Chapter 7: The Skeleton

The skeletal system forms the structural foundation of the human body, organized into two functionally distinct divisions that work together to support, protect, and enable movement. The axial skeleton, comprising eighty bones arranged along the body's central axis, houses and protects vital organs including the brain, spinal cord, heart, and lungs. The skull consists of twenty-two bones divided into eight cranial bones that form the protective braincase and fourteen facial bones that define the face and protect sensory openings. These cranial bones are held together by immovable fibrous joints called sutures, while in infants, flexible soft spots called fontanelles allow the skull to deform during birth and accommodate rapid brain growth. The vertebral column contains twenty-six bones organized into distinct regions: seven cervical vertebrae support the head and allow its movement through specialized structures like the atlas and axis; twelve thoracic vertebrae connect to the ribs; five lumbar vertebrae bear the greatest weight; and the sacrum and coccyx complete the column by anchoring the pelvis. The thoracic cage, formed by twelve pairs of ribs and the sternum, protects the heart and lungs while facilitating breathing through its structure of true ribs, false ribs, and floating ribs with varying degrees of sternal attachment. The appendicular skeleton comprises one hundred twenty-six bones of the limbs and their supporting girdles. The pectoral girdle, consisting of the clavicle and scapula, connects each arm to the trunk and supports the humerus, which articulates with the radius and ulna of the forearm. The hand contains eight carpal bones in the wrist, five metacarpals, and fourteen phalanges distributed across fingers and thumbs. The pelvic girdle, formed from three fused bones—the ilium, ischium, and pubis—creates the hip bones and works with the sacrum to form the complete pelvis, which bears significant structural differences in females to facilitate reproduction. The lower limbs include the femur as the body's largest bone, articulating with the patella and tibia at the knee, while the fibula provides lateral support and muscle attachment without bearing weight. The foot contains seven tarsal bones, five metatarsals, and fourteen phalanges, with the longitudinal arch serving as a crucial shock-absorbing structure during weight-bearing activities.