LEARNING
difficulties, and
cause
kidney
problems
and
hearing
difficulties. In adults, it can
harm
the
kidneys, and
the
nervous and
digestive
systems.
DIAGNOSIS AND TREATMENT
Lead
poisoning
may
be
confirmed
by
blood
and
urine tests.
C h e l a t i n g a g e n t s
,
such
as
p e n i c i l l a m i n e
,
may
be
prescribed;
they
bind
to
the
lead
and
enable
the
body
to
excrete
it at a
faster rate.
le a rn in g
The
process by
which
know
ledge
or
abilities are
acquired, or by
which
behaviour is modified.
Various theories about learning
have
been
proposed. Behavioural theories
emphasize
the
role of
c o n d i t i o n i n g
,
and
cognitive
theories are
based
on
the
con-
cept that learning
occurs through
the
building
of abstract “cognitive”
models,
using
mental capacities such
as insight,
m e m o r y
,
i n t e l l i g e n c e
,
and
understanding.
No
one
theory, however, can
account
for the
complexities of learning.
le a rn in g d iffic u lt ie s
L
Problems w
ith
l e a r n i n g
,
which
result
from
a
range
of mental and
physical
problems. Possible
causes include
d e a f -
n e s s
,
s p e e c h d i s o r d e r s
,
and
disorders of
v i s i o n
,
as well as genetic and
chromoso-
mal problems. Learning
difficulties may
be
either general or specific.
In general learning
difficulties, all
aspects of mental and
physical function-
ing
may
be
affected. Depending
on
the
severity
of the
problem
, a
child
w
ith
general learning
difficulties may
need
to
be
educated
in a
special school.
Specific learning
difficulties include
d y s l e x i a
(difficulty in reading
and/or
writing), dyscalculia
(inability to
solve
mathematical problems), and
dysgra-
phia
(a
writing disorder). An
affected
child
has problems in a
particular area
of learning
but has normal intelligence.
Treatment includes correction
of any
physical problems that interfere
w
ith
learning, such
as vision
or hearing
problems, and
specialized
teaching.
le a rn in g d is a b ilit y
A
l e a r n i n g d i f f i c u l t y
.
L e b e r’s h e re d it a ry o p tic a tro p h y
See
o p t i c a t r o p h y
.
le c ith in
A
p h o s p h o l i p i d
(fatty
substance) that is
an
important component of cell mem
-
branes. Lecithin
is found
in foods such
as egg
yolk, offal, and
whole grains, and
can
be
extracted
from
soya
to
be
used
as an
emulsifier in processed
foods. (See
also
n u t r i t i o n . )
le e c h
A
type
of bloodsucking
worm
w
ith
a
flattened
body
and
a
sucker at each
end.
Leeches of various types mainly inhabit
tropical forests and
waters. They
bite
painlessly, introducing
their saliva
into
the
wound
before
sucking
blood. Leech
saliva
contains an
anticlotting
substance
called
hirudin, which
may
cause
the
wound
to
bleed
for hours.
Use of leeches to drain blood
Leeches such as this are used in medicine (to drain
a haematoma from the outer ear following injury,
for example). The bites are painless, but the saliva
contains an anticlotting agent, and the wound may
bleed for several hours.
Leeches are
sometimes used
in medi-
cine in order to
drain
a
h a e m a t o m a
(collection
of blood) from
a
wound. In
addition, sometimes leeches are
used
in an
effort to
improve
the
circulation
in tissues after surgery.
le ft a triu m
The
upper left chamber of the
h e a r t
. In
each
heartbeat, the
left atrium
fills w
ith
oxygenated
blood, which
travels from
the
lungs via the
pulmonary veins. The
m i t r a l v a l v e
then
opens, allow
ing
blood
to
be
squeezed
into
the
l e f t v e n t r i c l e ,
to
be
pumped
around
the
body.
le ft b u n d le b ra n c h b lo c k
See
h e a r t b l o c k .
le ft v e n t ric u la r fa ilu re
A
disorder in which
the
left ventricle
fails to
empty
normally, despite
in-
creased
pressure
from
the
blood
w
ithin
it. (See
also
h e a r t f a i l u r e . )
le g , b ro k e n
See
f e m u r , f r a c t u r e o f , f i b u l a - , t i b i a .
L e g g -C a lv e -P e rth e s ’ d is e a s e
See
P e r t h e s ’ d i s e a s e .
le g io n n a ir e s ’ d is e a s e
A
form
of
p n e u m o n i a
that is caused
by
L
e g i o n e l l a
p n e u m o p h i l a
, a
bacterium
that
breeds in warm
, moist conditions and
stagnant water. Legionnaires’ disease
can
occur in outbreaks. The
source
of infec-
tion
is often
an
air-conditioning
system
in a
large
public building; the
disease
is
contracted
by
the
inhalation
of droplets
of contam
inated
water.
The
first symptoms include headache,
muscular and
abdom
inal pain, diarrhoea,
and
a
dry
cough. Over the
next few
days,
pneumonia
develops, resulting
in high
fever, shaking
chills, coughing
up
of
thick
s p u t u m
(phlegm
), drowsiness, and
sometimes
d e l i r i u m
.
Liver and
kidney
damage
may
occur.
Treatment is w
ith
antibiotic drugs
such
as
c l a r i t h r o m y c i n
and
r i f a m p i c i n
.The
majority of people
recover but mortali-
ty
rates are
higher among
the
elderly.
le g , s h o r t e n in g o f
Shortening
of the
leg
is usually
caused
by
faulty
healing
of a
fractured
f e m u r
(thigh-bone)
or
t i b i a
(shin).
Other
causes are
an
abnormality
present from
birth, surgery
on
the
leg, or muscle
weakness associated
w
ith
p o l i o m y e l i t i s
or
another neurological disorder.
le g s , r e s t le s s
See
r e s t l e s s l e g s
.
le g u lc e r
An
open
sore
on
the
leg
that is slow
to
heal, usually
resulting
from
poor blood
circulation
in the
area.
TYPES
There
are
two
main
types of leg
ulcer:
venous (also
called
gravitational or sta-
sis) and
arterial. Venous ulcers are
by
far
Venous ulcer on the leg
This type of ulcer, also known as a stasis ulcer, is
caused by impaired drainage of blood from the leg
by the veins. It is usually accompanied by oedema
(fluid accumulation) in the lower leg.
462
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