Seven true ribs, five false ones, and two that float free
Every rib is anchored to the spine, but only the top seven pairs reach the breastbone on their own. Three more borrow cartilage from the rib above, which lets the lower chest swing like a bucket handle as you breathe. The last two pairs stop short entirely, attached to nothing at the front.
The human thoracic cage combines 12 pairs of ribs, their costal cartilages, the sternum and the 12 thoracic vertebrae. It shields the heart, lungs and great vessels, anchors muscles of the neck, arms, back and upper abdomen, and in four-limbed animals it also houses the breathing muscles, the diaphragm and intercostals, that power active inhalation and forced exhalation.
Anatomists sort the ribs by how they meet the sternum. Pairs one to seven, the true ribs or costae verae, each have their own cartilage running to the breastbone. Pairs eight to twelve are false ribs; of these, eight to ten join via the cartilage of the rib above, and that springy arrangement produces the bucket-handle motion of breathing. The eleventh and twelfth, the floating ribs, are small, delicate and tipped with cartilage, and connect only to the spine.
Individual ribs differ more than their neat numbering suggests. A typical rib has a head, a neck about 3 cm long and a shaft that bends in two directions while twisting along its length. The first rib is the most curved and usually the shortest, broad and flat, with grooves for the subclavian vein and artery. The tenth attaches squarely to its own vertebra rather than between two, and the front of the ninth lies level with the first lumbar vertebra, a landmark called the transpyloric plane. The second rib's cartilage meets the sternum at the sternal angle, which makes it easy to find by touch.
Ribs begin as cartilage formed from embryonic somites and gradually turn to bone. Testosterone during puberty widens the male chest. Counts vary: about 1 in 200 to 500 people have an extra cervical rib, more often women, and it can press on the nerves to the arm. Engineers modelling rib reconstruction found carbon-fibre-reinforced PEEK composites a promising replacement for titanium implants.
Source: Rib cage
