In human physiology, any of a series of short muscles that extend between the ribs and serve to draw them together during inspiration and forced expiration or expulsive actions. A set of external and internal intercostalis muscles is found between each vertical pair of ribs on each side of the chest. The Intercostales (Intercostal muscles) are two thin planes of muscular and tendinous fibers occupying each of the intercostal spaces. They are named external and internal from their surface relations, the external being superficial to the internal. The Intercostales externi (External intercostals) are eleven in number on either side. They extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end in thin membranes, the anterior intercostal membranes, which are continued forward to the sternum. Intercostalgia pain appears while motion, deep breathing or on expanding the chest.