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

Distal Attachment
Posteriorly the fibers pass obliquely infero-laterally, while anteriorly they pass infero-medially to insert onto the superior border of the twelfth rib from the tubercle, where it blends with the posterior fibers of the superior costotransverse ligament, to the free end of the costal cartilage, to continue as the external intercostal membrane attaching to the sternum.  

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

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