Bma Illustrated Medical Dictionary

the retinaat the back of the eye and to

protect the front of the eye. It is kept

moist by tears produced by the lacrimal

glandand the mucus- and fluid-secret-

ing cells in the eyelids and conjunctiva.

cornea, disorders of Injuries or dis-

eases affecting the cornea, the outer

shell of the eyeball. Injuries include

corneal abrasions,which sometimes be-

come infected and progress to a corneal

ulcer. Penetrating corneal injuries can

cause scarring, which may lead to im-

pairment of vision. Chemical injuries can

result from contact with a corrosive

substance and require immediate flush-

ing of the eye with water.

In actinic keratopathy, the outer layer

of the cornea is damaged by ultraviolet

light. In exposure keratopathy, damage

is due to reduced protection by the tear

film and blink reflex. The cornea can also

be infected by viruses, bacteria, and fungi,

the herpes simplexvirus being especially

dangerous. True inflammation of the cor-

nea (called keratitis) is uncommon as

the cornea contains no blood vessels.

Other disorders include: keratomalacia

as a result of vitamin A deficiency; ker-

atoconjunctivitis sicca(dry eye); corneal

dystrophies such as keratoconus; and

oedema, in which fluid builds up in the

cornea and impairs vision. Rare con-

genital defects include microcornea

(smaller cornea than normal) or mega-

locornea (bigger than normal) and

buphthalmos, or “ox-eye’’, in which the

entire eyeball is distended as a result of

glaucoma. Degenerative conditions of the

cornea such as calciumdeposition, thin-

ning, and spontaneous ulceration occur

mainly in the elderly, and are more

common in previously damaged eyes.

corneal abrasionA scratch or defect

in the epithelium(outer layer) of the

corneacaused by a small, sharp particle

in the eye (see eye, foreign body in) or

by an injury. Corneal abrasions usually

heal quickly but may cause severe pain

and photophobia. Treatment includes

covering the eye with a patch, analgesic

drugsto relieve pain, and, if the eye

muscles go into spasm, eyedrops con-

taining cycloplegic drugs (which paralyse

the ciliary muscle, preventing accommo-

dation).Antibioticeyedrops are usually

given to prevent bacterial infection,

which can lead to a corneal ulcer.

corneal graftThe surgical transplanta-

tion of donor corneal tissue to replace a

damaged cornea. In most grafts, tissue is

taken from a human donorafter death.

The success rate of corneal grafts is

generally high, because the cornea has

no blood vessels; this reduces access

for white blood cells, which can cause

rejectionof the donor tissue.

corneal transplantSee corneal graft.

corneal ulcerA break, erosion, or open

sore in the cornea commonly caused by

a corneal abrasion. It may also be due to

chemical damage, or infection with bac-

teria, fungi, or viruses (particularly with

the herpes viruses). Eye conditions such

as keratoconjunctivitis siccaand eyelid

deformities such as entropionor ectro-

pionincrease the risk of an ulcer.

Corneal ulcers are revealed by intro-

ducing fluoresceindye into the eye.

Infections and predisposing eye condi-

tions are treated according to their

cause. A superficial, noninfectious ulcer

usually heals quickly; if it fails to do so,

it may be treated with a “bandage’’ con-

tact lens or tarsorrhaphy.

coronaryAny structure that encircles

like a crown. The term usually refers to

the coronary arteries. It is also some-

times used as a nonmedical term for a

heart attack (see myocardial infarction).

coronary arteryEither of the 2 main

arteriesthat supply the tissues of the

heartwith oxygen-rich blood. These are

CORNEA, DISORDERS OF CORONARY ARTERY

C

Coronary

vein

Left

circumflex

artery

Inferior

vena cava

Right

main

coronary

artery

Pulmonary

veins

Left

anterior

descending

artery

CORONARY ARTERY

Left main

coronary

artery

Descending

aorta

Superior

vena cava

Aorta