Facts on File Encyclopedia of Health and Medicine

eye The organ of vision. The paired eyes work in
coordination to present NERVEimpulses the BRAIN
interprets as dimensional (stereovisual) images.
The function of sight requires close integration
among the structures of the eye, the neurologic sys-
tem, and the muscular system. Each eye is a fluid-
filled, elongated globe of fibrous tissue, about^1 ⁄ 4
inch from front to back and 1 inch from top to bot-
tom and side to side, contained within the protec-
tive cavity of the orbital socket in the skull. The
OPTIC NERVE, the second cranial nerve, provides a
direct pathway from the back of the eye to the
brain. Six muscles move each eye up and down,
from side to side, and in rotation. These muscles
direct the eye toward objects within the VISUAL FIELD
and hold the eyes steady.
The process of vision begins when lightwaves
enter the eye through the CORNEA, a transparent
portion of the eye’s tough outer layer, the sclera.
The cornea’s convex front surface initially refracts
the lightwaves for preliminary focusing. The
cornea is soft and flexible but fixed; it does not
adjust or move. The LENS, a transparent and flexi-
ble convex disk behind the cornea, further refracts
the lightwaves. Tiny muscles at the edge of the
lens, the ciliary muscles, cause the lens to thicken
or flatten to adjust the degree of refraction for
optimal focus. The resulting light pattern strikes
the RETINA, activating the specialized cells that

detect color (cones) and brightness (rods). These

cells convert the light to nerve impulses that con-

verge at the back of the retina at the optic disk,

their portal to the optic nerve. The optic nerve

conveys the signals to the brain, which interprets

them as images.

COMMON CONDITIONS OF THE EYE

AMBLYOPIA ASTIGMATISM

BLEPHARITIS CATARACT

CONJUNCTIVITIS CORNEAL INJURY

DRY EYE SYNDROME EYE STRAIN

FLOATERS GLAUCOMA

HORDEOLUM HYPEROPIA

ISCHEMIC OPTIC NEUROPATHY KERATITIS

KERATOCONUS MYOPIA

NIGHT BLINDNESS PRESBYOPIA

RETINAL DETACHMENT RETINITIS PIGMENTOSA

RETINOPATHY STRABISMUS

VISION IMPAIRMENT

For further discussion of the eye within the

context of ophthalmologic structure and function

please see the overview section “The Eyes.”

See also AGING, VISION AND EYE CHANGES THAT

OCCUR WITH; CRANIAL NERVES.

eye pain Sensations discomfort involving the EYE

and its supporting structures. Eye PAINmay vary

eye pain 89

COMMON CAUSES OF EYE PAIN

Quality of Pain Possible Causes Medical Attention Required
itchy or scratchy sensation DRY EYE SYNDROME, ALLERGIC self-care such as artificial tears or
CONJUNCTIVITIS, dirty contact ANTIHISTAMINE MEDICATIONS; timely doctor’s
lenses,EYE STRAIN assessment if symptoms persist after self-care
efforts to relieve them

burning and PHOTOPHOBIA, may CONJUNCTIVITIS, HORDEOLUM, prompt doctor’s assessment; infections require
include discharge CHALAZION, BLEPHARITIS, KERATITIS, antibiotic medications
DACRYCYSTITIS, ENTROPION

burning, photophobia, excessive BULBOUS KERATOPATHY, CICATRICIAL immediate medical attention
tearing, visible blisters on surface PEMPHIGOID, corneal abrasion
of the eye

sharp, deep, or intense pain that may ORBITAL CELLULITIS, trauma to eye, medical emergency
increase with, or prevent, eye GLAUCOMA, optic neuritis, chemical
movement or flash BURNS