Bony landmark · Chest

Costal margin

Arcus costalis

The costal margin is the curved lower edge of the ribcage — the arch you can trace with your fingers from the tip of the breastbone out and down to the side. It is not bone but the joined cartilages of the seventh to tenth ribs. It is the direct handover between breathing and bracing: the diaphragm attaches along its inner surface, while the transversus abdominis, internal oblique and rectus abdominis attach along its outer surface from below.