The Skeletal System: Bone Structure, Types, Joints, and Functions

The skeletal system provides the body's structural framework.

Bone Tissue

• Compact (cortical) bone: Dense outer cortex; osteons (Haversian systems) with central canals for vessels/nerves.
• Cancellous (spongy/trabecular) bone: Porous inner network; contains red marrow (hematopoiesis). High surface area for metabolic exchange.

Bone Shapes

• Long (femur, humerus) – leverage.
• Short (carpals, tarsals) – stability.
• Flat (skull, ribs, scapulae) – protection, broad muscle attachment.
• Irregular (vertebrae, pelvis) – complex shapes.
• Sesamoid (patella) – embedded in tendons.

Bone Markings

Surface features: articular surfaces, processes (muscle/tendon attachment), foramina (vessels/nerves), fossae, sulci.

Joints (Articulations)

• Fibrous: Sutures (skull) – immovable.
• Cartilaginous: Symphyses (intervertebral discs), synchondroses (epiphyseal plates) – slight movement.
• Synovial (diarthroses): Freely movable; capsule, synovial fluid, articular cartilage. Types: hinge, pivot, ball-and-socket, saddle, condyloid, plane.

Divisions

• Axial: Skull, vertebral column, thoracic cage (80 bones).
• Appendicular: Limbs + girdles (126 bones).

Functions

• Support, protection, movement (levers), mineral homeostasis (Ca²⁺, PO₄³⁻), hematopoiesis, triglyceride storage (yellow marrow), endocrine (osteocalcin, FGF23).