yawning An involuntary act, or reflex
action, usually associated with drowsi-
ness or boredom. The mouth is opened
wide and a slow, deep breath is taken
through it in order to draw air into the
lungs. The air is then slowly released.
Yawning is accompanied by a momen-
tary increase in the heart-rate, and, in
many cases, watering of the eyes.
The purpose of yawning is unknown,
but one theory suggests it is triggered
by raised levels of carbon dioxide in the
blood; thus, its purpose could be to
reduce the level of carbon dioxide and
increase that of oxygen in the blood.
yaws An infectious disease that tends
to be found throughout poorer subtrop-
ical and tropical areas of the world.
Yaws is caused by a
ral-shaped bacterium), and it spreads
principally in conditions of poor hygiene.
The infection is almost always acquired
in childhood, and it mainly affects the
skin and bones.
The bacteria enter the body through
abrasions in the skin. Three or 4 weeks
after infection, an itchy, raspberry-like
growth appears at the site of infection,
sometimes preceded by fever and pains.
Scratching spreads the infection and
causes more growths to develop else-
where on the skin. Without treatment,
the growths heal slowly over the course
months, but recurrence is
common. In about 10 per cent of un-
treated cases, widespread tissue loss
eventually occurs. This may eventually
lead to gross destruction of the skin,
bones, and joints of the legs, nose,
palate and upper jaw.
Yaws can be cured by a single large
dose of a
given as an
injection into muscle.
Y chromosome A
is present in every normal male body
cell. It is paired with an
and is absent in every female body cell.
Each sperm carries either a single X or a
single Y chromosome.
Unlike the X chromosome, the Y chro-
mosome carries little genetic material.
Its major function is to stimulate the
development of the
There are no significant diseases related
to abnormalities of the Y chromosome,
but hairy ears is a trait thought to be
determined by a Y-linked gene.
yeasts Types of
in which the body
of the fungus comprises individual cells
that occur either singly, in pairs, or in
longer chains. Certain yeasts can cause
infections of the skin or mucous mem-
branes; the most important of these
disease-causing yeasts is
yellow fever An infectious disease of
short duration and variable severity that
is caused by a virus transmitted by mos-
quitoes. In severe cases, the skin yellows
, from which the name
yellow fever derives. The infection may
be spread from monkeys to humans in
forest areas through various species of
mosquito; and in urban areas it can be
transmitted between humans by
Today, yellow fever is contracted only in
Central America, parts of South America,
and a large area of Africa. Eradication of
the causative mosquito from populated
areas has greatly reduced its incidence.
Yellow fever is characterised by a sud-
den onset of fever and headache, often
with nausea and nosebleeds and, despite
the high fever, a very low heart-rate. In