What the skeleton does
- Support: gives the body its shape and holds it upright.
- Protection: the skull guards the brain, the ribs guard the heart and lungs, and the vertebrae guard the spinal cord.
- Movement: muscles pull on bones, which act as levers around joints.
- Making blood cells: red bone marrow produces red and white blood cells and platelets.
- Storage: bones store calcium and phosphate, and yellow marrow stores fat.
Axial and appendicular skeleton
The axial skeleton (80 bones) forms the central axis: the skull, the vertebral column, the ribs and the sternum. The appendicular skeleton (126 bones) includes the limbs and the girdles that attach them: the shoulder girdle (clavicles and scapulae) and the pelvic girdle (hip bones). More than half of all your bones are in your hands and feet.
Types of bone
- Long bones (femur, humerus, finger bones) are longer than they are wide and act as levers.
- Short bones (carpals, tarsals) are roughly cube-shaped and give stability with some movement.
- Flat bones (skull, ribs, sternum, scapula) are thin and protect organs or anchor muscles.
- Irregular bones (vertebrae, pelvis, mandible) have complex shapes suited to their jobs.
- Sesamoid bones (the patella) develop inside tendons.
Inside a bone
Bone is a living tissue made of collagen fibres (for flexibility) hardened with calcium phosphate minerals (for strength). A long bone has a hard outer layer of compact bone, a honeycomb of spongy bone at the ends, and a central cavity containing marrow. Its surface is covered by a membrane called the periosteum, rich in nerves and blood vessels. The ends of bones at joints are capped with smoothcartilage to reduce friction.
Joints
A joint is where two or more bones meet. Fixed (fibrous) joints, such as the sutures of the skull, allow no movement. Cartilaginous joints, such as those between vertebrae, allow a little. Most joints in the limbs are synovial joints, with a fluid-filled capsule that lets them move freely. Their shape decides the movement:
| Synovial joint | Movement | Examples |
|---|---|---|
| Ball-and-socket | In all directions, including rotation | Shoulder, hip |
| Hinge | Bending and straightening in one plane | Elbow, knee, fingers |
| Pivot | Rotation around an axis | Atlas–axis (neck), radius–ulna |
| Gliding (plane) | Small sliding movements | Between carpals and tarsals |
| Saddle | Two directions, like a rider on a saddle | Base of the thumb |
Ligaments join bone to bone and hold joints together; tendons join muscle to bone.
Growth and repair
A newborn has around 300 bones, many partly made of cartilage. As a child grows, cartilage is replaced by bone and several bones fuse, such as the vertebrae of the sacrum and the three parts of each hip bone. Long bones lengthen atgrowth plates near their ends, which close in the late teens. Bone is constantly remodelled throughout life: cells called osteoclasts remove old bone and osteoblasts build new bone, which is how broken bones heal.
Key terms
- Vertebral column
- The spine: 33 vertebrae, some fused, protecting the spinal cord.
- Synovial joint
- A freely movable joint with a capsule of lubricating fluid.
- Ligament / tendon
- Connects bone to bone / connects muscle to bone.
- Bone marrow
- Soft tissue inside bones; red marrow makes blood cells.
Common misconceptions
- “Bones are dead and dry.” Living bone has blood vessels, nerves and cells, and is constantly rebuilt.
- “Babies have fewer bones than adults.” They have more; many fuse during growth.
- “Muscles push bones.” Muscles can only pull; they work in pairs to move joints both ways.
Educational model only. This 3D skeleton is simplified and not anatomically exact, and the joint ranges are typical values. It cannot be used to assess injuries or medical conditions and is not medical advice. If you think you have a bone or joint injury, see a qualified healthcare professional. See oureducational and medical disclaimer.
Key takeaways
- The adult skeleton has 206 bones, split into the axial and appendicular skeletons.
- It supports, protects, enables movement, stores minerals and makes blood cells.
- Bones are classified as long, short, flat, irregular or sesamoid.
- Joint shape decides movement: hinge, ball-and-socket, pivot, gliding or saddle.
Model note: bones are built from simple shapes to keep the page fast. Small bones are grouped (for example, all eight carpals of a wrist highlight together).