upper left side of the belly. An adult’s
stomach is about 10 inches (25 centimeters)
long. It can expand to hold as
much as 1 quart (0.9 liter) of food.
Glands in the stomach produce gastric
juices. These juices partly break down
food. A lining of mucus protects the
inside of the stomach from the gastric
juices.
Stomach muscles mix food and gastric
juices together. They also move the food
toward the small intestine. Food may
remain in the stomach from about one
hour to more than five hours, depending
on the type of food. Fats remain in the
stomach much longer than starches and
sugars do. After the stomach empties,
the stomach muscles continue to move.
This causes hunger pangs.
Stomachs of Other Animals
The stomachs of some animals have
more than one chamber, or section.
Each chamber has its own job. Cows
and other animals that eat grassy food
have four separate chambers in their
stomachs. Birds have three chambers.
#More to explore
Digestive System • Food and Nutrition
• Intestines
Stone Age
The Stone Age was a time in prehistory
when humans made and used stone
tools. (Prehistory is the time before
people invented writing.) Early humans
began using stones as simple tools about
2 million years ago. Humans used
mainly stone tools until about 10,000
years ago. However, the Stone Age began
and ended at different times in various
parts of the world.
Scientists divide the Stone Age into two
main periods: the Paleolithic Period and
the Neolithic Period. Sometimes scientists
put another period, called the
Mesolithic, in between these two.
Paleolithic Period
Paleolithic means “old stone age.” At
the beginning of the Paleolithic Period,
early humans made chopping tools out
of stones. These tools did not change
much for thousands of years. Then
humans learned to chip flakes off stone.
With this method they made better
tools, such as scrapers and chisels. By
about 40,000 years ago humans were
attaching stone blades to handles made
of bone or antler.
Some Paleolithic people lived in caves.
Others lived under rock overhangs or
The stomach is one of the main organs of
the human digestive system. It is connected
to the esophagus at one end and the small
intestine at the other end.
188 Stone Age BRITANNICA STUDENT ENCYCLOPEDIA
out in the open. They gathered wild
plants to eat. Their tools helped them to
hunt animals. Groups moved with the
seasons to find food. In the later Paleolithic
Period some groups made small
sculptures out of clay, stone, or bone.
Some groups painted or carved designs
on rocks or cave walls.
Neolithic Period
Neolithic means “new stone age.” The
Neolithic Period began about 10,000
years ago. During this period people
ground and polished stone to make
more useful tools. They also learned to
grow crops and to tame animals. They
began to settle in villages. They learned
to make pottery and to weave cloth and
baskets.
The Neolithic way of life first appeared
in southwestern Asia. Over several thousand
years it spread northward to
Europe and eastward to India and East
Asia. People in the Americas developed
Neolithic skills on their own. By about
6500 BC North Americans were growing
corn, beans, and squash.
End of the Stone Age
By about 5,000 years ago people in
Greece and China were making tools
from bronze (a mixture of copper and
tin). This development marked the end
of the Stone Age and the beginning of
the Bronze Age. The Bronze Age began
later in other parts of the world. Some
places never had a Bronze Age. In the
Americas, for example, the development
BRITANNICA STUDENT ENCYCLOPEDIA Stone Age 189
of agriculture and towns brought an end
to the Stone Age.
#More to explore
Bronze Age • Cave • Human Origins
• Prehistoric Life
Stonehenge
Stonehenge is a circular group of huge
stone blocks in southern England. It is
located near the city of Salisbury in the
county ofWiltshire. People built Stonehenge
in stages between 5,000 and
3,500 years ago. No one knows for sure
why Stonehenge was built. Many scholars
think that it was a place of religious
worship.
Today only ruins of Stonehenge are still
standing. Hundreds of years ago builders
took many of the stones for use in other
places. Other stones have fallen down.
But the ruins still give an idea of what
Stonehenge looked like in prehistoric
times.
The largest stones were arranged in two
groups. One was a circle of flat stones
that rested on other stones that stood
upright. The other group was inside the
circle. It was a horseshoe-shaped group
of five pairs of upright stones, with a flat
stone on top of each pair. These stones
weighed as much as 50 tons each.
Stonehenge also included smaller stones
that weighed up to 4 tons each. Some of
these stones had to be moved 240 miles
(385 kilometers) from the place where
they were mined. No one knows how
this was done.
A straight path called the Avenue led
away from the circles. An upright stone
called the Heel Stone still stands near
the center of the Avenue. On the longest
day of the year (usually June 21), a person
standing in the center of the stone
circle can see the sun rise directly above
the Heel Stone. This has led some
