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Structure of Joints.—The meeting of two bones forms a joint (Exp. 4). Some of the joints are immovable. The skull bones join in zigzag lines called sutures, formed by the interlocking of sawlike projections (Fig. 35). These immovable joints are necessary for the protection of the brain, which is the most delicate of the organs. The brain attains almost its full size by the seventh year of life; its bony case needs to grow very little after that. The joints of the pelvis are also immovable. All movable joints have two cartilages, and as the bones turn, one cartilage slips over the other. There is an intermediate class of joints found between the vertebræ and where the ribs join the breastbone. These joints depend for their motion upon the flexibility and compressibility of their cartilages. They are called mixed, or elastic, joints, and allow slight motion. Such a joint has only one cartilage.

Kinds of Movable Joints.—The movable joints are found chiefly in the limbs. When one end of the bone is rounded and fits into a cuplike hollow, the joint allows motion in all directions, and is known as a ball-and-socket joint. The hip joints and shoulder joints are examples. A hinge joint allows motion in only two (opposite) directions; for example, the to-and-fro motion of the elbow. A pivot joint allows a rotary motion; examples, the first vertebra on the second, one bone of forearm upon the other. A gliding joint consists of several bones that slide upon one another, as at the wrists and ankles.

The Four Features presented by a Movable Joint (Fig. 36).—If not held in place, the bones would slip out of their sockets, hence there are ligaments, or tough bands,