~--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