The skeletal system is the body's supporting framework of bones, together with cartilage, a firm but flexible supporting tissue, and ligaments, strong bands that connect bones. The adult skeleton conventionally contains 206 bones and provides support, protection, movement, mineral storage and a site for blood cell formation.
What are the functions of the skeletal system?
Bones are living organs rather than inert supports. Their shape and arrangement provide a framework, while their internal tissues contribute to mineral storage and blood cell formation.
The skeletal system has five principal functions:
- Support: bones maintain the body's framework and provide attachment sites for muscles and other soft tissues. The skeleton supports the head and trunk and transfers body weight towards the ground.
- Protection: bony walls surround vulnerable structures. The head, spinal canal and chest illustrate how bones can enclose organs while leaving openings for nerves and blood vessels.
- Movement: bones act as levers at joints, places where skeletal elements meet. Skeletal muscles, the muscles responsible for voluntary body movement, pull on these levers through their attachments.
- Mineral storage: bone tissue contains calcium and phosphate minerals that contribute to its hardness. These minerals also form a store available to the body.
- Blood cell formation: red bone marrow, blood-forming tissue within bones, produces blood cells. In adults, it is concentrated mainly in bones of the central skeleton and near the shoulder and hip ends of certain limb bones.
What is the difference between the axial and appendicular skeleton?
The axial skeleton forms the central framework of the body. It includes the skull, the bony framework of the head; the vertebral column, the segmented support of the back; and the thoracic cage, the framework surrounding the chest. The hyoid bone in the neck and the small bones of the middle ears also belong to this division.
The appendicular skeleton consists of the limb girdles and the limbs themselves. A girdle is a bony arrangement that connects a limb to the trunk: the pectoral girdle connects the upper limb, and the pelvic girdle connects the lower limb.
The conventional adult count is 206 bones. Some bones that are separate during development fuse as the skeleton matures, and normal anatomical variation means that an individual's count may differ. The usual adult count treats each fused sacrum, the bone at the base of the spine, as a single bone.
Which bones belong to each skeletal region?
The regions below organise the skeleton by location. The girdles belong to the appendicular skeleton even though they lie close to the trunk.
| Region | Main bones | Division |
|---|---|---|
| Head | Skull, including the lower jaw | Axial |
| Back | Vertebral column, including the sacrum and coccyx, or tailbone | Axial |
| Chest | Each rib, the sternum or breastbone, and the chest portion of the vertebral column | Axial |
| Pectoral girdle | Clavicle or collarbone, and scapula or shoulder blade | Appendicular |
| Upper limb | Humerus in the arm; radius and ulna in the forearm; wrist, palm and finger bones | Appendicular |
| Pelvic girdle | A hip bone on each side | Appendicular |
| Lower limb | Femur in the thigh; patella at the knee; tibia and fibula in the leg; ankle, foot and toe bones | Appendicular |
How do the skull and vertebral column support and protect the body?
The skull has a braincase that encloses the brain and a facial skeleton that supports the face. Its bones also form the walls of the eye sockets and much of the nasal cavity. Most skull bones meet at sutures, joints held together by fibrous connective tissue, a tissue rich in strong fibres.
The mandible, or lower jaw, moves at joints with the skull. This arrangement allows jaw movement while the braincase remains a relatively rigid enclosure.
The vertebral column consists of vertebrae, the individual bones of the spine. Its regions are cervical in the neck, thoracic in the chest, lumbar in the lower back, sacral at the back of the pelvis and coccygeal at the tailbone.
Openings in successive vertebrae form the vertebral canal, which contains the spinal cord, its coverings and spinal nerve roots. In adults, the spinal cord usually ends near the level of the first or second lumbar vertebra. Between most adjacent vertebral bodies—the weight-bearing front portions—intervertebral discs provide flexible connections. These discs contain fibrocartilage, a tough form of cartilage reinforced by collagen fibres, and a softer central core.
What forms the thoracic cage?
The thoracic cage consists of the thoracic portion of the vertebral column, usually 12 pairs of ribs, their costal cartilage and the sternum. Costal cartilage is the cartilage extending from the front end of a rib. The cage surrounds the heart and lungs and provides attachment sites for muscles involved in breathing.
The ribs connect behind to the thoracic vertebrae. In front, the upper ribs connect to the sternum through their own costal cartilage, while others connect indirectly through neighbouring cartilage. The lowest ribs have no anterior connection to the sternum.
The sternum lies at the front of the chest. Its parts are the manubrium, the broad upper portion; the body, the elongated middle portion; and the xiphoid process, the small lower projection. The joints and cartilage of the cage allow movement during breathing rather than forming a completely rigid box.
How do the pectoral and pelvic girdles connect the limbs?
The pectoral girdle consists of a clavicle and scapula on each side. The clavicle connects the sternum to the scapula and helps hold the shoulder away from the chest. The scapula provides muscle attachment surfaces and a socket that meets the upper end of the humerus.
The clavicle provides the upper limb's only direct bony connection to the axial skeleton. Muscles support the scapula against the chest wall, allowing it to move as the arm changes position.
The pelvic girdle consists of the hip bones, which connect behind to the sacrum and meet in front. Each hip bone develops from the ilium, its broad upper part; the ischium, its lower posterior part; and the pubis, its anterior part. These fuse around the acetabulum, the socket receiving the head of the femur.
The bony pelvis includes the hip bones, sacrum and coccyx. It transfers trunk weight to the lower limbs and surrounds the pelvic cavity, the space containing pelvic organs.
How are the bones of the limbs arranged?
In anatomical position—the body upright, palms facing forwards—the radius lies on the thumb side of the forearm and the ulna on the little-finger side. The humerus meets these bones at the elbow. The hand contains carpals, the wrist bones; metacarpals, the palm bones; and phalanges, the finger bones.
The femur connects the hip to the knee. Below the knee, the tibia is the main weight-bearing bone of the leg, while the fibula lies on its outer side and contributes to the ankle. In anatomical terminology, the leg means the region between knee and ankle, not the whole lower limb.
The patella lies within the tendon of the quadriceps muscle at the front of the knee. A tendon is connective tissue that transmits a muscle's pull to its attachment. The foot contains tarsals, the ankle, hindfoot and midfoot bones; metatarsals, the main bones extending towards the toes; and phalanges, the toe bones.
How do cartilage and ligaments work with bones?
At many freely movable joints, articular cartilage covers the contacting bone surfaces. This smooth covering reduces friction and distributes loads. Cartilage also provides flexible connections elsewhere, including the costal cartilage and parts of the intervertebral discs.
Ligaments connect bones and help restrain excessive movement at joints. They differ from tendons, which transmit muscle force. Joint stability also depends on the shapes of the bones and the surrounding muscles.
Within a bone, compact bone forms a dense outer layer, while spongy bone forms an internal network of bony struts surrounding marrow spaces. The periosteum, a connective tissue covering, surrounds most external bone surfaces except those covered by articular cartilage.
In the 3D model next to this article, you can rotate the skeleton and isolate a structure to compare its position with neighbouring bones.