Carolyn Warner, age 12, of Akron, Ohio, for her question:

HOW ARE THE BODY'S JOINTS HELD TOGETHER?

A place where two or more bones meet in the skeleton of the body is called a "joint." Bones are held together at the joint by strong ligaments that attach above and below the joint. A ligament is fibrous tissue that comes in cords, bands or sheets.

Movable joints are protected from wear and tear in several ways. A smooth layer of gristle, called "cartilage," covers the ends of bones that move over one another. The elasticity of this cartilage helps to lessen the force of sudden shock.

Also, the smooth quality of the cartilage makes a joint move easily. A liquid called synovial fluid keeps the joints moist and lubricated.

There are three main kinds of movable joints:

Hinge joints are those which permit a forward and backward joint in one plane, like the motion of a door on its hinges. The joints at the knee and fingers are hinge joints.

Pivot joints allow a rotating motion, such as the movement of the head from side to side. The elbow has both hinge and pivot joints.

Ball and socket joints allow the greatest freedom of movement. These joints are made up of a large round end of a long bone that fits into the hollow of another bone. The hip and shoulder have ball and socket joints.

The arms can move more freely than the legs because of the way the joints are arranged and because the shoulder blade is only loosely attached to the vertebral column by muscles and thin ligaments.

Another unique joint is the hip. A number of ligaments circle the bone like a collar to keep the joint in place.

Joints may be movable or fixed. Fixed joints are seams between bones that lie directly against each other. In case of a blow or an accident, fixed joints may absorb just enough shock to keep the bones from breaking. The joints of the cranium or covering of the brain are fixed joints and they protect the brain.

Joints are often sprained or dislocated. A sprain occurs when the ligaments around a joint are torn or badly stretched.

Serious sprains are painful and if neglected may result in stiffening of the joint.

Dislocated joints should be treated as soon as possible by a doctor.

Inflammation of the joints may result from infections or from disturbances such as arthritis.