Sperm are produced within the seminif-
erous tubules of the testesand mature in
the epididymis. Production and develop-
ment of sperm cells is dependent on
testosteroneand on gonadotrophin hor-
monessecreted by the pituitary gland.
Sperm production starts at puberty.
spermatic cordThe structure in males
that runs from the abdomen to the
scrotum and contains the vas deferens.
(See also reproductive system, male.)
spermatoceleA harmless cyst of the
epididymiscontaining fluid and sperm.
spermatozoaSee sperm.
spermicidesContraceptive preparations
that kill sperm. They are usually recom-
mended for use with a barrier device.
SPF Sun protec-
tion factor, the
degree of protec-
tion a sunscreen
provides against
sunburn. It is a
measure of the
amount of UVB
radiation a sun-
screen absorbs:
the higher the
number, the more
the protection.
sphenoid bone
The bat-shaped
bone in the cen-
tre of the base of
the cranium.
spherocytosis,
hereditaryAn inherited disorder in
which there are a large number of
unusually small, round red blood cells
(spherocytes) in the circulation. These
abnormal red cells are readily broken up
when blood passes through the spleen.
At times, the rate of red cell destruction
exceeds the rate at which new cells can
be made in the bone marrow, leading to
symptoms of anaemia. Other symptoms
include jaundiceand enlargement of the
spleen. Occasionally,crises occur (usu-
ally triggered by infection) in which all
symptoms worsen.Gallstonesare a fre-
quent complication. Splenectomyusually
leads to permanent improvement.
sphincterA ring of muscle around a
natural opening or passage that acts as
a valve, regulating inflow or outflow.
sphincter, artificialA surgically creat-
ed valve or other device used to treat or
prevent urinary or faecal incontinence.
sphincterotomyA surgical procedure
that involves cutting the muscle that
closes a body opening or that constricts
the opening between body passages.
sphygmomanometer An instrument
used for measuring blood pressure. A
cuff attached to the device is wrapped
around the person’s arm and inflated
until it compresses the main artery in
the arm. The cuff is deflated while the
doctor listens to the blood flow through
a stethoscope. The sphygmomanometer
records the pressure on a mercury-filled
glass column or a digital display.spider bitesNearly all spiders produce
venom, which they use to kill their prey.
However, only a few species, such as the
black widow in North America, are harm-
ful to humans. Antivenoms are available
for many dangerous spider bites.
spider naevusA red, raised pinhead-
sized dot, from which small blood
vessels radiate, due to a dilated minor
artery and its connecting capillaries.
Small numbers of spider naevi are com-
mon in children and pregnant women,
but in larger numbers, they may indicate
liver disease. (See also telangiectasia.)
spina bifidaA congenitaldefect that is a
type of neural tube defectin whichpart
of 1 or more vertebrae fails to develop
completely. As a result, a portion of the
spinal cordis left exposed.SPERMATIC CORD SPINA BIFIDA
S
SPHYGMOMANOMETERStethoscopeSphygmomanometerInflatable
cuff