eyes moist. Eyelashes catch dust and
other particles that could enter the eye.
Six muscles hold the eyeball in place and
move it up, down, and sideways.
The eyeball is filled with a clear jelly.
Three coats, or layers, surround this
jelly. The outer coat consists of the
sclera, or white part of the eye, and the
cornea. The cornea is the window of the
eye, through which light passes.
The middle coat holds the eye’s main
blood supply. The iris, or colored part of
the eye, makes up the front part of this
In 1971 U.S. astronauts spent almost three
days on the Moon. They traveled on the
Moon’s surface using the Lunar Roving
Vehicle.
The lens of a human eye focuses light on the
retina.
BRITANNICA STUDENT ENCYCLOPEDIA Eye 175
coat. Light passes through an opening,
called the pupil, in the center of the iris.
The iris opens and closes to allow more
or less light through the pupil. Behind
the iris sits the lens. The lens focuses
light onto the inner coat of the eye. The
muscle that holds the lens in place also
adjusts its shape. To focus on objects that
are near, the lens thickens. To focus on
objects that are far away, the lens flattens.
The inner coat of the eye is called the
retina. Cells called rods and cones are
built into the retina. These cells collect
light. As the cells collect light, they trigger
electrical signals. These signals travel
through the optic nerve to the brain.
Problems with the Eye
When the eyeball is too long from front
to back, a person cannot see faraway
objects clearly. This condition is called
nearsightedness. Similarly, farsighted
people have trouble seeing nearby
objects. Their eyeballs are too short.
Glasses, contact lenses, or surgery can
correct these problems. Blindness happens
when the eye’s lens becomes cloudy
or when the retina is damaged.
Eyes of Other Animals
Vertebrates, or animals with a backbone,
each have two eyes similar to human
eyes. But there are differences, which
depend on the animal’s habitat and
behavior. For example, the pupils of
animals active at night are often shaped
like slits. This allows the pupils to close
almost completely to protect the eyes
from bright light.
Invertebrates, or animals without a
backbone, have various other kinds of
eyes. Some mollusks, including octopuses,
have large eyes that are similar to
vertebrate eyes. Other mollusks and
spiders have simple eyes called ocelli.
Insects have compound eyes. These are
made up of many lenslike parts.Worms
and some protozoans merely have eyespots
that are sensitive to light.
#More to explore
Blindness • Brain • Lens • Light
Many animals
have two eyes
that look in
different directions.
This
makes it
harder for
enemies to
sneak up on
them.
A gecko narrows its pupils to protect its
eyes from bright light. Geckos and other
animals that are active at night often have
slit-shaped pupils.
The eye of a giant Pacific octopus is large
and complex. It gives the octopus a sharp
sense of sight.
176 Eye BRITANNICA STUDENT ENCYCLOPEDIA
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Figs are called fruits, but they
are actually inside-out flowers.
(See Fig.)
Fireflies make light by mixing
chemicals in their body with
oxygen from the air.
(See Firefly.)
The flower of a plant is where
the plant’s seeds are produced.
(See Flower.)
A substance called cholesterol
can build up in blood vessels
and block the flow of blood.
(See Food and Nutrition.)
Fossil fuels, such as petroleum,
are the remains of plants and
animals that lived long ago.
(See Fossil Fuel.)
Benjamin Franklin said, “Early
to bed and early to rise, makes
a man healthy, wealthy, and
wise.”
(See Franklin, Benjamin.)
Fff
Fable
A fable is a kind of story that teaches a
lesson. Fables are usually entertaining
tales featuring animals that talk and
behave as people do. Because they have
humanlike qualities, the animals show
how foolish or wise people can be. For
example, “The Three Little Pigs” teaches
that hard work is important: The house
built by the hardest-working pig is the
only one that survives the wolf’s attacks.
Some of the oldest fables came from
India and Greece thousands of years ago.
Many fables that are familiar in English
today—including “The Hare and the
Tortoise” and “TheWolf in Sheep’s
Clothing”—are said to have been written
in ancient Greece by a man named
Aesop. These tales have been translated
