3 The Skeletal System

The skeletal system and its roles

The Skeletal system is the body’s internal framework. It includes bones, cartilage, joints, and ligaments. The adult skeleton is commonly described as having 206 bones, although the number can vary slightly among individuals.

Together, skeletal structures support the body, protect organs, store minerals, produce blood cells, and work with skeletal muscles to create movement. Bones also act as levers: when skeletal muscles contract, they pull on bones across joints.

Bone tissue and marrow

Bone is living connective tissue that is continually maintained and remodeled by specialized cells. Its matrix combines collagen fibers, which add flexibility, with mineral crystals—mainly calcium phosphate—which give bone hardness and strength.

Most bones contain both Compact bone and Spongy bone. Compact bone is dense and forms much of a bone’s outer layer; in a long bone, it is especially thick along the shaft. Spongy bone consists of a lattice of bony struts called Trabeculae. This arrangement provides strength while keeping the skeleton relatively light.

Spaces in spongy bone can contain red marrow, where red blood cells, white blood cells, and platelets are produced. Yellow marrow, found mainly in the central cavities of adult long bones, stores fat.

The structure of a long bone

A typical long bone, such as the femur, has a shaft called the Diaphysis and expanded ends called the Epiphyses. The shaft surrounds a marrow cavity. The outer surface is covered by the periosteum, a membrane containing blood vessels and nerves that also provides attachment sites for tendons and ligaments.

At joint surfaces, smooth articular cartilage covers the bone ends, reducing friction and helping absorb impact. In a growing bone, a cartilage growth plate near each end enables the bone to lengthen. After growth ends, the plate becomes a bony line.

How bones support the body

Bones provide a framework that supports body tissues and helps maintain posture. They protect organs: the skull protects the brain, while the rib cage protects the heart and lungs. By acting as levers for skeletal muscles, bones also contribute to movement.

Red marrow produces blood cells, and bone stores minerals—particularly calcium and phosphate—that the body can release as needed. Yellow marrow stores fat, providing energy storage.

Axial and appendicular skeleton

The Axial skeleton forms the body’s central axis and contains 80 bones. It includes the skull, the small auditory bones of the middle ear, the hyoid bone, the vertebral column, and the thoracic cage—the ribs and sternum. These bones primarily support the head and trunk and protect the brain, spinal cord, and organs in the chest.

The Appendicular skeleton contains 126 bones: the bones of the upper and lower limbs and the girdles that attach them to the axial skeleton. The pectoral, or shoulder, girdle consists of the clavicles and scapulae; the pelvic girdle consists of the two hip bones. The limbs and their girdles enable reaching, grasping, standing, and walking.

Joint structure and movement

A joint, or articulation, is where two or more bones meet, or where bone meets cartilage. Joints can be described structurally, by the material connecting the bones, and functionally, by how much movement they allow.

Structural classifications include:

• Fibrous joints: Bones are joined by fibrous connective tissue. Skull sutures are an example and allow almost no movement.

• Cartilaginous joints: Bones are joined by cartilage. Intervertebral discs connect vertebral bodies and permit limited movement.

• Synovial joints: Bones meet within a fluid-filled joint cavity. These joints are freely movable and include the shoulder, hip, knee, and elbow.

Functional classifications describe how much movement a joint allows. Synarthroses are essentially immovable, amphiarthroses are slightly movable, and diarthroses are freely movable. All synovial joints are diarthroses.

Features and types of synovial joints

At a synovial joint, articular cartilage cushions the bone ends, and a surrounding capsule encloses the joint. Synovial fluid lubricates the moving surfaces. Ligaments connect bone to bone and help stabilize the joint.

Synovial joints also differ in the movements they permit. Hinge joints, such as the elbow, mainly allow bending and straightening. Pivot joints, such as the joint between the first two neck vertebrae, allow rotation. Ball-and-socket joints, such as the shoulder and hip, allow movement in several directions.

Common skeletal movements

Movement names describe how a body part changes position. The range of motion depends on the joint’s structure and is limited by factors such as ligaments and surrounding tissues.

• Flexion decreases the angle at a joint, as when bending the elbow. Extension increases the angle or straightens a joint. Hyperextension is extension beyond the anatomical position.

• Abduction moves a limb away from the body’s midline, as when raising an arm sideways. Adduction brings it toward the midline.

• Rotation turns a bone around its long axis, as when turning the head from side to side. Circumduction moves a limb in a circle by combining flexion, abduction, extension, and adduction.

• At the ankle, dorsiflexion lifts the toes toward the shin, while plantar flexion points the foot downward.

• Pronation and supination rotate the forearm so the palm faces down or up, respectively.

• Inversion turns the sole of the foot inward; eversion turns it outward.

• Elevation raises a body part, such as the shoulders in a shrug; depression lowers it. Protraction moves a part forward, and retraction moves it backward.

• Opposition brings the thumb across the palm toward a fingertip, enabling many grasping actions.

For example, raising an arm to reach a shelf involves shoulder abduction; bringing it back to the side involves adduction. The shoulder’s ball-and-socket structure allows a wide range of motion, while the elbow’s hinge structure mainly permits flexion and extension.