The skeletal system is divided into the axial and appendicular sections. The axial skeleton forms the central, upright axis of the body, comprising 80 bones. This core structure provides the main support for the trunk and head, and the appendicular skeleton (limbs) connects to it.
Bones of the Head and Face
The skull consists of 22 bones divided into the cranium and the facial bones. The cranium houses and protects the brain within a cavity formed by eight plate-shaped bones. These include the unpaired frontal, occipital, ethmoid, and sphenoid bones, along with the paired parietal and temporal bones. These bones meet at immovable joints called sutures.
The face is structured by 14 bones that provide cavities for the sense organs and attachment points for facial muscles. Prominent facial bones include the mandible (lower jaw), the only movable bone of the skull, and the maxillae (upper jaw). The zygomatic bones create the cheekbones, and the nasal bones form the bridge of the nose.
The axial framework also includes associated bones of the head and neck. Six tiny auditory ossicles—the malleus, incus, and stapes—are located within the middle ear and transmit sound vibrations. The single hyoid bone, located below the mandible, is a bone that does not articulate with any other bone. It serves as a movable base for the tongue and an attachment point for neck muscles.
The Vertebral Column
The vertebral column, or spine, extends from the base of the skull to the pelvis, consisting of 26 bones in the adult. The spine is organized into five distinct regions, each with a specific number of individual vertebrae.
The uppermost region is the cervical spine (C1–C7), consisting of seven vertebrae that support the head and neck. Below this are the 12 thoracic vertebrae (T1–T12), which articulate with the ribs. The five largest vertebrae, the lumbar vertebrae (L1–L5), form the lower back and bear the greatest amount of the body’s weight.
The sacrum and coccyx are formed from fused vertebrae below the lumbar region. The sacrum is a single triangular bone resulting from the fusion of five sacral vertebrae, connecting with the pelvic bones. The coccyx, or tailbone, is also a single bone formed from fused coccygeal vertebrae. Intervertebral discs, located between adjacent vertebrae, act as shock absorbers and allow for subtle movements.
The Thoracic Cage
The thoracic cage, or rib cage, forms the bony enclosure of the chest. It is composed of the sternum and 12 pairs of ribs. The sternum, or breastbone, is the elongated flat bone at the center front of the chest, divided into three parts: the superior manubrium, the central body, and the inferior xiphoid process.
The 12 pairs of ribs are anchored posteriorly to the 12 thoracic vertebrae. Ribs are classified based on their connection to the sternum. The first seven pairs are “true ribs” because their costal cartilages connect directly to the sternum.
Ribs eight through twelve are “false ribs” because their cartilages do not connect directly to the sternum. Ribs eight, nine, and ten attach to the cartilage of the rib above them. The last two pairs, ribs eleven and twelve, are called “floating ribs” because they do not connect to the sternum or to the cartilage of any other ribs.
Core Functions of the Axial Skeleton
The axial skeleton performs two primary roles: protection and support. Protection involves safeguarding the body’s delicate organs. The skull forms a hard, bony vault that encases the brain.
The vertebral column creates a canal that shields the spinal cord. The thoracic cage encloses and protects the heart and lungs, providing a robust barrier. This framework maintains the integrity of the central nervous and cardiorespiratory systems.
The second function is support and posture, as the axial skeleton serves as the central pillar of the body. It provides a stable base for the attachment of muscles that control the movements of the head, neck, and trunk.
By maintaining upright posture, the axial bones efficiently transmit the weight of the upper body down to the lower limbs. This central bony axis also stabilizes the appendicular skeleton, allowing for range of motion in the arms and legs.