The transverse muscle of thorax (Musculus transversus thoracis) is a triangular sheet, with transversely oriented fibers, lying on the inside of the sternum and the sternal costal cartilages. It originates from the sternal ligament and insert on the costochondral junction of the second to eight ribs.
Origin: By an aponeurosis on the lateral internal surface of the sternum, from the second sternebra to the caudal end of the xiphoid process.
Insertion: Second through seventh costal cartilages.
Action: Expiration. Pulls the ribs inward when contracting
Nerve: Intercostal nerves