Muscle · Chest

Transversus thoracis

Musculus transversus thoracis

A flat fan on the inner surface of the front chest wall, spreading from the back of the lower sternum and the xiphoid process upward and outward into the costal cartilages of the second to sixth ribs. It is the anterior member of the same deep layer as the subcostals behind, and it is continuous below with transversus abdominis — so the deepest sheet of the abdominal wall does not stop at the ribs, it carries on up behind the breastbone. Its pull draws the costal cartilages down, which is a forced-expiration action rather than a quiet-breathing one.