- The inner part of an organ, such as the renal medulla
or the adrenal medulla (Chapter 8).
Megakaryocyte(MEH-ga-KA-ree-oh-sight) A cell in the
red bone marrow that breaks up into small fragments
called platelets, which then circulate in peripheral blood
(Chapter 11).
Meiosis(my-OH-sis) The process of cell division in which
one cell with the diploid number of chromosomes divides
twice to form four cells, each with the haploid number of
chromosomes; forms gametes (Chapter 3).
Meissner’s plexus(MIZE-ners PLEK-sus) The autonomic
nerve plexus in the submucosa of the organs of the ali-
mentary tube; regulates secretions of the glands in the
mucosa of these organs (Syn.—submucosal plexus)
(Chapter 16).
Melanin(MEL-uh-nin) A protein pigment produced by
melanocytes. Absorbs ultraviolet light; gives color to
the skin, hair, iris, and choroid layer of the eye (Chap-
ter 5).
Melanocyte(muh-LAN-o-sight) A cell in the lower epider-
mis that synthesizes the pigment melanin (Chapter 5).
Melanoma(MEL-ah-NOH-mah) Malignant pigmented
mole or nevus (Chapter 5).
Melatonin(mel-ah-TOH-nin) A hormone produced by the
pineal gland; influences sleep cycles (Chapter 10).
Membrane(MEM-brayn) A sheet of tissue; may be made of
epithelial tissue or connective tissue (Chapter 4).
Meninges(me-NIN-jeez) The connective tissue mem-
branes that line the dorsal cavity and cover the brain and
spinal cord (Chapter 1).
Meningitis (MEN-in-JIGH-tis) Inflammation of the
meninges, most often the result of bacterial infection
(Chapter 8).
Menopause(MEN-ah-paws) The period during life in
which menstrual activity ceases; usually occurs between
the ages of 45 and 55 years (Chapter 20).
Menstrual cycle(MEN-stroo-uhl SIGH-kuhl) The peri-
odic series of changes that occur in the female reproduc-
tive tract; the average cycle is 28 days (Chapter 20).
Menstruation (MEN-stroo-AY-shun) The periodic dis-
charge of a bloody fluid from the uterus that occurs at
regular intervals from puberty to menopause (Chapter
20).
Mesentery(MEZ-en-TER-ee) The visceral peritoneum
(serous) that covers the abdominal organs; a large fold
attaches the small intestine to the posterior abdominal
wall (Chapter 1).
Mesoderm (MEZ-oh-derm) The middle primary germ
layer of cells of an embryo; gives rise to muscles, bones,
and connective tissues (Chapter 21).
Metabolic acidosis(MET-uh-BAH-lik ass-i-DOH-sis) A
condition in which the blood pH is lower than normal,
caused by any disorder that increases the number of
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