nerve). Nerve-cell groups in the brain-
stem, known collectively as the reticular
formation, alert the higher brain centres
to sensory stimuli that may require a
conscious response. Our sleep/wake cycle
is controlled by the reticular formation.
The brainstem is susceptible to the
same disorders that afflict the rest of
the central nervous system (see brain,
disorders of). Damage to the medulla’s
vital centres is rapidly fatal; damage
to the reticular formation may cause
coma. Damage to specific cranial nerve
nuclei can sometimes lead to specific
effects. For example, damage to the 7th
cranial nerve (the facial nerve) leads to
facial palsy. Degeneration of the sub-
stantia nigra in the midbrain is thought
to be a cause of Parkinson’s disease.
brain syndrome, organicDisorder of
consciousness, intellect, or mental func-
tioning that is of organic (physical), as
opposed to psychiatric, origin. Causes
include degenerative diseases, such as
Alzheimer’s disease; infections; certain
drugs; or the effects of injury, stroke, or
tumour. Symptoms range from mild
confusion to stupor or coma. They may
also include disorientation, memory
loss, hallucinations, and delusions (see
delirium). In the chronic form, there is a
progressive decline in intellect, memory,
and behaviour (see dementia). Treat-
ment is more likely to be successful
with the acute form. In chronic cases,
irreversible brain damage may already
have occurred. (See also psychosis.)
brain tumourAn abnormal growth in
or on the brain. Tumours may be primary
growths arising directly from tissues
within the skull or metastases (second-
ary growths) that have spread from
tumours elsewhere in the body. The
cause of primary brain tumours is not
known. About 60 per cent are gliomas
(frequently cancerous), which arise from
the brain tissue. Other primary tumours
include meningiomas, acoustic neuromas,
and pituitary tumours. Most of these
tumours are noncancerous, but their
size can cause local damage. Certain
types of primary brain tumour mainly
affect children. These include 2 types of
glioma called medulloblastomaand cere-
bellar astrocytoma. Primary brain tumours
virtually never spread (metastasize)
outside the central nervous system.
Symptoms include muscle weakness,
loss of vision, or other sensory disturb-
ances, speech difficulties, and epileptic
seizures. Increased pressure within the
skull can cause headache, visual distur-
bances, vomiting, and impaired mental
functioning. Hydrocephalusmay occur.
When possible, primary tumours are
removed by surgery after opening the
skull (see craniotomy). In cases where a
tumour cannot be completely removed,
as much as possible of it will be cut
away to relieve pressure. For primary
and secondary tumours, radiotherapy
or anticancer drugsmay also be given.
Corticosteroid drugs are often pre-
scribed temporarily to reduce the size of
a tumour and associated brain swelling.
branThe fibrous outer covering of grain
that cannot be digested. The fibre is
used as a bulk-forming laxativeto pre-
vent constipation (see fibre,dietary).
branchial disordersDisorders due to
abnormal development, in an embryo,
of the branchial arches (paired segmen-
ted ridges of tissue in each side of the
throat). They include branchial cyst and
branchial fistula. A branchial cyst is a soft
swelling, containing a pus-like or clear
fluid, that appears on the side of the
neck in early adulthood. Treatment is by
surgical removal. A branchial fistula oc-
curs between the back of the throat and
the external surface of the neck, where
it appears as a small hole, usually noted
at birth. A hole in the neck that does
not extend to the back of the throat is a
branchial cleft sinus. A branchial fistula
or cleft sinus may discharge mucus or
pus and may be removed surgically.
brash, waterSee waterbrash.
Braxton Hicks’ contractionsShort
relatively painless contractions of the
uterus during pregnancy. They may be
felt in late pregnancy and are some-
times mistaken for labour pains.
breakbone fever A tropical viral ill-
ness, also called dengue, that is spread
by mosquitoes.
breakthrough bleedingBleeding or
staining (“spotting”) from the vagina
between periods in women taking an
oral contraceptive. The bleeding is mostBRAIN SYNDROME, ORGANIC BREAKTHROUGH BLEEDING
B