The rib cage includes the breastbone and twelve pairs of ribs. It serves two purposes: to protect the heart and lungs from injury; and to take part in the movements of breathing. The latter function involves some degree of motion of the rib cage. All the ribs are attached behind to the vertebral column in a fashion that permits of a little motion up and down. All except the last two are fastened in front, seven pairs to the breastbone, three pairs each to the rib above it. In breathing the breastbone and ribs are moved up and down by muscles attached to them. The shoulder girdle is made up of the collar bones and shoulder blades. Each collar bone is fastened at its inner end to the upper edge of the breastbone; this is the only direct contact the shoulder girdle has with any other part of the skeleton of the trunk; at the point where collar bone and shoulder blade meet, there is a shallow cup into which the head of the skeleton of the arm fits. The arrangement is favorable to great freedom of movement of the arm. Not only is the shoulder joint very flexible owing to the shallowness of the cup into which the arm bone fits, but the shoulder blade itself is capable of a considerable range of movement. This is because it is imbedded in and held in place by muscles. If one watches a person with bare shoulders while he raises his arms, it will be seen that the shoulder blades do not move much while the arms are being lifted to the horizontal position, but as that point is passed they begin to swing outward rapidly, so that when the arms are high above the head the shoulder blades are in a quite different position from that which they have when the arms are down. The hip girdle consists of five bones of the vertebral column welded firmly together to make up what is called the sacrum, and two other large bones known as the innominate bones, each of which, in turn, is made up of three bones tightly fused together. The innominate bones are firmly joined to the sacrum at the back and they meet in front, also in a firm joint. The hip girdle or _pelvis_ is rigid, suiting it to bear the strains that come upon it on account of its position at the junction of the legs with the trunk. At the outer side of each innominate bone is a cup, much deeper than the corresponding cup of the shoulder girdle, and into this fits the head of the skeleton of the leg. The arrangement is a typical ball and socket, and has been much copied in machinery where a flexible joint is required. In a good many people the union of the innominate bones to the sacrum is not so firm but that it yields somewhat when strains are put on it. Ordinary strains in these cases produce severe backache. Heavy strains may cause an excessively painful as well as disabling dislocation. In either case medical attention is needed.