Lisa Schiefer, age 9, of Columbus, Ohio, for her question:

HOW MANY RIBS DOES A PERSON HAVE?

Enclosing the chest in the human body are 24 rib bones, 12 on each side of the body. Each is connected to the backbone by small 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 it with cartilage or gristle. The lowest two are attached only to the backbone. They are known 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 of ribs while a two toed sloth has 24 pairs.