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

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

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