The skeletal system is an intricate framework organized into two major divisions based on their distinct locations and functional roles. The axial skeleton forms the central support structure for the body.
Defining the Central Framework of the Body
The axial skeleton forms the central, vertical axis of the body, comprised of 80 individual bones in an adult human.
This central core provides the main structure for the head, neck, and trunk. Its physical location spans from the skull down to the base of the spine and across the chest, establishing the body’s midline and giving rigidity to the torso.
The Three Major Component Groups
The axial skeleton is organized into three distinct groups of bones, starting with the skull. The skull is divided into the cranium (eight bones that encase the brain) and 14 facial bones. The hyoid bone in the neck and the six auditory ossicles within the middle ear are also counted as part of this complex.
The second major component is the vertebral column, or spine. This column is composed of 24 individual vertebrae, plus the fused bones of the sacrum and the coccyx. The vertebrae are categorized into seven cervical bones (neck), twelve thoracic bones (upper back), and five lumbar bones (lower back).
The third major group is the thoracic cage, which surrounds the chest cavity. It is formed by the sternum (breastbone) and the 24 ribs that extend from the thoracic vertebrae. The ribs attach to the sternum either directly, indirectly, or not at all, creating an enclosure that expands during respiration.
Essential Roles of Support and Protection
The primary function of the axial skeleton is to offer a robust defense for the body’s most delicate internal organs. The bony plates of the cranium form a protective vault for the brain, while the vertebral column creates a hollow canal to shield the spinal cord. Similarly, the entire thoracic cage works as a flexible shield to enclose and safeguard the heart and the lungs.
The axial structure also serves a role in support and posture. It acts as a central pillar, holding the body upright against gravity. The bones of the trunk provide surface areas for the attachment of muscles that control movement of the head, neck, and torso.
Distinguishing Axial from Appendicular Anatomy
The skeletal system is completed by the appendicular division, which contrasts with the axial framework. The appendicular skeleton includes the 126 bones of the upper and lower limbs (arms, hands, legs, and feet). These bones are specialized for movement and locomotion.
This division connects to the central axis via the pectoral and pelvic girdles. The pectoral girdle attaches the upper limbs, and the pelvic girdle links the lower limbs to the trunk. The axial skeleton provides central stability, while the appendicular skeleton provides mobility.