lungs, and other organs in the upper
body.
The skeletal system also protects the
central nervous system, made up of the
The human skeleton is mostly bone but has
some cartilage. The skeleton supports and
protects the body.
96 Skeletal System BRITANNICA STUDENT ENCYCLOPEDIA
brain and the spinal cord. Inside the
head, the skull surrounds and protects
the brain. The vertebrae, or bones of the
spine, surround and protect the nerves
of the spinal cord.
Movement
The third major job of the skeletal system
is movement. The skeleton works
with the muscles to allow the body to
move in many different ways. Many
muscles are attached to at least two
bones in the skeleton. When the brain
tells a muscle to move, the muscle pulls
on or pushes its attached bones.
How the Skeletal System Is
Connected
Tough, leathery tissues called ligaments
hold together the bones of the skeleton.
Muscles also hold together some of the
bones. In most cases, however, a cord
called a tendon connects muscles to
bones.
At the points where bones connect, the
bones have a protective covering called
cartilage. If two hard bones constantly
rubbed together without this covering,
they would wear down and become
damaged. Cartilage is tough enough to
protect the bones but elastic enough
not to become damaged itself. It is
found at the ends of many bones and
in joints, such as the knees and the
hips. Cartilage also connects parts of
the rib cage. The cartilage allows the
ribs to move in and out during
breathing.
Cartilage can be damaged, however, and
it is not easily repaired. As people get
older, the cartilage in some joints can
wear down. This allows the bones of
the joints to rub together painfully. In
such cases doctors can replace the joints
with artificial joints made of plastic or
metal.
#More to explore
Bone • Exoskeleton • Muscle • Nervous
System
Skiing
Skiing is a winter sport that involves the
use of skis. Skis are long, mostly flat
strips of metal, wood, or plastic. Skiers
attach them to their shoes or boots and
use them to glide over snow, often down
a mountain slope. Many people ski for
fun. Skiing can be fast and exciting, but
it can also be dangerous.
Skiing events are a major feature of the
Winter Olympic Games. Four types of
skiing events are held at the Olympics:
An infant’s
skeleton is
made of
cartilage that
is gradually
replaced by
bone as the
infant grows
into an adult.
A view of a knee joint shows the different
kinds of connective tissue that hold together
the human skeleton.
BRITANNICA STUDENT ENCYCLOPEDIA Skiing 97
events include downhill and slalom
racing. Downhill racing is the fastest and
most dangerous of all skiing events. The
skier goes down a long, steep course that
may have ruts and large bumps. Along
the course are gates formed by single
poles or pairs of poles. The skier must ski
between the gates and stay on the course.
The winner is the skier who completes
the course in the shortest time.
Slalom racing is similar to downhill racing.
However, a slalom course is shorter
and more winding than a downhill
course. The slalom course requires the
skier to make high-speed turns. The
giant slalom and the supergiant slalom,
or super-G, take place on longer versions
of a slalom course.
Freestyle Skiing
Freestyle skiing combines skiing and
acrobatics. There are three freestyle
events: acro, aerials, and moguls. An
acro skier performs jumps, flips, and
spins while skiing on a gently sloping
hill. An aerial skier does similar tricks
in the air after jumping off a ramp. A
mogul skier races over and around large
bumps of snow, called moguls. The
mogul skier also makes two jumps and
does tricks in the air. In freestyle events
judges score the skiers on different
skills.
Snowboarding
Snowboarding developed in the United
States from the sports of surfing and
skateboarding. It is like surfing on snow.
A snowboard looks much like a skateboard
without wheels.
Snowboarding has slalom, giant slalom,
and supergiant slalom (or super-G)
competitions. These events are similar to
the Alpine slalom events. The snowboarder
who completes the course in the
shortest time is the winner.
There are also freestyle snowboarding
events. They take place on a half-pipe,
which is a ramp built from snow. The
ramp looks like the lower half of a huge
flat-bottomed tube with one open end.
A freestyle skier on a moguls course goes
over many bumps. On an aerials course,
the skier jumps from a ramp to perform flips
and twists in the air.
BRITANNICA STUDENT ENCYCLOPEDIA Skiing 99
Snowboarders ski from side to side of
the ramp, picking up speed. They fly up
from the top of the ramp and do tricks
in the air. Judges rate the performances.
History
Skis found in Sweden and Finland are
thought to be between 4,000 and 5,000
years old. A rock carving of two men on
skis, found in Norway, dates from 2000
BC. People in China have skied since at
least the AD 600s. Skiing was used in
warfare in Norway as early as 1200.
Troops in Sweden, Finland, Russia, and
Poland also used skis.
Cross-country is the oldest form of sport
skiing. Cross-country competitions
began in Norway in the 1840s. By the
