drink enough may need to have fluids
pumped into their veins. If people eat
natural poisons, they must go to the
Salmonella bacteria that cause illness are
seen through a microscope.
46 Food Poisoning BRITANNICA STUDENT ENCYCLOPEDIA
hospital right away to have their stomachs
emptied.
#More to explore
Bacteria • Food and Nutrition
Foot
All animals with a backbone and legs
have a foot attached to the end of each
leg. Feet are made up of bones,
muscles, and connective tissue. At the
front, feet are divided into digits, or
toes. Hardened skin—in the form of
claws, nails, or hooves—covers the ends
of the digits.
While an animal is standing, its feet
touch the ground and support its
weight. The structure of feet also allows
animals to walk, run, and jump.
The Human Foot
The human foot has a heel, a middle
portion called an instep, and toes. The
bottom of the foot is called the sole. The
ball of the foot is the part of the sole
behind the toes.
Each foot has 26 bones. The bones form
two arches that allow humans to stand
and walk upright. One arch extends
from heel to toes. It absorbs the shock of
walking. The other arch reaches across
the ball of the foot. It helps to spread the
weight of the body over the foot. The
joints, or places where the bones meet,
let the foot bend easily.
Feet of Other Animals
Some animals—including raccoons,
monkeys, and apes—can use their feet
to grasp things. But like other animals,
they also use their feet to get around.
While walking, some animals, such as
bears and baboons, put the whole foot
on the ground. Others, including cats
and dogs, never put their heels down.
Still other animals, such as horses and
deer, walk on their hooves, which are at
the tip of one or two toes. Running on
tiptoe allows these animals to move very
quickly.
Mollusks, including snails and clams,
have a muscle called a foot that allows
them to move. What are called feet in
insects are just the lower parts of their
legs.
#More to explore
Hoof • Leg • Nail and Claw
A podiatrist is
a doctor who
specializes in
the care of
human feet.
Humans place their whole foot on the
ground while walking. Dogs keep their
heels off the ground.
BRITANNICA STUDENT ENCYCLOPEDIA Foot 47
Football
Millions of people in the United States
consider autumn to be football season.
Football is a very popular sport that is
played on high school fields, college
campuses, and in professional stadiums.
Football involves running, passing, kicking,
and body contact. A football game
is played between two teams of 11 players
each. A team scores points by moving
a ball across the other team’s goal
line. The team that scores more points
wins.
This game is sometimes called
American football. It is played mainly
in the United States. Canadian football
is very similar to the U.S. game. But in
most countries around the world the
word football refers to the sport also
known as soccer.
Playing Field and Equipment
American football is played on a grassy,
rectangular field. The field is 120 yards
(110 meters) long and 160 feet (49
meters) wide. White lines called end
lines and sidelines mark the edges of the
field.
Each end of the field has a goal line. The
goal lines are 100 yards (91 meters)
apart. The 10-yard areas between the
goal lines and the end lines are called the
end zones. A pair of goalposts connected
by a crossbar stands at the back of each
end zone.
Most of the action in a football game
takes place between the goal lines. White
lines run across this part of the field
every 5 yards (4.6 meters). These lines
help to show how far a team has moved
the football. Two sets of short lines,
called hash marks, cross these lines.
Officials use the hash marks to place the
ball on the field after play is stopped.
Because football is a rough game, players
wear equipment to protect themselves.
They wear padding on their shoulders,
hips, thighs, and knees. They also wear a
mouth guard and a helmet with a face
mask.
A football is an oval with pointed ends.
It is filled with air and usually covered
with leather.
Playing the Game
A football game begins with a kickoff.
One team, called the kicking team, kicks
the football to the other team, called the
receiving team. A player on the receiving
A football player carries the ball
while a player for the other team
tries to stop him.
48 Football BRITANNICA STUDENT ENCYCLOPEDIA
team catches the ball. The player then
runs toward the kicking team’s goal line.
The kicking team tries to stop him. Its
players can tackle, or throw down, the
ballcarrier. They can also push him out
of the playing area. This is called being
out of bounds.
The game continues from the spot
where the ballcarrier was stopped. An
official places the ball on the field. The
teams line up facing each other on
opposite sides of the ball. An imaginary
line called the line of scrimmage runs
through the football to each sideline. It
separates the two teams.
A football team has some players who
play offense and others who play
defense. The two teams take turns playing
offense and defense.
The team with the ball plays offense. It
tries to move the ball down the field for
