The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end in thin membranes, the external intercostal membranes, which are continued forward to the sternum.

The internal intercostal muscles relax while the external muscles contract causing the expansion of the chest cavity and an influx of air into the lungs.

They are thicker than the internal intercostals, and their fibers are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front.

Continuation with the external oblique or serratus anterior: A supracostalis muscle, from the anterior end of the first rib down to the second, third or fourth ribs occasionally occurs.

This article incorporates text in the public domain from page 403 of the 20th edition of Gray's Anatomy (1918)