zen 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 shoulde