Sperm are produced within the seminif-
erous tubules of the
and mature in
Production and develop-
ment of sperm cells is dependent on
secreted by the
Sperm production starts at
spermatic cord The structure in males
that runs from the abdomen to the
scrotum and contains the
reproductive system, male
spermatocele A harmless cyst of the
containing fluid and sperm.
spermicides Contraceptive preparations
They are usually recom-
mended for use with
degree of protec-
sunburn. It is a
measure of the
amount of UVB
radiation a sun-
number, the more
bone in the cen-
tre of the base of
hereditary An 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
At times, the rate of red cell destruction
exceeds the rate at which new cells can
be made in the bone marrow, leading to
. Other symptoms
and enlargement of the
spleen. Occasionally, crises occur (usu-
ally triggered by infection) in which all
are a fre-
leads to permanent improvement.
sphincter A ring of muscle around a
natural opening or passage that acts as
a valve, regulating inflow or outflow.
sphincter, artificial A surgically creat-
ed valve or other device used to treat or
prevent urinary or faecal
sphincterotomy A surgical procedure
that involves cutting the muscle that
closes a body opening or that constricts
the opening between body passages.
used for measuring
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
spider bites Nearly 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.
for many dangerous spider bites,
spider naevus A red, raised pinhead-
sized dot, from
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
spina bifida A
defect that is a
neural tube defect
in which part
fails to develop
completely. As a result, a portion of the
is left exposed.
a barrier device.
glass column or a digital display.