rib

A curved, twisted strip of bone passing around the thorax from the vertebral column to articulate indirectly with the sternum by its costal cartilage. There are twelve pairs in humans, of which the eleventh and twelfth are not attached at the front (floating ribs). As they pass around the thorax the ribs curve downwards as well as forwards. They provide protection for the lungs, heart, and great vessels. During breathing they move to bring about changes in the volume of the thorax.

In entomology the costa refers to the vein on the leading edge of the wing (See Glossary of Lepidopteran terms) In mammals, one generally thinks of ribs occurring only in the chest. However, fused-on remnants of ribs can be traced in development in neck vertebrae (cervical ribs) and sacral vertebrae.

In reptiles, ribs sometimes occur in all vertebrae from the neck to the sacrum.

Fish can have up to four ribs on each vertebra and this can easily be seen in the herring, although not all fish have this many.

Humans (both male and female) have 24 ribs (12 sets). This was noted by the Flemish anatomist Vesalius in 1543 (De Humani Corporis Fabrica ) setting off a wave of controversy, since it was assumed based on the story of Adam and Eve, that men's ribs would number one fewer than women's.