External intercostal muscles
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. These muscles work in unison when inspiration (inhalation) occurs. The intercostal muscles relax while external muscles contract causing the expansion of the chest cavity and an influx of air into the lungs.
Each arises from the lower border of a rib, and is inserted into the upper border of the rib below. In the two lower spaces they extend to the ends of the cartilages, and in the upper two or three spaces they do not quite reach the ends of the ribs.
They are thicker than the Intercostales interni, and their fibers are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front (the example is often used of sticking one's hands in their pocket and noting the direction of the fingers pointing downward and medially).
Variations
Continuation with the Obliquus externus or Serratus anterior: A Supracostalis muscle, from the anterior end of the first rib down to the second, third or fourth ribs occasionally occurs.
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