Proximal Attachment
The eighth intercostal intimus muscle lies deep to the eighth intercostal internus in the middle two-quarters of the intercostal space.  It is a more substantial muscle than those of the upper intercostal spaces and arises from the internal aspect of the eighth rib.

Distal Attachment
Anteriorly the fibers pass infero-laterally, while laterally they pass infero-medially to insert onto the internal aspect of the ninth rib.  

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

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