=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 lower part of each limb. Twenty-eight small bones are found in the hand, while twenty-seven are present in the foot. =How the Bones may be Injured.=--In the young some of the entire bones and parts of many others are soft like gristle. For this reason, the bones of the young seldom get broken, but they are easily bent and pressed out of their natural shape. On this account you should hold the body erect in sitting and walking. Bending over the table or desk day after day is not only likely to cause round shoulders, but is sure to squeeze up the lungs and other organs so they cannot do their best work. Sitting at a table or desk, so that one shoulder is higher than the other or carrying books at the side, so that they rest on the hip may cause a curve sidewise in the backbone. Tight clothing about the waist presses the ribs out of shape and hurts the other organs within the body. [Illustration: FIG. 82.--How the bones are held together. A piece has been cut out of the tough ligament to show the cup of the hip bone into which the head of the thigh bone fits.]