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

External intercostal membrane

Unlike the other two intercostal muscles, the external intercostal muscle does not retain its muscular character all the way to the sternum, and so the tissue in this location is called the external intercostal membrane....

 , 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.

They are thicker than the Intercostales interni, and their fibers are directed obliquely downward and laterally on the back of the thorax

Thorax

The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

, 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.