The human skeleton includes two divisions, the axial skeleton and the appendicular skeleton. The axial skeleton includes the bones that form the body's longitudinal axis or center, including the cranium, auditory ossicles, hyoid bone, vertebral column, and thoracic cavity. The 80 bones of the axial skeleton are located in the head and trunk. The function of the axial skeleton is to provide support and protection for the brain, the spinal cord, and the organs in the central body cavity. It provides a surface for the attachment of muscles that move the head, neck, and trunk, performs respiratory movements, and stabilizes parts of the appendicular skeleton. The axial skeleton runs vertically through the upper body. Its structure encases the brain, protects other body organs, and provides strength and support. The axial skeleton also helps enable a wide variety of functions. Breathing, gathering sensory information, including by sight and hearing, and maintaining an upright posture all depend on the axial skeleton. Bones of the axial skeleton also protect the spinal cord by completely surrounding it. Protection is perhaps the most important function of the axial skeleton. The axial skeleton is the base where bones that form structures, called girdles, connect limbs to the body. Cartilage and strong ligaments connect leg bones to the hip, or pelvic girdle. Muscles connect the vertebral column to arms at the shoulder, or pectoral girdle. Both girdles permit limbs to move freely.
The appendicular skeleton consists of all the bones of the limbs—the arms and legs.