Proximal Attachment
The fifth intercostal externus muscle arises from the lower border of the fifth rib.

Distal Attachment
Posteriorly the fibers pass obliquely infero-laterally, while anteriorly they pass infero-medially, to insert onto the superior border of the sixth rib.  It extends from the tubercle, where it blends with the posterior fibers of the superior costotransverse ligament, to the costochondral junction, where it continues as the external intercostal membrane attaching to the sternum.  

Nerve Supply
The fifth intercostal externus muscle is supplied by the fifth intercostal nerve.

Action
The fifth intercostal externus is most active during inspiration, but its primary role is with the fifth intercostal internus to stiffen the chest wall during respiration.