Angela Bruno, age 10, of Montgomery, Ala., for her question:

HOW MANY RIBS DOES A PERSON HAVE?

Enclosing the chest in the human body are 24 rib bones. There are 12 ribs on each side of the body, each connected to the verterbrjl column or backbone by small joints called costovertebral joints.

In the front of the body, the uppermost seven ribs on each side connect to the sternum or breastbone and are called true ribs. The five lower ribs, called false ribs, do not connect directly to the sternum. Each of the upper three false ribs is attached to the rib above with cartilage or gristle. The lowest two are attached only to the backbone. They are know as floating ribs.

The spaces between the ribs, called intercostal spaces, contain arteries, veins, muscles and nerves.

Most vertebrates or animals with backbones have ribs. The number, however, varies. Some whales have only nine pairs while a two toed sloth has 24 pairs.

The ribs form a cage around the chest cavity that protects the heart and lungs. They also move up and down and, together with the diaphragm, control the movement of air in and out of the lungs.