Functions and divisions
The skeleton supports the body, protects organs (the skull shields the brain, the rib cage the heart and lungs), provides levers for muscles, stores calcium and phosphate, and houses red bone marrow, where blood cells form. Yellow marrow stores fat.
An adult skeleton has 206 bones, grouped into two divisions:
- Axial skeleton (80 bones): skull, hyoid, ear ossicles, vertebral column and thoracic cage (ribs and sternum).
- Appendicular skeleton (126 bones): the limbs and the girdles that attach them to the axis.
Bone structure
A typical long bone has a shaft, the diaphysis, and two ends, the epiphyses, covered by smooth articular cartilage at joints. A tough membrane, the periosteum, wraps the outside. The hard outer shell is compact bone, and the inside is lighter spongy (cancellous) bone, whose struts run along lines of stress. The central medullary cavity holds marrow.
Bone cells
- Osteoblasts build new bone matrix.
- Osteocytes are mature cells that maintain existing matrix.
- Osteoclasts break down matrix and release calcium.
Bone is constantly remodelled by this balance. Parathyroid hormone raises blood calcium by stimulating bone breakdown, while calcitonin has the opposing effect.
Bone formation
In intramembranous ossification, bone forms directly within a membrane of connective tissue, as in the flat bones of the skull roof. In endochondral ossification, a hyaline cartilage model is gradually replaced by bone, as in most bones of the limbs.
Joints
Joints are classified by structure. Fibrous joints, such as skull sutures, are held by dense connective tissue and barely move. Cartilaginous joints, such as those between vertebrae, are united by cartilage. Synovial joints have a fluid-filled joint cavity and allow free movement. Their types include the hinge (elbow), pivot (between the first two neck vertebrae) and ball-and-socket (shoulder and hip).
Worked walk-through: counting the 206
Check that the divisions add up.
- Skull 22, hyoid 1, ear ossicles 6, vertebral column 26, and thoracic cage 25 (24 ribs and the sternum): 22 + 1 + 6 + 26 + 25 = 80 axial bones.
- Pectoral girdles: 2 clavicles and 2 scapulae = 4. Each upper limb has 30 bones, so 60 together. Pelvic girdle: 2 hip bones. Each lower limb has 30 bones, so 60 together.
- Appendicular total: 4 + 60 + 2 + 60 = 126.
- Whole skeleton: 80 + 126 = 206.
Infants have around 270 bones at birth, because many fuse during growth. This shows that the skeleton is a living, changing organ rather than a fixed frame.