Axial skeleton

The skeletal system is a framework of bones and a few cartilages. Bone has a very hard matrix because of calcium salts; cartilage has a slightly pliable matrix because of chondroitin salts.

The human skeleton has 206 bones and a few cartilages, grouped into the axial and the appendicular skeleton.

The axial skeleton has 80 bones lying along the body's central axis: those of the skull, the vertebral column, the sternum and the ribs.

The skull has 22 bones: 8 cranial bones that form the cranium around the brain, and 14 facial bones at the front. A single U-shaped hyoid lies at the base of the buccal cavity, and each middle ear has three ossicles: malleus, incus and stapes.

The skull joins the top of the vertebral column by two occipital condyles, so it is called dicondylic.

The vertebral column is built from 26 vertebrae on the dorsal side. Counting down from the skull: 7 cervical, 12 thoracic, 5 lumbar, then one fused sacral and one fused coccygeal. Almost all mammals, humans included, have seven cervical vertebrae.

Each vertebra has a central neural canal for the spinal cord. The first vertebra, the atlas, articulates with the occipital condyles. The column protects the spinal cord, supports the head and gives attachment to the ribs and back muscles.

The sternum, or breastbone, is a flat bone lying in the midline on the front (ventral) side of the thorax.

Humans have 12 pairs of ribs. Every rib is a thin, flat bone; at the back it joins the vertebral column and at the front the sternum. Its dorsal end carries two articular surfaces, so it is called bicephalic.

The first 7 pairs are true ribs, joined to the sternum by hyaline cartilage. Pairs 8, 9 and 10 join the seventh rib through hyaline cartilage instead of the sternum: vertebrochondral or false ribs. Pairs 11 and 12 have no ventral connection: floating ribs.

The thoracic vertebrae, the ribs and the sternum make up the rib cage between them.