Proximal Attachment
The ninth intercostal intimus muscle lies deep to the ninth 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 ninth rib.

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

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

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