Lumen (LOO-men) The space within an artery, vein, intestine, renal
tubule, or other tubular structure.
Lungs Main organs of respiration that lie on either side of the heart
in the thoracic cavity.
Lunula (LOO-noo-la) The moon-shaped white area at the base of a
Luteinizing hormone (LH) (LOO-te-in'-Tz-ing) A hormone secreted
by the anterior pituitary that stimulates ovulation, stimulates
progesterone secretion by the corpus luteum, and readies the
mammary glands for milk secretion in females; stimulates testos-
terone secretion by the testes in males.
Lymph (LIMF) Fluid confined in lymphatic vessels and flowing
through the lymphatic system until it is returned to the blood.
Lymph node An oval or bean-shaped structure located along lym-
Lymphatic capillary (lim-FAT-ik) Closed-ended microscopic lymphatic
vessel that begins in spaces between cells and converges with
other lymphatic capillaries to form lymphatic vessels.
Lymphatic tissue A specialized form of reticular tissue that contains
large numbers of lymphocytes.
Lymphatic vessel A large vessel that collects lymph from lymphatic
capillaries and converges with other lymphatic vessels to form
the thoracic or right lymphatic ducts.
Lymphocyte (LIM-fo-sTt) A type of white blood cell that helps carry
out cell-mediated and antibody-mediated immune responses;
found in blood and in lymphatic tissues.
Lysosome (LT-so-som) An organelle in the cytoplasm of a cell,
enclosed by a single membrane and containing powerful diges-
Lysozyme (LT-so-zTm) A bactericidal enzyme found in tears, saliva,
perspiration, nasal secretions, and tissue fluids.
Macrophage (MAK-ro-faj) Phagocytic cell derived from a monocyte;
may be fixed or wandering.
Macula (MAK-u-la) A discolored spot or a colored area. A small,
thickened region on the wall of the utricle and saccule that con-
tains receptors for static equilibrium.
Macula lutea (MAK-u-la LOO-te-a) The yellow spot in the center of the
Major histocompatibility (MHC) antigens Surface proteins on white
blood cells and other nucleated cells that are unique for each per-
son (except for identical siblings); used to type tissues and help
prevent rejection of transplanted tissues. Also known as human
leukocyte antigens (HLAs).
Malignant (ma-LIG-nant) Referring to diseases that tend to become
worse and cause death, especially the invasion and spreading of
Mammary gland (MAM-ar-e) Modified sudoriferous (sweat) gland of
females that produces milk for the nourishment of the young.
Marrow (MAR-o) Soft, spongelike material in the cavities of bones.
Red bone marrow produces blood cells; yellow bone marrow
contains adipose tissue that stores triglycerides.
Mast cell A cell found in areolar connective tissue that releases his-
tamine, a dilator of small blood vessels, during inflammation.
Mastication (mas'-ti-KA-shun) Chewing.
Matter Anything that occupies space and has mass.
Mature follicle (FOL-i-kul) A large, fluid-filled follicle containing a
secondary oocyte and surrounding granulosa cells that secrete
estrogens. Also called a Graafian follicle (GRAF-e-an).
Meatus (me-A-tus) A passage or opening, especially the external
portion of a canal.
Mechanoreceptor (me-KAN-o-re-sep-tor) Sensory receptor that
detects mechanical deformation of the receptor itself or adjacent
cells; stimuli so detected include those related to touch, pres-
sure, vibration, proprioception, hearing, equilibrium, and blood
Medial (ME-de-al) Nearer the midline of the body or a structure.
Mediastinum (me'-de-as-TT-num) The broad, median partition
between the pleurae of the lungs, that extends from the sternum
to the vertebral column in the thoracic cavity.
Medulla (me-DUL-la) An inner portion of an organ, such as the
medulla of the kidneys.
Medulla oblongata (me-DUL-la ob'-long-GA-ta) The most inferior part
of the brain stem. Also termed the medulla.
Medullary cavity (MED-u-lar'-e) The space within the diaphysis of a
bone that contains yellow bone marrow. Also called the marrow
Medullary rhythmicity area (rith-MIS-i-te) The neurons of the respi-
ratory center in the medulla oblongata that control the basic
rhythm of respiration.
Meiosis (me-O-sis) A type of cell division that occurs during produc-
tion of gametes, involving two successive nuclear divisions that
result in daughter cells with the haploid
number of chromo-
Meissner corpuscle (MTZ-ner) The sensory receptor for the sensation
of touch; found in dermal papillae, especially in the palms and
soles. Also called a corpuscle of touch.
Melanin (MEL-a-nin) A dark black, brown, or yellow pigment found
in some parts of the body such as the skin, hair, and pigmented
layer of the retina.
Melanocyte (MEL-a-no-sTt') A pigmented cell, located between or
beneath cells of the deepest layer of the epidermis, that synthe-
Melanocyte-stimulating hormone (MSH) A hormone secreted by
the anterior pituitary that stimulates the dispersion of melanin
granules in melanocytes in amphibians; continued administration
produces darkening of skin in humans.
Melatonin (mel-a-TON-in) A hormone secreted by the pineal gland
that helps set the timing of the body's biological clock.
Membrane (MEM-bran) A thin, flexible sheet of tissue composed of
an epithelial layer and an underlying connective tissue layer, as in
an epithelial membrane, or of areolar connective tissue only, as in
a synovial membrane.
Membranous labyrinth (mem-BRA-nus LAB-i-rinth) The part of
the labyrinth of the internal ear that is located inside the bony