~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. ~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. ~20.~ A person's height depends upon the length of his bones. The use of alcohol and tobacco by a growing boy has a tendency to stunt the growth of his bones, so that they do not develop as they should. SUMMARY. 1. There are about two hundred bones in the body. 2. All together they are called the skeleton.