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The Epic Interactive Encyclopedia 1998
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Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
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Locomotion
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1992-09-04
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Locomotion (moving around) is of enormous
importance to all animals. Animals need to
move around to find food, to find a mate, to
escape from predators, and so on. Most
animals move around on four legs, as this is
a very efficient way of getting around. The
cheetah, for example, can run at up to 60
miles per hour on its four legs. Human
beings, however, move around on two legs.
Humans are not unique in this: all of the
flightless birds walk on two legs, and,
among the mammals, kangaroos and wallabies
hop around on two legs. No-one is quite sure
why humans evolved this trait, which appeared
approximately 4 million years ago in our
evolutionary history. It seems to have been
useful for several reasons, one of which is
that it leaves the hands free for carrying
things, which enabled humans to develop into
the tool-using species that we are today.
Locomotion relies on the interplay of three
systems. The skeletal system provides a
supporting framework for the body. The
muscular system provides the power which
moves everything around, and the nervous
system controls the muscular system to make
fine movements and balancing possible.
Muscles have the unique property of being
able to contract when stimulated. Each muscle
is made up of hundreds of muscle cells, which
are long and thin. Each muscle cell is
connected to a nerve fibre, which provides
the signal to contract. At this signal, the
muscle cell contracts to become shorter and
fatter. When all the muscle cells pull
together, the whole muscle becomes shorter
and fatter, and the pull is strong. This is
why a strong man's arms bulge as he lifts a
heavy weight. Not all of the cells in a
muscle contract at once. If this were the
case, the pull would be so strong it might
tear the muscle. Instead, complex neurone
patterns in the spinal cord ensure that the
muscle cells pull in turn so that each gets a
chance to rest. In addition, even when a
muscle is fully relaxed, some of the cells
are still contracting. This is called muscle
tone. If you put your finger in your ear, the
low rumble you can hear is the sound of your
muscles working. The human body has thousands
of muscles. The forearm and hand together
have 40 muscles, which all have individual
functions. Some muscles are very large, such
as the gluteus maximus muscle, which forms
the bulk of the buttock. The longest muscle
in the body is the sartorius muscle in the
front of the thigh. Some are very tiny, such
as the muscles in the eye which allow it to
focus. Muscles which bend limbs are called
flexors. A good example of a flexor is the
biceps muscle, which bends the elbow. Muscles
which straighten limbs are called extensors.
A good example of this is the triceps muscle,
which straightens the elbow. Bending and
straightening limbs is what makes us move
around. When we walk, we hold one leg
straight, bend the other, swing it forwards
and put it down, transfer our weight onto it,
then hold it straight while we move the first
leg. This is a very complicated process, but
it is completely unconscious in most people.
This is because it is so well controlled by
the brain that we no longer need to think
about how we walk, we just simply do it. The
parts of the brain which control walking are
the basal ganglia and cerebellum, but the
spinal cord is also extremely important.
Equally important in locomotion are the
joints between the bones which provide free
movement. Most joints involved in movement
are called synovial joints. In a synovial
joint, the two bone ends are covered in
smooth white cartilage, which is rubbery to
absorb shock and slippery to allow smooth
movement. The cartilage surfaces slide
smoothly over each other, lubricated by
synovial fluid which is produced by the
joint. Synovial joints are so smooth that the
friction between the two surfaces is less
than between two plates of wet ice. Most
joints are quite complicated in their range
of movements. The knee, for example, is not a
simple hinge but allows a little sideways
movement and a little rotation as well. Very
mobile joints such as the shoulder allow a
wide range of movement in all directions.
Subject by: Dr Aidan M. O'Donnell