A bone grows like any other part of the body. It is made of living cells like woven threads. Lime is mixed among the cells, and makes them stiff like starch among the threads of a linen collar. Blood tubes go through every part of the bone so as to feed the cells. The living cells form one third of the bone, while the lime forms two thirds. =182. Broken bones.=--Bones are very hard, and yet they can bend a little without breaking. Most of them are curved a little, and so they will spring instead of breaking when they are pressed hard. But sometimes they break. Then a person must wear a splint and bandage to keep the bones in place until they grow together again. The living cells will mend a bone in about a month. An old person's bones are more tender than a child's, and will not spring much without breaking. An old man is afraid of falling and breaking his bones, while a child falls a dozen times a day without danger. The bones of some children bend too easily. When they stand, the bones of their legs bend a little. After a while they grow in the crooked shape, and the child is bow-legged. =183. Joints.=--Some bones are hinged upon each other. A bone hinge is a _joint_. The rings of the backbone are held together by very tough pads of flesh. Each pad lets the backbone bend only a little, but altogether they let us bend our backs in any direction. These pads are like rubber springs in a wagon, and keep our bodies from being jarred too much. The finger and toe joints, the wrists and ankles, the elbows and the knees, bend back and forth like a hinge. Tough bands of flesh bind the bones together. The ends of the bones are rounded and smooth. They fit together and make perfect hinges. The joints are oiled by a fluid like the white of an egg. In old people this fluid sometimes dries up. Then the joints become stiff, and creak like a squeaking hinge. [Illustration: =Hinge joint of the elbow.= 1 humerus 2 ulna] The shoulders and hips can be moved in every direction. The upper ends of the arm and leg bones are round like half a ball. They fit into cups on the shoulder and hip bones. They are very smooth, and are oiled like the hinge joints. The joints are made to work very smoothly and easily. =184. Bones out of joint.=--When the ends of bones are torn away from each other, the bone is out of joint. Then the bone cannot be moved without great pain. It should be put back in place at once and kept there by splints and bandages. A person is less liable to have his joints out of place than he is to have his bones broken. =185. Sprains.=--Sometimes a joint is turned too much. This stretches the flesh around the joint, and makes it very tender and painful. This is a _sprain_. When you sprain a joint, you should put it in hot water for an hour or two. Then keep it still for a few days.