The axial skeleton

The skeleton is made of bones (a hard matrix strengthened with calcium salts) and a few cartilages (a bendable matrix with chondroitin salts; that is why your ear can bend). An adult human has 206 bones, in two divisions: the axial skeleton (along the body's centre line) and the appendicular skeleton (the limbs and their girdles).

The axial skeleton has 80 bones: the skull, vertebral column, sternum and ribs.

• Skull: 22 bones: 8 cranial bones forming the hard case around the brain, and 14 facial bones forming the front of the skull. Also counted here: a U-shaped bone at the base of the mouth, the hyoid, and three tiny ear ossicles in each middle ear: the malleus, incus and stapes.
• The skull meets the backbone at two smooth knobs, the occipital condyles, so the human skull is dicondylic.

• Vertebral column: 26 vertebrae, from top to bottom: cervical (neck) 7, thoracic (chest) 12, lumbar (lower back) 5, sacral 1 (several fused into one), and coccygeal 1 (also fused). The first vertebra, which holds up the skull, is the atlas. Each vertebra has a central hole, the neural canal, through which the spinal cord runs. Almost all mammals, even the giraffe, have seven neck vertebrae.
• Sternum (breastbone): a flat bone down the middle of the front of the chest.
• Ribs: 12 pairs, each with two heads (bicephalic) joined to the backbone. The first 7 pairs are true ribs, joined directly to the sternum by hyaline cartilage. The 8th, 9th and 10th pairs are false ribs (vertebrochondral): they join the cartilage of the 7th rib, not the sternum. The 11th and 12th pairs are floating ribs: not joined at the front at all.

The thoracic vertebrae, the ribs and the sternum together form the rib cage, which protects the heart and lungs.