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.
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.
A central rib of the left side. Inferior aspect.
Diagram of the course and branches of a typical intercostal nerve.