thought to stimulate erotic desire and
enhance sexual performance. For cent-
uries, various substances (most notably
oysters and rhinoceros horn) have been
used as aphrodisiacs. In fact, no sub-
stance has a proven aphrodisiac effect.
aphthous ulcerSee ulcer,aphthous.
apicectomySurgical removal of the tip
of a tooth root. Apicectomy may be per-
formed as part of root-canal treatment.
aplasia Absent or severely reduced
growth and development of any organ
or tissue. For example, in bone marrow
aplasia, the rate of cell division in the
bone marrow is reduced, leading to in-
sufficient blood-cell production (see
anaemia,aplastic). Some birth defects,
such as stunted limbs (see phocomelia),
occur as a result of incomplete tissue
formation during prenatal development.
aplastic anaemiaSee anaemia,aplastic.
apnoeaCessation of breathing, either
temporarily or for a prolonged period.
Breathing is an automatic process
controlled by the respiratory centre in
the brainstem. Failure of this centre to
maintain normal breathing is known as
central apnoea. It may occur in babies,
particularly those who are premature,
and can be detected by an apnoea
alarm. Central apnoea can also result
from brainstem damage, for example
following a strokeor head injury.
In obstructive apnoea, breathing is pre-
vented by a blockage in the airway. The
most common type is sleep apnoea, in
which blockage of the upper airway
occurs repeatedly during sleep.
Deliberate temporary apnoea occurs
in breath-holding attacks. Another type
of apnoea occurs in Cheyne–Stokes res-
piration, in which cycles of deep, rapid
breathing alternate with episodes of
breathing stoppage.
Treatment of apnoea depends on the
cause. In newborn babies, it resolves as
they mature. In stroke or head injury,
artificial ventilation may be needed
temporarily until recovery occurs.
apocrine gland A gland that dis-
charges cellular material in addition to
the fluid it secretes. The term is usually
applied to the type of sweat glandsthat
appear in hairy body areas after puberty.
(See also eccrine gland.)
apolipoproteinA group of proteins
that are constituents of lipoproteins, the
carriers of fat in the bloodstream. Apo-
lipoproteins are also involved in the
growth and repair of nerve tissues.
aponeurosisA wide sheet of tough,
fibrous tissue that acts as a tendon,
attaching a muscle to a bone or a joint.
apophysisAn outgrowth of bone at the
site of attachment of a tendon to bone.
Inflammation may also occur, as in
Osgood–Schlatter disease.
apoplexyAn outdated term for a stroke.
apoptosisThe natural process of pro-
grammed cell death. Apoptosis occurs
in embryonic development, when the
shaping of body parts is taking place
and continues throughout life in the
constant cycle of death and renewal of
body cells. Failure of apoptosis is impli-
cated in the development of cancers.
apothecaryAn old term for a pharmacist.
appendicectomySurgical removal of the
appendix to treat acute appendicitis.
appendicitisAcute inflammation of the
appendix. The cause is usually not
known, but appendicitis is sometimes
caused by obstruction of the appendix
by a lump of faeces. The 1st symptom is
usually vague discomfort around the
navel. Within a few hours, this develops
into severe, more localized pain, which is
usually most intense in the lower right-
hand side of the abdomen. Symptoms
may differ if the appendix is not in the
most common position. For example, if
the appendix impinges on the ureter,
the urine may become bloodstained.
The usual treatment for appendicitis
is appendicectomy, which is often per-
formed endoscopically (see minimally
invasive surgery). If the treatment is
delayed, an inflamed appendix may
burst, releasing its contents into the
abdomen. This leads to peritonitisand,
in some cases, an abscess.
appendixA small, narrow tube that
projects out of the caecum (the 1st part
of the colon) at the lower right-hand
side of the abdomen. It may lie behind
or below the caecum, or in front of or
behind the ileum (part of the small
intestine). The appendix has no known
function, but it contains a large amount
of lymphoid tissue which provides aAPHTHOUS ULCER APPENDIX
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