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

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

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

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