The ribs are connected to
spine and protect the heart and lungs. A joint is where
or more bones meet. Some joints don't move much, like those in the skull — they allow the brain to grow during childhood, then knit together for protection. Other joints, like the elbow, work like a hinge, opening and closing in one direction. Still others allow for more complicated movement — a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement. Joints are classified by their range of movement: Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which move slightly during birth and then fuse together as the skull finishes growing. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone. Partially movable, or cartilaginous, joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility. Freely movable, or synovial (pronounced: sih-NO-vee-ul), joints move in many directions. The
joints of the body — such as those found at the hip, shoulders, elbows, knees, wrists, and ankles — are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily.
kinds of freely movable joints play a big part in voluntary movement: Hinge joints allow movement in one direction, as seen in the knees and elbows. Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side. Ball-and-socket joints allow the greatest freedom
movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.