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 low