The axial and appendicular components of the skeletal system of an adult human consist of 206 individual bones arranged to form a strong, flexible body framework. The adult skeletal system consists of approximately 206 bones. The exact number of bones differs from person to person depending on age and genetic factors. At birth, the skeleton consists of about 270 bones. As further bone development (ossification) occurs during infancy, the number increases. During adolescence, however, the number of bones decreases, as separate bones gradually fuse. Each bone is actually an organ that plays a part in the total functioning of the skeletal system. The science concerned with the study of bones is called osteology. Some adults have extra bones within the sutures (joints) of the skull called sutural (wormian) bones. Additional bones may develop in tendons in response to stress as the tendons repeatedly move across a joint. Bones formed this way are called sesamoid bones. Sesamoid bones, like the sutural bones, vary in number. The patellae (“kneecaps”) are two sesamoid bones all people have. For convenience of study, the skeleton is divided into axial and appendicular portions, are shown in (fig. 3.1.) The axial skeleton consists of the bones that form the axis of the body and support and protect the organs of the head, neck, and trunk. The components of the axial skeleton are as follows:
FIGURE 3.1 The human skeleton. (a) An anterior view and (b) a posterior view. The axial portion is colored light blue.
1. Skull. The skull consists of two sets of bones: the cranial bones that form the cranium, or braincase, and the facial bones that support the eyes and nose and form the bony framework of the oral cavity.
2. Auditory ossicles. Three auditory ossicles (“ear bones”) are present in the middle-ear chamber of each ear and serve to transmit sound impulses.
3. Hyoid bone. The hyoid bone is located above the larynx (“voice box”) and below the mandible (“jawbone”). It supports the tongue and assists in swallowing.
4. Vertebral column. The vertebral column (“backbone”) consists of 26 individual bones separated by cartilaginous intervertebral discs. In the pelvic region, several vertebrae are fused to form the sacrum, which is the attachment portion of the pelvic girdle. A few terminal vertebrae are fused to form the coccyx (“tailbone”).
5. Rib cage. The rib cage forms the bony and cartilaginous framework of the thorax. It articulates posteriorly with the thoracic vertebrae and includes the 12 pairs of ribs, the flattened sternum, and the costal cartilages that connect the ribs to the sternum.

The appendicular skeleton is composed of the bones of the upper and lower extremities and the bony girdles that anchor the appendages to the axial skeleton. The components of the appendicular skeleton are as follows:
1. Pectoral girdle. The paired scapulae (“shoulder blades”) and clavicles (“collarbones”) are the appendicular components of the pectoral girdle, and the sternum (“breastbone”) is the axial component. The primary function of the pectoral girdle is to provide attachment for the muscles that move the brachium (arm) and antebrachium (forearm).
2. Upper extremities. Each upper extremity contains a proximal humerus within the brachium, an ulna and radius within the antebrachium, the carpal bones, the metacarpal bones, and the phalanges (“finger bones”) of the hand.
3. Pelvic girdle. The two ossa coxae (“hipbones”) are the appendicular components of the pelvic girdle, and the sacrum is the axial component. The ossae coxae are united anteriorly by the symphysis pubis and posteriorly by the sacrum. The pelvic girdle supports the weight of the body through the vertebral column and protects the viscera within the pelvic cavity.
4. Lower extremities. Each lower extremity contains a proximal femur (“thighbone”) within the thigh, a tibia (“shinbone”) and fibula within the leg, the tarsal bones, the metatarsal bones, and the phalanges (“toe bones”) of the foot. In addition, the patella is located on the anterior surface of the knee joint, between the thigh and leg.
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