HOW THE BODY MOVES
=The Need of a Framework.=--The body needs a stiff framework made of
bones for three purposes. One purpose is to give it shape, a second
purpose is to help the body move, and a third one is to protect from
injury some of the delicate organs, such as the heart and brain.
The bones are nowhere separate but are joined together with tough
bands named _ligaments_. All the bones together form the _skeleton_.
All animals from fish to man have a skeleton. Many of the lower
creatures, such as worms and flies, have no bony skeleton. Most of
these move sluggishly or have a hardened outer covering, like beetles
and wasps. The skeleton of animals such as the cat, rabbit, or cow,
has about the same number of bones as man, and they are arranged in
the same way.
=Of what a Bone is Made.=--Although the bones are so hard, they are
not dead. They contain blood, have feeling, and are just as much
alive as the softer parts of the body. It is the lime that makes them
stiff. This can be eaten out by putting the bone in strong vinegar or
other acid for a few days. A long bone will then become so limber that
it can be tied into a knot.
The living part of a bone can be burned out by placing it on hot coals
for a half hour. At the end of this time the bone will look just as
before, but when it is touched, will crumble to pieces.
=Forms of Bones.=--The bones of the legs and arms are hollow. This
form gives the greatest strength with the least weight. You can prove
this by using two sheets of paper. Roll one sheet and fold the other
one. Hang weights on both ends of each and use the finger for a
support in the middle.
The cavity of these bones is filled with a soft white substance called
_marrow_. This is largely fat. Each bone is surrounded by a tough
membrane to which the muscles are attached.
=Arrangement of the Bones.=--The bones of the head form the _skull_.
The other bones of the body not belonging to the _limbs_ make up the
_trunk_. There are over two hundred bones in the entire body. Eight of
these form a case for the brain. Fourteen give shape to the face. A
chain of twenty-six bones named _vertebrae_ forms the backbone.
[Illustration: FIG. 81.--Photograph of the bones of the skeleton.]
Twelve pairs of _ribs_ encircle the chest. They are fastened behind to
the backbone. The front parts of the ribs are made of pieces of
gristle. The seven upper pairs are joined to the breastbone. The five
lower pairs are named _false ribs_.
The _collar bone_ is in front of the shoulder and behind it is the flat
_shoulder blade_. There is one bone in the upper part of each arm and
leg and two bones in the lower part of each limb. Twenty-eight small
bones are found in the hand, while twenty-seven are present in the foot.
=How the Bones may be Injured.=--In the young some of the entire bones
and parts of many others are soft like gristle. For this reason, the
bones of the young seldom get broken, but they are easily bent and
pressed out of their natural shape. On this account you should hold the
body erect in sitting and walking. Bending over the table or desk day
after day is not only likely to cause round shoulders, but is sure to
squeeze up the lungs and other organs so they cannot do their best work.
Sitting at a table or desk, so that one shoulder is higher than the
other or carrying books at the side, so that they rest on the hip may
cause a curve sidewise in the backbone. Tight clothing about the waist
presses the ribs out of shape and hurts the other organs within the
body.
[Illustration: FIG. 82.--How the bones are held together. A piece has
been cut out of the tough ligament to show the cup of the hip bone
into which the head of the thigh bone fits.]
=Caring for Broken Bones.=--When a bone of the arm or leg is broken, the
muscles tend to make the ends shove over each other. The broken ends are
sometimes sharp, and if the limb is bent, these may tear through the
flesh. This may be prevented by binding a board firmly on opposite sides
of the limb across the broken part. This will hold the bones in place
until the surgeon comes and will also allow the patient to be moved.
The surgeon will set the broken bones by bringing the ends together
and holding them in place by sheets of wood or metal firmly held by a
bandage. In a few days the membrane around the bone begins to grow new
bone to join the broken parts.
=How the Bones are joined together.=--The two general classes of
joints are the _movable_ and _immovable_. Except the lower jaw, the
bones of the skull are so tightly joined together that there is no
motion between them. The bones of the wrist and back have but little
movement. The freest motion is at the shoulder joint, where the round
head of one bone fits into the shallow cup of another. This is called
a _ball and socket joint_. Such a joint is found also at the hip. At
the elbow and knee the bones move back and forth like a hinge and
these are named _hinge joints_.
=Working Parts of a Joint.=--The ends of the bones are covered with a
thin layer of gristle. The bones are held in place by several strong
bands called _ligaments_ (Fig. 82). These entirely surround the joint.
On their inner sides is a delicate membrane which gives out a slippery
fluid to make the joint work easily.
The ligaments are sometimes strained, stretched, or torn by a fall.
The joint then swells because the watery part of the blood collects
there. A sprained limb should be elevated to prevent swelling. Bathing
it in very hot water is helpful.
=The Muscles.=--The muscles form the lean meat in any animal. They make
up about one half the weight of the body. Each muscle is a bundle of
thousands of little threads held together by other finer threads, while
the whole is surrounded by a thin sheet. Little bundles formed of
several of these threads called fibers may be seen in a piece of cooked
beef picked to pieces. There are over five hundred muscles in the body.
[Illustration: FIG. 83.--Fifty of the muscles just under the skin.
Note the white cords, the tendons in the regions of the hands and feet.]
[Illustration: FIG. 84.--The biceps muscle contracted above and
relaxed or loosened below.]
Some of the muscles are more than a foot long and have the shape of a
ribbon. Some are circular like those around the mouth, eyes, and
stomach, while others are large in the middle and taper toward the ends.
=How the Muscles are fastened to the Bones.=--The two ends of a muscle
are attached to different bones. In many cases the muscle is not
joined directly to the bone, but is connected to a tough white cord
called a _tendon_. The tendon is then fixed to the bone.
Several of the muscles in the forearm run into tendons in the wrist
because if the muscle part were to extend along the wrist, this part
of the arm would be large and clumsy instead of graceful and slender.
Some of these tendons may be seen to move by bending the wrist and
then working the fingers.
=How the Muscles do their Work.=--A tiny nerve thread runs from the
spinal cord or brain to every muscle thread. Messages sent through the
nerve threads to the muscles make them act. A muscle can act in only
two ways (Fig. 84). It can become shorter or longer. When it gets
shorter, we say it _contracts_. When it stretches out, it is said to
_relax_.
A muscle cannot contract more than one fourth of its length. To pull
the forearm up, the brain sends a message to the muscle fixed by one
end at the shoulder and by the other end to a bone at the elbow. The
muscle at once becomes shorter and thicker, as may be felt by placing
the fingers on it. Although it shortens only two inches it is fastened
to the bone so near the elbow that it draws the hand up two feet.
PRACTICAL QUESTIONS
1. Of what use are the bones?
2. What animals have bony skeletons?
3. What can you say of the form of bones?
4. How many bones in the body?
5. Name six bones.
6. What part of the arm has two bones side by side?
7. How many ribs have you?
8. Explain how a broken bone should be cared for.
9. Point out and name two kinds of joints.
10. What are ligaments?
11. Of what is a muscle made?
12. How many muscles in the body?
13. How many tendons can you feel in your wrist?