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.
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.
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.