Encyclopedia of Health and Medicine

THE EYES
The eyes conduct the function of vision. Practitioners who provide care for the eyes and vision may be ophthalmologists
(medical doctors who specialize in ophthalmology, providing medical and surgical treatment for diseases of the EYE) or
optometrists (doctors of optometry who specialize in diagnosing and correcting REFRACTIVE ERRORS of vision). This section,
“The Eyes,” presents a discussion of the structures of the eye and how they function to provide the sense of sight,
an overview of VISION HEALTH and disorders, and entries about the health conditions that can affect the eyes and vision.
EYELIDS
LACRIMAL (TEAR) GLANDS
CONJUNCTIVA
SCLERA
CORNEA
CHOROID
RETINA
RODS
CONES
MACULA
OPTIC DISK
Structures of the Eye
IRIS
PUPIL
ANTERIOR CHAMBER
AQUEOUS HUMOR
LENS
POSTERIOR CHAMBER
CILIARY BODY
CILIARY PROCESSES
CILIARY MUSCLES
VITREOUS HUMOR
OPTIC NERVE
Functions of the Eye
Ancient philosophers viewed the eyes as the windows
to the soul, based on the belief that the
PINEAL GLAND, located deep within the BRAIN, held
the soul. Their rudimentary understanding of
anatomy and physiology led them to conclude
that the optic nerves connected the pineal gland
and the soul directly to the outside world through
the eyes. Though modern knowledge of the
body’s structure and function clarifies that no
such physical pathway exists, ancient scientists
were not entirely off track. The pineal gland does
appear to receive direct information from the
external environment, which influences its production
of MELATONIN, a HORMONE related to the
body’s circadian cycles (cycles of wakefulness and
sleep). Researchers do not fully understand the
mechanisms of this, and it is possible the OPTIC
NERVE plays some role. However, the primary
function of the optic NERVE is to provide a direct
65
conduit from the EYE to the brain through which
the brain receives about two thirds of the information
it processes about the environment outside
the body.
The eye resides within the protective enclosure
of the orbit, a socket of BONE in the skull. Thin
pads of fat cover the orbital bones to cushion the
eye. A small opening in the back of the orbit
allows passage of the optic nerve and the blood
vessels that supply the eye. The eyelids, upper and
lower, blink—automatically open and close—15
to 20 times a minute to rinse the eye with tears.
Reduced blink rate is a characteristic of neurologic
disorders such as PARKINSON’S DISEASE; increased
blink rate occurs with eye irritation such as CON-
JUNCTIVITIS and diseases such as MENINGITIS. The
tears then drain from the lacrimal sac at the inner
corner of the eye into the upper NOSE. The eyelids
also close to protect the eye from hazards such as
foreign objects and very bright light, and to cover
the eye during sleep to keep it moist. The eyelashes,
extending from the eyelids, also help keep
foreign objects from striking the eye and the eyebrows
channel sweat around the eyes.
Six muscles attach the eye to the orbit, functioning
in pairs as well as in coordination with
one another to move the eye. These muscles integrate
into the sclera, the fibrous outer layer of the
eye, and extend to the back of the orbit where
they anchor to the bone. When one MUSCLE in a
pair contracts, the other relaxes. Typically both
eyes move in tandem, which allows the eyes to
simultaneously focus on the same object. This