The axial skeleton forms the central framework of the human body, supporting and protecting organs and the central nervous system. Positioned along the body’s midline, it includes bones of the head, neck, and trunk. This core maintains posture and provides attachment points for muscles involved in movement and respiration.
The Skull and Hyoid Bone
The skull, a complex bony structure, consists of two main groups of bones: the cranium and the facial bones. The cranium forms the superior aspect of the skull, protecting the brain, its surrounding membranes, and associated blood vessels. Its eight bones include the single frontal, occipital, sphenoid, and ethmoid bones, and the paired parietal and temporal bones. These bones are tightly fused in adults by immovable fibrous joints called sutures, though they are separate in children to allow for brain growth.
The facial skeleton comprises 14 bones that shape the face and protect sensory organs like the eyes, nose, and mouth. These bones include paired zygomatic (cheekbones), lacrimal (inner eye corner), nasal (bridge of nose), maxilla (upper jaw), palatine (hard palate), and inferior nasal conchae bones. The unpaired facial bones are the vomer, forming part of the nasal septum, and the mandible, which is the lower jaw and the only movable bone of the skull.
Below the skull, in the anterior neck, is the U-shaped hyoid bone. This bone does not articulate directly with any other bone in the body. Instead, it is suspended by muscles and ligaments, aiding tongue movement, swallowing, and speech production by providing attachment points for muscles of the mouth, tongue, and larynx.
The Vertebral Column
The vertebral column, also known as the spine or backbone, is a flexible column extending from the base of the skull to the pelvis. It consists of 33 individual vertebrae in youth, which reduce to 26 bones in adulthood as some fuse. The column supports the trunk, protects the spinal cord, and allows for a wide range of motion.
The vertebral column is divided into five distinct regions. The cervical region, located in the neck, contains seven vertebrae (C1-C7). Below this, the thoracic region comprises 12 vertebrae (T1-T12) in the upper back, which articulate with the ribs. The lumbar region, forming the lower back, has five larger vertebrae (L1-L5) for weight-bearing.
Inferior to the lumbar region is the sacrum, a single bone formed by the fusion of five sacral vertebrae (S1-S5). At the end of the column is the coccyx, or tailbone, composed of three to five fused coccygeal vertebrae. Intervertebral discs, made of fibrocartilage, cushion most vertebrae, absorbing shock and contributing to spinal flexibility.
The Thoracic Cage
The thoracic cage, commonly known as the rib cage, forms the bony framework of the chest. It is composed of the sternum, 12 pairs of ribs, and the 12 thoracic vertebrae to which the ribs attach posteriorly. This structure protects organs within the chest cavity, including the heart and lungs. It also aids respiration by allowing expansion and contraction for breathing.
The sternum, or breastbone, is a flat, elongated bone situated in the central anterior chest. It consists of three main parts: the manubrium, the body, and the xiphoid process. The manubrium is the broad, superior portion, connecting with the clavicles and the first pair of ribs. The body forms the central and longest part, attaching to most of the remaining ribs. The xiphoid process is the small, pointed inferior tip.
The 12 pairs of ribs extend from the thoracic vertebrae towards the sternum. They are categorized based on their anterior attachment. True ribs (pairs 1-7) connect directly to the sternum via their own costal cartilages. False ribs (pairs 8-10) attach indirectly, with their cartilages joining the cartilage of the rib above them. The last two pairs, ribs 11 and 12, are known as floating ribs because they do not attach anteriorly to the sternum or to other ribs, ending freely in the abdominal wall.