Basic Anatomy: Parts and Structure of the Skeletal System
The adult human skeleton consists of 206 bones, though babies are born with approximately 270 bones, many of which are cartilage that gradually ossifies and fuses as they grow. The skeletal system includes not only bones but also cartilage, ligaments, and tendons that work together to create a functional framework for movement and support.
Bones are classified by shape into five categories, each designed for specific functions. Long bones, such as the femur, humerus, and tibia, act as levers for movement and bear weight. These bones feature a long shaft (diaphysis) with expanded ends (epiphyses) that form joints with other bones. Short bones, found in the wrists and ankles, provide stability and support while allowing complex movements. Flat bones, including the skull, ribs, and sternum, protect vital organs and provide broad surfaces for muscle attachment. Irregular bones, such as vertebrae and facial bones, have complex shapes that serve specialized functions. Sesamoid bones, like the kneecap (patella), develop within tendons and improve leverage for muscle actions.
The microscopic structure of bone reveals its remarkable engineering. Compact bone, forming the outer layer of all bones, consists of repeating units called osteons or Haversian systems. Each osteon contains concentric rings of bone matrix (lamellae) surrounding a central canal that houses blood vessels and nerves. This arrangement provides maximum strength while maintaining blood supply to bone cells. Spongy bone, found in bone ends and interiors, consists of a lattice-like network of thin plates called trabeculae. This design maximizes strength while minimizing weight—spongy bone is about 20% as dense as compact bone but provides significant structural support.
Bone tissue consists of an organic matrix (primarily collagen) that provides flexibility and an inorganic matrix (mainly calcium phosphate) that provides hardness and compression resistance. Without the organic matrix, bones would be brittle like chalk; without the inorganic matrix, they would be flexible like rubber. This combination creates a composite material stronger than either component alone.
Three types of bone cells maintain skeletal health. Osteoblasts are bone-building cells that secrete the organic matrix and promote mineralization. Osteocytes are mature bone cells embedded within the bone matrix that maintain bone tissue and detect mechanical stress. Osteoclasts are large, multinucleated cells that break down bone tissue, releasing minerals into the bloodstream. The balance between osteoblast and osteoclast activity determines whether bone mass increases, decreases, or remains stable.
The skeletal system divides into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body and includes the skull (protecting the brain), vertebral column (protecting the spinal cord and supporting the trunk), and rib cage (protecting thoracic organs). The skull consists of 22 bones: 8 cranial bones forming the braincase and 14 facial bones. The vertebral column contains 26 bones: 7 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 1 sacrum (5 fused bones), and 1 coccyx (3-4 fused bones).
The appendicular skeleton includes the arms, legs, and the structures that attach them to the axial skeleton. The shoulder girdle (clavicle and scapula) connects the arms to the trunk, while the pelvic girdle (hip bones) connects the legs. Each arm contains 30 bones: 1 humerus, 2 forearm bones (radius and ulna), 8 wrist bones (carpals), 5 hand bones (metacarpals), and 14 finger bones (phalanges). Each leg contains 30 bones: 1 femur, 2 lower leg bones (tibia and fibula), 7 ankle bones (tarsals), 5 foot bones (metatarsals), and 14 toe bones (phalanges).
Joints, where bones meet, are classified by their structure and movement capabilities. Fibrous joints, such as skull sutures, are immovable and held together by dense connective tissue. Cartilaginous joints, like those between vertebrae, allow slight movement and are connected by cartilage. Synovial joints, including the knee, shoulder, and hip, allow free movement and feature a joint cavity filled with lubricating synovial fluid.