The skeletal system provides the rigid framework on which the body is built. Its 206 adult bones serve multiple functions: they support soft tissues, protect vital organs like the brain and heart, enable movement by serving as attachment sites for muscles, store minerals such as calcium and phosphorus, and house bone marrow where blood cells are produced. Anatomists divide the skeleton into the axial skeleton, which forms the central core and includes the skull, vertebral column, and rib cage of about 80 bones, and the appendicular skeleton, comprising the limbs and their supporting girdles of about 126 bones. The shoulder girdle consists of the clavicle and scapula, with the scapula's glenoid cavity articulating the humerus for extensive arm mobility. The pelvic girdle is formed by two os coxae (each fusing ilium, ischium, and pubis) together with the sacrum and coccyx, supporting body weight and protecting pelvic organs. The lower limb contains the femur in the thigh, the patella at the knee, the tibia as the weight-bearing shin bone, and the fibula for lateral support, with tarsals, metatarsals, and phalanges making up the foot.
Bones are classified by shape into five categories that reflect their function. Long bones, such as the femur, are elongated with a shaft and built for leverage. Short bones like the carpals of the wrist are roughly cube-shaped and allow gliding movement. Flat bones, including those of the skull, are thin and provide broad surfaces for protection or muscle attachment. Irregular bones, exemplified by vertebrae with their complex shapes, fit together for support and protection of the spinal cord. Sesamoid bones, such as the patella, form within tendons to reduce friction at joints. A typical long bone shows the general structural plan of skeletal elements: a diaphysis shaft, two epiphyses at the ends, a metaphyseal growth zone, an outer periosteum, an inner endosteum, and a medullary cavity containing marrow, with compact bone surrounding a core of spongy bone trabeculae. A typical vertebra shows similar adaptation for its irregular form: a body for weight bearing, a vertebral arch formed by pedicles and laminae, a spinous process and two transverse processes for muscle attachment, and a vertebral foramen that, together with the others, creates the spinal canal; vertebrae are grouped into cervical, thoracic, and lumbar regions with regional variations in size.
Where bones meet, articulations (joints) determine the range and type of movement. Fibrous joints are immovable, joined by dense connective tissue as seen in the sutures between skull bones. Cartilaginous joints permit slight movement, as in intervertebral discs. Synovial joints are the most mobile, characterized by a joint capsule, synovial membrane that secretes lubricating fluid, articular cartilage covering the bone ends, surrounding ligaments for stability, and bursae to cushion adjacent structures; hinge joints like the knee and ball-and-socket joints like the shoulder are common subtypes. The skull itself reflects this organization, with eight cranial bones (frontal, two parietal, occipital, two temporal, sphenoid, and ethmoid) protecting the brain and fourteen facial bones (including the maxillae, mandible, and zygomatics) shaping the face, while numerous foramina throughout the skull allow passage of cranial nerves and blood vessels.