Transverse thoracic muscle
A vestigial muscle arising from posterior surfaces of the lower third of the body of the sternum and the xiphoid process and the costal cartilages of the last 3 or 4 ribs and attaches to the posterior surface of the costal cartilages of the 2nd to 6th ribs.
Source: The Anatomical Atlas of CardiacMRI, Editor Mohan Sivananthan - Leeds General Infirmary – used with permission