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

Distal Attachment
Posteriorly the fibers pass obliquely infero-laterally, while anteriorly they pass infero-medially to insert onto the superior border of the seventh 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 sixth intercostal externus muscle is supplied by the sixth intercostal nerve.

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