[TA] internal muscle of thorax; origin, dorsal surface of xiphoid process and lower portion of dorsal surface of body of sternum; insertion, second to sixth costal cartilages; action, contributes to depression of ribs, narrowing chest; nerve supply, intercostal. SYN: musculus transversus thoracis [TA], musculus triangularis sterni, sternocostalis muscle, transverse muscle of thorax.