Bones are living organs that form the skeletal system, supporting the body, protecting internal structures and providing attachment sites for muscles. They are classified by shape as long, short, flat, irregular or sesamoid; a typical long bone has a shaft and expanded ends, with dense outer bone surrounding an internal cavity and spongy bone.
What are the five types of bones?
Bone classification describes overall shape rather than size alone. A small bone can still be a long bone if it has a recognisable shaft and ends. Shape helps explain how a bone supports weight, protects structures or contributes to movement.
| Bone type | Main shape or defining feature | Examples |
|---|---|---|
| Long | Longer than it is wide, typically with a shaft and expanded ends | The femur in the thigh and the humerus in the upper arm |
| Short | Approximately cube-shaped, with similar dimensions in different directions | A typical carpal bone in the wrist |
| Flat | Relatively thin, often broad or curved | The sternum at the front of the chest and the scapula at the back of the shoulder |
| Irregular | A complex shape that does not fit the other shape categories | A vertebra, a bone of the vertebral column |
| Sesamoid | Develops within a tendon, a band of connective tissue that transmits muscle pull | The patella at the front of the knee |
Long bones act as levers during movement, while many flat bones provide broad surfaces for muscle attachment or protection. Short bones contribute to support and controlled movement in regions such as the wrist. Irregular bones have specialised shapes suited to their position.
Sesamoid bones are distinguished by their relationship to a tendon rather than by a single outline. They can alter the direction of tendon pull and keep a tendon away from an underlying joint surface.
What is the difference between compact and spongy bone?
Compact bone is dense bone tissue that forms the outer layer of a bone. This outer layer is also called the cortex. It is especially thick around the shaft of a long bone, where it resists bending and twisting.
Spongy bone is an internal network of thin plates and struts called trabeculae. The spaces between trabeculae contain bone marrow, the soft tissue within bones. Despite its name, spongy bone is firm, mineralised tissue rather than soft or sponge-like material.
The arrangement of trabeculae helps distribute forces through a bone without filling its entire interior with dense tissue. Compact and spongy bone are therefore different arrangements of bone tissue, not separate categories of bones.
Both occur in long bones, but their proportions vary by region. In a typical long bone, the shaft has a substantial compact wall around its central cavity, whereas the expanded ends contain abundant spongy bone beneath a thinner compact covering. Flat bones commonly have spongy bone between layers of compact bone.
What are the main parts of a long bone?
The parts of a typical long bone can be understood by following it from its shaft towards either end, distinguishing the central shaft, the transition regions and the expanded ends.
- Diaphysis: the shaft, or elongated central portion, of a long bone. Its thick compact wall surrounds the medullary cavity.
- Epiphysis: an expanded end of a long bone. It contains mainly spongy bone enclosed by a compact outer layer, and its joint surfaces are covered by articular cartilage, a smooth supporting tissue.
- Metaphysis: the transition region between the shaft and an expanded end. In a growing bone, it lies next to the growth plate, a layer of cartilage where lengthwise growth occurs.
- Medullary cavity: the central space within the shaft. It contains bone marrow rather than being an empty channel.
- Periosteum: a connective tissue membrane covering most of the outer bone surface. It is absent where articular cartilage covers a joint surface and at some tendon and ligament attachment sites.
- Articular cartilage: the cartilage covering the bone surfaces that meet at a freely movable joint. It provides a smooth contact surface and helps distribute loads.
In a growing long bone, cartilage at the growth plate is progressively replaced by bone as the bone lengthens. When growth is complete, the plate is replaced by bone and may leave a visible epiphyseal line. The metaphysis remains a recognisable region of the adult bone.
What covers the outside and lines the inside of a bone?
The periosteum has an outer fibrous layer and an inner layer containing cells involved in bone formation. Blood vessels and nerves travel through it, and fibres anchor it to the underlying bone. Many tendon and ligament attachments blend with this covering; a ligament is a band of connective tissue that usually connects bones at a joint.
Articular cartilage replaces the periosteal covering at joint surfaces. In the usual freely movable joint, it is hyaline cartilage, a type of cartilage with a smooth, glass-like tissue appearance. It has no blood vessels and receives nourishment mainly from the fluid within the joint.
The endosteum is a thin cellular lining on internal bone surfaces. It lines the medullary cavity and covers the surfaces of trabeculae. Together, the periosteum and endosteum provide surfaces where bone tissue can be added or removed during growth and normal remodelling, the ongoing renewal of bone.
What do common bone markings mean?
Bone markings are named features on a bone's surface. Some form joint surfaces, some provide attachment sites, and others allow structures to pass through or lie against the bone. The name describes the feature's shape, but does not always identify its complete function.
- Condyle: a rounded prominence that forms a joint surface. The condyles at the lower end of the femur articulate with the tibia, meaning that their surfaces meet at a joint.
- Tubercle: a small rounded projection, commonly associated with a tendon or ligament attachment.
- Foramen: an opening through a bone that allows a structure, such as a nerve or blood vessel, to pass.
- Fossa: a depression or hollow in a bone. It may receive another structure or contribute to a joint surface.
- Crest: a prominent ridge of bone, often providing an attachment site.
These terms are best learned alongside the bone's position and neighbouring structures. A projection used for attachment differs from a smooth joint surface, even when both are prominent landmarks on the same bone.
What are the axial and appendicular skeletons?
The adult skeleton is divided into the axial skeleton and the appendicular skeleton. Its precise composition can vary between individuals, particularly through the presence of small additional bones and variable sesamoid bones.
The axial skeleton forms the central axis of the body. It includes the skull, the vertebral column and the thoracic cage, the framework formed by the ribs and sternum with the thoracic vertebrae. The small bones of the middle ear and the hyoid bone, which supports the tongue in the neck, also belong to the axial skeleton.
The appendicular skeleton consists of the limbs and the girdles that connect them to the axial skeleton. The shoulder girdle includes the scapula and clavicle, or collarbone. The pelvic girdle consists of the hip bones, which connect the lower limbs to the central skeleton.
This division is separate from classification by bone shape. The sternum is both a flat bone and an axial bone; the humerus is both a long bone and an appendicular bone. Shape describes the individual bone, while axial or appendicular describes its place in the skeleton.
How can you recognise a long bone in anatomical position?
Start by finding the elongated shaft and the expanded ends. Then distinguish smooth joint surfaces from roughened areas where tendons or ligaments attach. A long bone need not be perfectly straight, and its ends need not have matching shapes.
Direction terms help describe its parts: proximal means nearer the limb's attachment to the trunk, while distal means farther from that attachment. These terms identify the ends without depending on how the bone is turned in an illustration.
In the 3D model next to this article, you can select a bone, isolate it, rotate it and zoom. When studying a long bone, distinguish its shaft, ends and surface markings, while keeping the distinction between a bone's overall form and its internal tissue arrangement.