EYE, EXAMINATION OF
moved by six delicate muscles. The eye
has a tough, white outer layer known as
the
sclera
.
At the front of the sclera,
the
cornea
(a transparent, thin-walled
dome) serves as the main “lens” of the
eye and does most of the focusing.
Behind the cornea is a shallow cham-
ber filled with watery fluid (called the
aqueous hum our), at the back of w hich
is the
iris
(the coloured part of the eye)
with its
pupil
(circular opening), w hich
appears black. Tiny muscles alter the
size of the pupil in response to changes
in light intensity in order to control the
amount of light entering the eye.
Behind the iris, and in contact with
it, is the crystalline lens, w hich is sus-
pended by fibres from a circular ring of
muscle: the
ciliary body
.
Contraction of
the ciliary body changes the shape of
the lens, enabling fine focusing.
The main cavity of the eye contains a
clear gel (the vitreous humour) and is
located directly behind the lens. On the
inside of the back of the eye is the retina,
a complex structure of nerve tissue that
is extremely sensitive to light. The retina
requires a constant supply of oxygen
and glucose. To meet this need, a thin
network of blood vessels, the choroid
plexus, lies immediately under it. The
choroid
is continuous at the front with
the ciliary body and the iris; these three
parts constitute the uveal tract.
CONJUNCTIVA AND EYELID
The eyeball is sealed off by a trans-
parent, flexible membrane called the
conjunctiva
,
w hich is attached to the skin
at the corners of the eye and forms the
inner lining of the eyelids. The conjunc-
tiva contains mucus-secreting glands.
These, together with the meibomian
glands in the eyelids (which secrete an
oily fluid), provide the tear film that
protects the cornea and conjunctiva.
Blinking, a protective
reflex
,
helps to
spread the tear film evenly over the
cornea to enable clear vision.
eye, artificial
A
prosthesis
used to replace an
eye
that has been surgically removed. An
artificial eye is worn purely for cosmetic
and psychological reasons and fits neat-
ly behind the eyelids into the cavity
from
w hich
the
natural
eye
was
removed. Some movement of the artifi-
cial eye may be achieved by attaching
the muscles that normally move the eye
to the remaining conjunctival mem-
brane (see
conjunctiva)
or to a plastic
implant placed in the eye socket.
Often called “ glass eyes", artificial
eyes were once made of glass; now an
easily mouldable plastic material is used.
eye-drops
Medication in solution that is used in
the treatment of eye disorders or to aid
in their diagnosis. Examples of drugs
given in this form are
antibiotic drugs
,
antihistamine drugs,
drugs used either to
dilate (widen) or constrict (narrow) the
pupil, the circular opening in the centre
of the iris, and
corticosteroid drugs.
A specified number of eye-drops are
applied to the inside of the lower eyelid
after the lid has been drawn down
using the tip of a clean finger. Blinking
helps to spread the medicated solution
evenly around the eyeball.
eye, examination of
An inspection of the structures of the
eyes,
either as part of a
vision test
or to
make a diagnosis when an eye disor-
der is suspected.
E
DISORDERSOFTHE EYE
Many
eye
disorders are minor, but some
can cause loss of vision unless treated.
(See also
disorders of the cornea
box;
disordersof the retina
box.)
Congenitaldefects
Squint
(misalignment of the eyes) may
be congenital (present at birth). Other
examples of congenital defects are
nystagmus
(uncontrollable, jerky eye
movements),
albinism
(absence of
pigment in the iris), and developmental
defects affecting the cornea and retina.
A rare birth defect is
microphthalmos
(abnormally small size of one or both
eyes).
Cataract
(opacity of the lens) can
sometimes be seen in newborn infants.
Infection
Conjunctivitis
(inflammation of the
conjunctiva) is the most common eye
infection.
Trachoma
(a persistent disease
of the cornea or conjunctiva) or severe
bacterial conjunctivitis can impair
vision. Corneal infections can lead to
blurred vision or corneal
perforation
if
not treated early. Endophthalmitis
(infection within the eye) can occur as
a result of injury to the eye or infection
elsewhere in the body.
Impaired blood supply
Narrowing, blockage, or
inflammation
of
retinal blood vessels may cause visual loss.
Tumours
Malignantmelanoma
of the
choroid
is the
most common cancerous tumour of the
eye.
Retinoblastoma
(a cancer of the retina)
most commonly affects children.
Basal cell
carcinoma
affects the eyelid and may result
from excessive exposure to sunlight.
Nutritional disorders
Various
vitamin
deficiencies affect the eyes.
Vitamin A deficiency may lead to
xeroph-
thalmia
(corneal and conjunctival
dryness),
night blindness,
or, eventually,
keratomalacia
(corneal softening and
disintegration).
Autoimmune disorders
Uveitis
(inflammation of the iris, choroid,
and/or ciliary body) may be caused by
infection or an
autoimmune disorder
(in
which the body’s immune system attacks
its own tissues), such as
ankylosing
spondylitis
and
sarcoidosis
.
Degeneration
Macular degeneration
of the retina is
common in elderly people, as is cataract.
Focusing disorders
Myopia
(shortsightedness),
hypermetropia
(longsightedness), and
astigmatism
are
relatively common.
Presbyopia
is a pro-
gressive, age-related loss of the ability
to focus at close range.
Amblyopia
(poor
vision in one eye unrelated to structural
abnormality) is often due to squint.
Other disorders
Glaucoma
(increased pressure within the
eyeball), can lead to permanent visual
loss. In
retinal detachment,
the retina lifts
away from the eye’s underlying layer.
INVESTIGATION
An
ophthalmoscope
and
slit-lamp
are
used to view the eye.Vision is evaluated
using
Snellen charts
and
refraction
tests.
291
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