intussusceptionor volvulusof the small
intestine. Diagnosis of Meckel’s divertic-
ulum may be made by using technetium
radionuclide scanning. If complications
occur, they are treated by surgical re-
moval of the diverticulum.
meconiumThe thick, sticky, greenish-
black faeces passed by infants in the
first day or two after birth. It consists of
bile, mucus, and shed intestinal cells.
Occasionally, the fetus passes meco-
nium into the amniotic fluid in the
uterus. This is more common in babies
who experience fetal distress during
labour or who are over 40 weeks’ gesta-
tion. Meconium in the amniotic fluid
may be inhaled when the baby starts to
breathe, sometimes blocking the air-
ways and damaging the lungs.
In some babies with cystic fibrosis, the
meconium is so thick and sticky that
it blocks the intestine (see intestine,
obstruction of).
medialA medical term that means “sit-
uated towards the midline of the body”.
Less commonly, the term refers to the
middle layer of a body structure.
median nerveOne of the main nerves
of the arm. It is a branch of the brachial
plexusand runs down the arm from the
shoulder into the hand. The median
nerve controls the muscles that carry
out bending movements of the wrist,
fingers, and thumb, and that rotate the
forearm palm-inwards. The nerve also
conveys sensations from the thumb and
first 3 fingers, and from the region of
the palm at their base.
Damage to the nerve may result from
injury to the shoulder, a Colles’ fracture
just above the wrist, or pressure on the
nerve where it passes through the wrist
(carpal tunnel syndrome). Symptoms of
damage include numbness and weak-
ness in areas controlled by the nerve.
mediastinoscopyInvestigation of the
mediastinumby means of an endoscope
inserted through an incision in the neck.
Mediastinoscopy is used mainly to
perform a biopsyof a lymph node. The
sample is removed by tiny blades on
the endoscope.
mediastinumThe membranous parti-
tion between the lungs and the other
structures within the chest cavity. These
structures include the heartand associ-
ated blood vessels, trachea, oesophagus,
thymusgland, lymph nodes, lymphatic
vessels, and nerves.
medicationAny substance prescribed to
treat disease. (See also drug; medicine.)
medicineThe study of human diseases,
their causes, frequency, treatment, and
prevention. The term is also applied to
a substance prescribed to treat illness.
medicolegalRelating to aspects of med-
icine and law that overlap. Among the
matters on which medicolegal experts
advise are the laws concerning damages
for injuries due to medical negligence
or malpractice, evidence concerning the
extent of injury in a civil action, the use
of paternity tests, the mental compet-
ence of people who have drawn up wills,
and restrictions on the mentally ill.
Medicolegal issues also include an
individual’s right to die (see brain death;
euthanasia; living will); the necessity for
informed consentto any surgical pro-
cedure; the legal aspects of artificial
insemination, in vitro fertilization, steril-
ization, and surrogacy; and a patient’s
right to confidentialityconcerning his or
her illness. (For the medical aspects of
criminal law, see forensic medicine.)
meditationConcentrating on an object,
a word, or an idea with the aim of induc-
ing an altered state of consciousness.
At its deepest level, meditation can
resemble a trance. More commonly, it is
a calming therapy and can be a way of
reducing stress levels and treating stress-
related disorders. A common form of
meditation practised in the west is tran-
scendental meditation (TM).
medroxyprogesteroneA progestogen
drugused to treat endometriosisand
certain types of breast cancerand uter-
ine cancer (see uterus, cancer of). It is
sometimes used to treat menstrual dis-
orders such as amenorrhoea(absence
of menstruation). Medroxyprogesterone
can also be used as a contraceptive,
administered by injection at 3-monthly
intervals (see contraception, hormonal
methods of). Possible adverse effects
include weightgain, swollen ankles, and
breast tenderness.
medullaThe innermost part of an organ
or other body structure; for example,MECONIUM MEDULLA
M