Webster's New World Medical Dictionary

medulla, adrenal
The innermost portion of the
adrenal gland, which makes epinephrine (adrena-
line) and norepinephrine (noradrenaline). These
hormones cause the breakdown of the storage
product glycogen to the sugar glucose in the liver,
facilitate the release of fatty acids from adipose (fat)
tissue, cause narrowing (constriction) of most
blood vessels in the body, and increase the output of
the heart.
medulla oblongata
The base of the brain, which
is formed by the enlarged top of the spinal cord. The
medulla oblongata directly controls breathing,
blood flow, and other essential functions.
medulloblastoma
A type of brain tumor that
tends to occur in children, arise in the cerebellum
(in the lower part of the brain), and spread along
the spine. Medulloblastoma is the most common
type of primary brain tumor in childhood.
Medulloblastomas occasionally metastasize outside
the central nervous system, usually to bone.
Treatment involves surgery, radiotherapy, and
chemotherapy. 
MEDVAC
Acronym for medical evacuation. MED-
VAC typically refers to a team that has the skills nec-
essary for proper medical evacuation in emergency
situations. Also known as MEDEVAC.
mega-
Prefix meaning abnormally large, as in
megalocephaly (an overly large head) and megac-
ardia (an enlarged heart).
megacolon
An abnormally enlarged colon.
Megacolon can arise as a serious congenital prob-
lem in the newborn, caused by abnormal innerva-
tion of the colon and frequently requiring surgery.
megakaryocyte
A giant cell in the bone marrow
that is the ancestor of blood platelets, which are
essential to normal blood clotting.
megavitamin therapy
The use of massive doses
of vitamins to treat disease. Because overuse of vita-
mins can cause disease, most physicians consider
megavitamin therapy controversial. See also
Appendix C, “Vitamins”;
orthomolecular medicine;
vitamin therapy.