extrinsic asthma
Any form of
precipitated by an
environmental factor. Extrinsic asthma
is usually due to an
to a foreign
substance, such as inhaled particles, cer-
tain foods, or a particular drug.
extrinsic factor
An alternative name for
vitamin B12.
A person whose main interest is in other
people and the outside world. Extroverts
are active, sociable, and have many inter-
ests. (See also
introvert; personality.)
The discharge of fluid from
blood vessels
into surrounding tissues. Exuded fluid
Most exudates are a result of inflamma-
tion. In inflamed tissue, the small blood
vessels widen and tiny pores in the ves-
sel walls enlarge, allowing fluid and
cells (mainly
white blood cells
to escape.
exudation cyst
(a fluid-filled lump or swelling)
formed when an existing body cavity
fills with fluid due to
(fluid-filled swelling in the scrotum)
is an example of an exudation cyst.
exudative retinitis
Coat’s disease
The organ of sight. The eye is a complex
organ, consisting of a series of structures
that focus an image on to the
light-sensitive inner layer at the back of
the eye) and nerve cells that convert this
image into electrical impulses. These
impulses are carried by the
optic nerve
the visual cortex (an area on the back
surface of the brain concerned with
) for interpretation.
The eyes work in conjunction with
one another, under the control of the
brain. They align themselves on a partic-
ular object so that a clear image of the
object is formed on each retina. If nec-
essary, the eyes may sharpen the image
by altering focus using an automatic
w hich
changes the shape of the
The eyeballs lie in pads of fat within the
(eye sockets) that provide
protection from injury. Each eyeball is
The eye is a complex organ that focuses light rays to form an
image on the retina, the light-sensitive inner layer at the back
of the eyeball. The cornea and lens focus the rays. The pupil
controls the amount of light entering the eye and the ciliary
body alters the shape of the lens to adjust the focus.
The retina has millions of nerve cells that respond to light;
they convert the image into a pattern of nerve impulses,
which are transmitted to the brain via the optic nerve.
S c l e r a
O p t ic n e r v e
V it r e o u s h u m o u r
L e n s
Movement of the eye
Movement of each eyeball occurs as the
result of contraction of one or more of the
muscles attached around it. There are six
ofthese muscles, each ofwhich pulls the
eye in a specific direction.
I r is
P u p il
C i l i a r y b o d y
L a c r im a l
d u c t s
The external eye
Behind the
transparent cornea
isthe fluid-filled
front chamber ofthe
eye. At the back of
this is the iris, with
the round pupil in
the centre.
B o n y e y e s o c k e t
S u p e r i o r r e c t u s
m u s c l e
L a c r im a l g l a n d
I r is
L a t e r a l r e c t u s
m u s c l e
I n f e r i o r o b l i q u e
m u s c l e
S c l e r a
P u p il
F a t
L a c r im a l
d u c t s
L a c r im a l
s a c
I n fe r io r
r e c t u s
m u s c l e
C o r n e a
I r is
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