Structures of the Eye
EYELIDS IRIS
LACRIMAL(TEAR) GLANDS PUPIL
CONJUNCTIVA ANTERIOR CHAMBER
SCLERA AQUEOUS HUMOR
CORNEA LENS
CHOROID POSTERIOR CHAMBER
RETINA CILIARY BODY
RODS CILIARY PROCESSES
CONES CILIARY MUSCLES
MACULA VITREOUS HUMOR
OPTIC DISK OPTIC NERVE
Functions of the EyeAncient philosophers viewed the eyes as the win-
dows 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 pro-
duction of MELATONIN, a HORMONErelated 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 NERVEis to provide a direct
conduit from the EYEto the brain through which
the brain receives about two thirds of the infor-
mation it processes about the environment out-
side the body.
The eye resides within the protective enclosure
of the orbit, a socket of BONEin 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-
JUNCTIVITISand 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 eye-
lashes, extending from the eyelids, also help keep
foreign objects from striking the eye and the eye-
brows channel sweat around the eyes.
Six muscles attach the eye to the orbit, func-
tioning in pairs as well as in coordination with
one another to move the eye. These muscles inte-
grate 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 MUSCLEin a
pair contracts, the other relaxes. Typically both
eyes move in tandem, which allows the eyes to
simultaneously focus on the same object. ThisTHE 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 ERRORSof vision). This sec-
tion, “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 HEALTHand disorders, and entries about the health conditions that can affect the eyes and vision.
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