Cross-section of a corn
A plug of dead skin cells extends through the
epidermis into the dermis, which has a nerve
supply. Pressure on the plug can cause pain.
that forms the front of the eyeball. The
cornea has two main functions. It helps
to focus light rays on to the
back of the eye, and protects the front
of the eye from debris and injury.
The cornea is joined at its circumfer-
ence to the
(white of the eye);
and the coloured
visible beneath it. There are five layers.
The outermost layer (the epithelium)
protects the eye and absorbs oxygen
and nutrients from tears. The central
layer (the stroma) is by far the thickest
and gives the cornea its form. The inner
layer (the endothelium) expels excess
fluid from the cornea, thus keeping the
For the cornea to stay healthy, it must
be kept moist and clean. It is kept moist
by a film of
w hich are produced
and by the mucus-
and fluid-secreting cells in the eyelids
Further protection is
provided by the eyelids, w hich blink or
close to keep out debris. In addition,
the cornea is very sensitive, and imme-
diately registers the presence of any
injury or foreign body.
A scratch or defect in the
(outer layer) of the
caused by a
small, sharp particle in the eye (see
foreign body in
or by an injury. Corneal
abrasions usually heal quickly but may
cause severe pain and
includes covering the eye with a patch
to relieve pain. If the cil-
iary muscles go into spasm, eyedrops
containing cycloplegic drugs may be
used to paralyse the
eyedrops are usually given to
prevent bacterial infection (w hich can
lead to a
The surgical transplantation of donor
corneal tissue to replace a damaged
In most grafts, tissue is taken
from a human
after death. The
success rate of corneal grafts is gener-
ally high, because the cornea has no
blood vessels; this reduces access for
white blood cells, w hich could other-
of the donor tissue.
A break, erosion, or open sore in the
It usually affects the outer layer
of the cornea, but in some cases may
penetrate down to the middle layer.
Corneal ulcers are commonly caused
They may also be
due to chemical damage, or infection
Eye conditions such
deformities such as
increase the risk of an ulcer.
Ulcers are revealed by introducing
dye into the eye. Infections
and predisposing eye conditions are
Various injuries or conditions can affect
the sensitive cornea.
(scratch) can become
infected and progress to a
Penetrating corneal injuries can cause
scarring, which may lead to im-
pairment of vision. Chemical injuries
can result from contact with a corrosive
substance such as an acid or alkali.
the outer layer
of the cornea is damaged by ultraviolet
light. In exposure keratopathy, damage
occurs as a result of reduced protection
from the tear film and blink reflex.
The cornea can be infected by viruses,
bacteria, or fungi. Some infections can
cause ulceration, the
virus being especially dangerous.
True inflammation of the cornea (called
) is uncommon because the
cornea contains no blood vessels.
Rare congenital defects include
microcornea (a cornea that is smaller
than normal) or megalocornea
(one that is bigger than normal) and
or “ox-eye’’, in which
the entire eyeball is distended as a
pressure in the eyeball).
Degenerative conditions of the cornea
include calcium deposition, thinning,
and spontaneous ulceration. Such
conditions occur mainly in elderly
people, and are more common in
previously damaged eyes.
Other disorders include:
which may result from vitamin A
(dry eye);corneal dystrophies, such
in which the cornea
becomes thinner and cone-shaped;
and oedema, in which fluid builds up
inside the cornea and impairs vision.