the mouth to the stomach. In a process
called peristalsis, the walls of the esophagus
move in waves to push the food
down the tube to the stomach.
Stomach
In the walls of the stomach, millions of
glands produce digestive juices. The
juices contain substances called enzymes
and hydrochloric acid. These substances
help to break down the food particles.
The stomach churns the food into a
thick liquid called chyme. This process
does not take place all at once. Starch
Many organs are involved in the human
digestive system.
BRITANNICA STUDENT ENCYCLOPEDIA Digestive System 47
and sugar are easily digested, so they
remain in the stomach for only a couple
of hours. Proteins remain for three to
five hours. Fats may remain in the stomach
even longer.
Small Intestine
From the stomach, peristalsis pushes the
chyme into the small intestine. The
small intestine is the longest part of the
digestive system. Food remains in the
small intestine for several hours.
Two large glands, the liver and the pancreas,
are connected to the small intestine
by ducts, or tubes. The glands send
digestive juices to the small intestine
through these ducts. Juice from the pancreas
helps to digest carbohydrates, proteins,
and fats. The liver makes bile,
which helps to digest fat. Glands in the
small intestine also produce digestive
fluids.
All these fluids break down most of the
remains of the food into simple chemicals.
These chemicals enter the bloodstream
through the walls of the small
intestine. The blood carries the chemicals
to the body’s cells. The body then
uses those chemicals as fuel for energy
and growth.
Large Intestine
Some substances, like plant fibers, cannot
be broken down. These substances
pass from the small intestine into the
large intestine, or colon. The large intestine
is much shorter and wider than the
small intestine. The waste material
slowly moves through the colon over a
period of 10 to 20 hours.
The first section of the colon absorbs
fluids and salts. The second section
removes more water. This process turns
the waste material from a liquid to a
solid. The last section of the colon is a
A grasshopper’s digestive system is basically
a long tube divided into sections.
A frog’s digestive system has many similarities
to the human digestive system.
48 Digestive System BRITANNICA STUDENT ENCYCLOPEDIA
holding area for waste. From there the
waste passes out of the body through the
anus.
#More to explore
Intestines • Mouth • Stomach
Dike
#see Levee and Dike.
Dili
Population
(2004 estimate)
51,700
Dili is the capital of East Timor, a country
in Southeast Asia. East Timor takes
up the eastern part of Timor Island. Dili
has been its capital for hundreds of
years. However, East Timor was not an
independent country until 2002.
Dili is East Timor’s largest city. It is also
the country’s main port and center of
trade.
People have lived on Timor Island for
thousands of years. Portuguese settlers
founded Dili in about 1520. It became
the capital of Portugal’s colony on the
island. East Timor declared its independence
from Portugal in 1975. However,
Indonesia soon captured East Timor.
In 1999 Indonesia began preparing to
allow East Timor to become independent.
Indonesian groups that opposed
this killed hundreds of East Timor’s
people. Dili was badly damaged, but it
later began to rebuild. In 2002 East
Timor became an independent country
with Dili as its capital.
#More to explore
East Timor
Dinosaur
Dinosaurs were the main animals on
Earth for more than 150 million years.
They were lizardlike reptiles. Some of
them were the largest and scariest creatures
that ever walked on land. The
word dinosaur comes from Greek words
meaning “terrible lizard.”
The last dinosaurs became extinct, or
died out, about 65 million years ago.
Fossils of dinosaurs were first discovered
in the early 1800s. Fossils are remains or
impressions of a plant or animal that
have been preserved in rock. By studying
these fossils, scientists know that dinosaurs
were the ancient cousins of today’s
crocodiles, snakes, and lizards. Scientists
also believe that today’s birds descended
from dinosaurs.
When and Where
Dinosaurs Lived
The study of the history and physical
features of Earth is called geology. Earth
began forming about 4.6 billion years
ago. Earth underwent many changes
from then until the present. Geologists
call this vast time period geologic time.
They break geologic time into smaller
time periods. Dinosaurs lived from
about 215 to 65 million years ago. They
At least 1,000
types of dinosaur
are
known. Paleontologists
(scientists who
study fossils)
discover new
types almost
every year.
BRITANNICA STUDENT ENCYCLOPEDIA Dinosaur 49
were common during the Triassic, Jurassic,
and Cretaceous geologic time periods.
Not all the species, or types, of
dinosaur lived at the same time. They
also did not all become extinct at the
same time. They died out gradually.
Dinosaurs lived throughout the world,
and their fossils have been found on
every continent. They lived in all different
kinds of environments as well, ranging
from tropical forests to dry, sandy
deserts.
Physical Features and Behavior
