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.