=Caring for Broken Bones.=--When a bone of the arm or leg is broken, the muscles tend to make the ends shove over each other. The broken ends are sometimes sharp, and if the limb is bent, these may tear through the flesh. This may be prevented by binding a board firmly on opposite sides of the limb across the broken part. This will hold the bones in place until the surgeon comes and will also allow the patient to be moved. The surgeon will set the broken bones by bringing the ends together and holding them in place by sheets of wood or metal firmly held by a bandage. In a few days the membrane around the bone begins to grow new bone to join the broken parts. =How the Bones are joined together.=--The two general classes of joints are the _movable_ and _immovable_. Except the lower jaw, the bones of the skull are so tightly joined together that there is no motion between them. The bones of the wrist and back have but little movement. The freest motion is at the shoulder joint, where the round head of one bone fits into the shallow cup of another. This is called a _ball and socket joint_. Such a joint is found also at the hip. At the elbow and knee the bones move back and forth like a hinge and these are named _hinge joints_. =Working Parts of a Joint.=--The ends of the bones are covered with a thin layer of gristle. The bones are held in place by several strong bands called _ligaments_ (Fig. 82). These entirely surround the joint. On their inner sides is a delicate membrane which gives out a slippery fluid to make the joint work easily. The ligaments are sometimes strained, stretched, or torn by a fall. The joint then swells because the watery part of the blood collects there. A sprained limb should be elevated to prevent swelling. Bathing it in very hot water is helpful. =The Muscles.=--The muscles form the lean meat in any animal. They make up about one half the weight of the body. Each muscle is a bundle of thousands of little threads held together by other finer threads, while the whole is surrounded by a thin sheet. Little bundles formed of several of these threads called fibers may be seen in a piece of cooked beef picked to pieces. There are over five hundred muscles in the body. [Illustration: FIG. 83.--Fifty of the muscles just under the skin. Note the white cords, the tendons in the regions of the hands and feet.] [Illustration: FIG. 84.--The biceps muscle contracted above and relaxed or loosened below.] Some of the muscles are more than a foot long and have the shape of a ribbon. Some are circular like those around the mouth, eyes, and stomach, while others are large in the middle and taper toward the ends. =How the Muscles are fastened to the Bones.=--The two ends of a muscle are attached to different bones. In many cases the muscle is not joined directly to the bone, but is connected to a tough white cord called a _tendon_. The tendon is then fixed to the bone.