[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 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.