BONES AND JOINTS
=180.= Bones make the body stiff and strong, and give it shape. Long
bones reach through the arms and legs, and little bones reach down the
fingers and toes. Rounded plates of bone form the head, and a pile of
bony rings makes up the backbone. Each bone is built to fit exactly
into its own place and to do its own work. In all there are over two
hundred bones in the body. They form one seventh of its weight.
=181. Form of bones.=--A bone is not like a solid piece of timber, but
is hollow like the frame of a bicycle. This makes it strong and light.
At its ends a bone is like a hard sponge covered with a firm shell.
This makes it too strong to be easily crushed, and keeps it light.
A bone grows like any other part of the body. It is made of living cells
like woven threads. Lime is mixed among the cells, and makes them stiff
like starch among the threads of a linen collar. Blood tubes go through
every part of the bone so as to feed the cells. The living cells form
one third of the bone, while the lime forms two thirds.
=182. Broken bones.=--Bones are very hard, and yet they can bend a
little without breaking. Most of them are curved a little, and so they
will spring instead of breaking when they are pressed hard. But
sometimes they break. Then a person must wear a splint and bandage to
keep the bones in place until they grow together again. The living
cells will mend a bone in about a month.
An old person's bones are more tender than a child's, and will not
spring much without breaking. An old man is afraid of falling and
breaking his bones, while a child falls a dozen times a day without
danger.
The bones of some children bend too easily. When they stand, the bones
of their legs bend a little. After a while they grow in the crooked
shape, and the child is bow-legged.
=183. Joints.=--Some bones are hinged upon each other. A bone hinge is a
_joint_. The rings of the backbone are held together by very tough pads
of flesh. Each pad lets the backbone bend only a little, but altogether
they let us bend our backs in any direction. These pads are like rubber
springs in a wagon, and keep our bodies from being jarred too much.
The finger and toe joints, the wrists and ankles, the elbows and the
knees, bend back and forth like a hinge. Tough bands of flesh bind the
bones together. The ends of the bones are rounded and smooth. They fit
together and make perfect hinges. The joints are oiled by a fluid like
the white of an egg. In old people this fluid sometimes dries up. Then
the joints become stiff, and creak like a squeaking hinge.
[Illustration: =Hinge joint of the elbow.=
1 humerus
2 ulna]
The shoulders and hips can be moved in every direction. The upper ends
of the arm and leg bones are round like half a ball. They fit into cups
on the shoulder and hip bones. They are very smooth, and are oiled like
the hinge joints. The joints are made to work very smoothly and easily.
=184. Bones out of joint.=--When the ends of bones are torn away from
each other, the bone is out of joint. Then the bone cannot be moved
without great pain. It should be put back in place at once and kept
there by splints and bandages. A person is less liable to have his
joints out of place than he is to have his bones broken.
=185. Sprains.=--Sometimes a joint is turned too much. This stretches
the flesh around the joint, and makes it very tender and painful. This
is a _sprain_. When you sprain a joint, you should put it in hot water
for an hour or two. Then keep it still for a few days.
=186. Why bones and joints grow wrong.=--While bones and joints are
growing they can be made to take any shape we please. They cannot be
bent all at once, but if we hold them in one way much of the time,
they will keep that shape. Some boys and girls sit with their backs
bent forward and lean against the desk as if they were too lazy to sit
up. When they grow up, they will be bent and round-shouldered. You
should sit and stand straight. Then you will grow tall and straight
and strong. A soldier has square shoulders and walks erect because he
is drilled until his bones and joints grow in the proper shape. As you
stand straight with your feet together, your two big toes, your two
ankles, and your two knees should touch each other.
If you wear tight shoes and press the toes out of shape, they will
soon grow so. Nearly every one's feet are out of shape from wearing
short, pointed shoes. Your toes should be straight and not cramped by
the shoe. If you wear narrow shoes, you may harm your feet. It is
better to have one's feet useful, even if they are large, than to make
them small and useless.
WHAT WE HAVE LEARNED
1. Bones make the body stiff, and give it form.
2. Some bones are long, some round, and some flat. All are hard
and springy.
3. Some bones are hinged together. The hinge is a joint.
4. The ends of bones in joints are rounded and smooth, and are
oiled with a liquid like the white of an egg.
5. Some bones are bound together by springy pads, as in the
backbone.
6. Bones can be broken. They will grow together again themselves.
7. Joints can be put out of place; then we must put them back.
8. If joints or bones are kept in wrong positions they will grow
into bad shapes. Tight shoes deform the feet.
[Illustration: =The muscular system.=]