Proximal Attachment
The eighth intercostal internus muscle arises from the floor of the subcostal groove and adjacent costal cartilage of the eighth rib, deep to the eighth intercostal externus muscle.

Distal Attachment
Its fibers are at nearly 90o to those of the eighth intercostal externus, passing infero-laterally anteriorly and infero-medially posteriorly to insert onto the superior border of the ninth rib.  It spans from the sternum anteriorly to the rib angle posteriorly, where it continues as the internal intercostal membrane, which is continuous with the anterior fibers of the superior costotransverse ligament.

Nerve Supply
The eighth intercostal internus muscle is supplied by the eighth intercostal nerve.

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