_breast-bone_, the _pelvis_, and the _back-bone_. The bones of the trunk
form a framework to support and protect the various organs within its
cavities.

~6. The Ribs.~--There are twelve _ribs_ on each side. The ribs join the
back-bone at the back. They are connected by cartilage to the
breast-bone in front. They look somewhat like the hoops of a barrel.
With the breast-bone and the back-bone they form a bony cage to contain
and protect the heart and the lungs.

~7. The Pelvis.~--The pelvis is at the lower part of the trunk. It is
formed by three bones, closely joined together. The large bones at
either side are called the hip-bones. Each hip-bone contains a deep
round cavity in which the upper end of the thigh-bone rests.

~8. The Back-bone.~--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.