OUR BONES AND THEIR USES.
~1. The Bones.~--In an earlier chapter we learned something about the
bones. This we must try to recall. You will remember that we called the
bones the framework of the body, just as the timbers which are first put
up in building a house are called its frame.
~2. The Skeleton.~--All the bones together make up the _skeleton_. (See
page 95.) There are about two hundred bones in all. They are of many
different shapes. They vary in size from the little bones of the ear,
which are the smallest, to the upper bone of the leg, which is the
largest in the body.
~3.~ The skeleton is divided into four parts: the _skull_, the _trunk_,
the _arms_, and the _legs_. We must learn something more about the bones
of each part.
~4. The Skull.~--The _skull_ is somewhat like a shell. It is made of a
number of bones joined together in such a way as to leave a hollow place
inside to hold the brain. The front part of the skull forms the
framework of the face and the jaws. In each ear there are three curious
little bones, which aid us in hearing.
~5. The Trunk.~--The bones of the trunk are, the _ribs_, the
_breast-bone_, the _pelvis_, and the _back-bone_. The bones of the trunk
form a framework to support and protect the various organs within its
cavities.
~6. The Ribs.~--There are twelve _ribs_ on each side. The ribs join the
back-bone at the back. They are connected by cartilage to the
breast-bone in front. They look somewhat like the hoops of a barrel.
With the breast-bone and the back-bone they form a bony cage to contain
and protect the heart and the lungs.
~7. The Pelvis.~--The pelvis is at the lower part of the trunk. It is
formed by three bones, closely joined together. The large bones at
either side are called the hip-bones. Each hip-bone contains a deep
round cavity in which the upper end of the thigh-bone rests.
~8. The Back-bone.~--The _back-bone_, or spinal column, is made up of
twenty-four small bones, joined together in such a way that the whole
can be bent in various directions. The skull rests upon the upper end of
the spinal column. The lower end of the back-bone forms a part of the
pelvis.
[Illustration: SKELETON OF A MAN.]
~9. The Spinal Canal.~--Each of the separate bones that make up the
back-bone has an opening through it, and the bones are so arranged, one
above another, that the openings make a sort of canal in the back-bone.
By the connection of the spinal column to the head, this canal opens
into the cavity of the skull. Through this canal there passes a peculiar
substance called the _spinal cord_, of which we shall learn more at
another time.
~10. The Arms.~--Each of the arms has five bones, besides the small
bones of the hand. They are the _collar-bone_, which connects the
shoulder to the breast-bone, the _shoulder-blade_, at the back of the
shoulders, the _upper arm-bone_, between the shoulder and the elbow, and
the two _lower arm-bones_, between the elbow and the wrist. There are
eight little bones in the wrist, five in that part of the hand next to
the wrist, and fourteen in the fingers and thumb.
~11. The Legs.~--The bones of the leg are the _thigh_ or _upper
leg-bone_, the _knee-pan_ or _knee-cap_, which covers the front of the
knee, the two bones of the _lower leg_, the _heel-bone_ and six other
bones in the _ankle_, five bones in that part of the foot next to the
ankle, and fourteen bones in the _toes_.
~12. Use of the Bones.~--The skeleton is not only necessary as a
framework for the body, but it is useful in other ways. Some of the
bones, as the skull, protect delicate parts. The brain is so soft and
delicate that it would be very unsafe without its solid bony covering.
The spinal cord also needs the protection which it finds in the strong
but flexible back-bone. The bones help to move our hands and arms, and
assist us in walking.
~13. The Joints.~--The places where two or more bones are fastened
together are called _joints_. Some joints we can move very freely, as
those of the shoulder and the hip. Others have no motion at all, as
those of the bones of the skull.
~14. Cartilage.~--The ends of bones which come together to form a joint
are covered with a smooth, tough substance, which protects the bone from
wear. This is called _gristle_ or _cartilage_. You have, no doubt, seen
the gristle on the end of a "soup-bone" or on one of the bones of a
"joint of beef."
~15.~ The joint contains a fluid to oil it, so that the ends of the
bones move upon each other very easily. If the joints were dry, every
movement of the body would be very difficult and painful.
~16.~ The bones are held together at the joints by means of strong bands
called _ligaments_.
~17. How the Bones are Made.~--The bones are not so solid as they seem
to be. The outside of most bones is much harder and firmer than the
inside. Long bones, like those of the arms and the legs, are hollow. The
hollow space is filled with _marrow_, in which are the blood-vessels
which nourish the bone.
~18. An Experiment.~--If you will weigh a piece of bone, then burn it in
the fire for several hours, and then weigh it again, you will find that
it has lost about one third of its weight. You will also notice that it
has become brittle, and that it seems like chalk.
~19. Why the Bones are Brittle.~--The hard, brittle portion of a bone
which is left after it has been burned contains a good deal of chalk and
other earthy substances, sometimes called bone-earth. It is this which
makes the bones so hard and firm that they do not bend by the weight of
the body. When we are young, the bones have less of this bone-earth, and
so they bend easily, and readily get out of shape. When we get old, they
contain so much bone-earth that they become more brittle, and often
break very easily.
~20.~ A person's height depends upon the length of his bones. The use of
alcohol and tobacco by a growing boy has a tendency to stunt the growth
of his bones, so that they do not develop as they should.
SUMMARY.
1. There are about two hundred bones in the body.
2. All together they are called the skeleton.
3. The skeleton is divided as follows:
_a._ The skull.
{ Ribs.
_b._ The trunk. { Breast-bone.
{ Pelvis.
{ Back-bone.
{ Collar-bone.
{ Shoulder-blade.
{ Upper arm-bones.
_c._ The arms. { Lower arm-bones.
{ Wrist.
{ Hand and fingers.
{ Thigh.
{ Knee-pan.
_d._ The legs. { Lower-leg bones.
{ Ankle, including heel-bone.
{ Foot and toes.
4. The bones are useful for support, protection, and motion.
5. The place where two bones join is called a joint.
6. The tough substance which covers the ends of many bones is called
cartilage or gristle.
7. The joints are enabled to work easily by the aid of a fluid secreted
for that purpose.
8. The ends of the bones are held together in a joint by means of
ligaments.
9. Bones are about two thirds earthy matter and one third animal matter.
10. The use of alcohol and tobacco may prevent proper development of the
bones.