Expansion
Formed by the cartilages of ribs 7 to 10, meeting at the xiphisternum
Expansion
- The costal margin is formed by the fused costal cartilages of ribs 7, 8, 9 and 10, with the tips of ribs 11 and 12 lying free
- The two margins meet at the xiphisternal joint, at about T9
- The angle between them is the subcostal (infrasternal) angle, normally about 70 to 90 degrees
The costal margin is the surface boundary between thorax and abdomen, but not the internal one. The diaphragm domes upwards well above it, so the liver, spleen, stomach and upper kidneys all lie within the rib cage. That mismatch is why lower rib fractures cause abdominal injury and why the spleen must enlarge substantially before it becomes palpable below the margin.
In emphysema the diaphragm flattens, the ribs are held elevated and the subcostal angle widens.