Axial skeleton
The axial skeleton forms the central core of the human endoskeleton, consisting of 80 bones that provide structural support along the body's longitudinal axis, including the skull, vertebral column, and thoracic cage.[1] It encompasses the bones of the head and trunk, distinguishing it from the appendicular skeleton, which includes the limbs and girdles.[2] The skull comprises 28 bones: 8 cranial bones that enclose the brain, 14 facial bones that support the features of the face, and 6 auditory ossicles in the middle ear. The hyoid bone (1) in the neck is also part of the axial skeleton.[3] The vertebral column, or spine, includes 26 vertebrae divided into cervical (7), thoracic (12), lumbar (5), sacral (5 fused into the sacrum), and coccygeal (4 fused into the coccyx) regions, forming a flexible column that encases the spinal cord.[1] The thoracic cage consists of 25 bones: the sternum (breastbone) and 24 ribs (12 pairs), which together protect the heart and lungs while facilitating breathing.[3] Key functions of the axial skeleton include protecting vital organs such as the brain, spinal cord, heart, and lungs from injury; providing attachment sites for muscles that enable posture, locomotion, and respiration; and serving as a framework for the body's overall support and weight distribution.[4] It also contributes to hematopoiesis through red bone marrow in its flat bones and stores minerals like calcium and phosphorus for metabolic needs.[5] In humans, this division of the skeleton underscores its evolutionary adaptation for upright posture and bipedalism.[6]Overview
Definition and Composition
The axial skeleton forms the central axis of the vertebrate body, comprising the core of the endoskeleton and consisting of the bones that support the head and trunk.[7] In adult humans, it includes exactly 80 bones, which provide structural support and protect vital organs such as the brain, spinal cord, and thoracic contents.[3] The composition of the axial skeleton is divided into three main regions: the bones of the head, the vertebral column, and the thoracic cage. The head region encompasses 29 bones, including the 22 bones of the cranium and face (8 cranial bones and 14 facial bones), 6 auditory ossicles (2 malleus, 2 incus, and 2 stapes), and 1 hyoid bone.[3][2] The vertebral column consists of 26 bones: 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 1 sacrum (fused from 5 vertebrae), and 1 coccyx (fused from 4 vertebrae).[3] The thoracic cage includes 25 bones: the sternum and 24 ribs (12 pairs).[3] In contrast to the axial skeleton, the appendicular skeleton comprises the 126 bones of the pectoral and pelvic girdles and the limbs, facilitating movement and interaction with the environment.[7] Evolutionarily, the axial skeleton originated in early vertebrates as an adaptation for body support and spinal cord protection, evolving from notochord-based structures in chordates.[8]Functions and Importance
The axial skeleton serves as the central framework of the body, primarily functioning to protect vital structures including the brain, spinal cord, heart, lungs, and abdominal organs. The skull encases the brain, the vertebral column shields the spinal cord, and the thoracic cage safeguards the thoracic and upper abdominal viscera from injury. Additionally, it acts as a central axis for muscle attachment, enabling posture maintenance and facilitating movements of the head, neck, and trunk.[9][10][11][12][13] It provides essential support for the appendicular skeleton through articulations at the rib cage and vertebral column, allowing the limbs to function in coordination with the core. The thoracic cage contributes to respiration by enabling rib movements that expand and contract the chest cavity during breathing, while the vertebral curvatures promote balance by distributing weight evenly and maintaining stability during upright positioning.[14][15][16][17] In locomotion, the axial skeleton functions as a rigid framework for weight-bearing, transmitting forces from the lower limbs to the upper body while the intervertebral discs and curvatures absorb shocks to minimize impact on the spine and organs. This structural integrity is crucial for efficient bipedal movement and overall mobility.[11][9][17] Comparatively, the human axial skeleton has adapted to emphasize upright posture, with enhanced lumbar lordosis and a more flexible vertebral column to support bipedalism, differing from the relatively straighter, horizontally oriented spine in quadrupedal vertebrates that prioritizes stability in four-limbed locomotion.[18][19]Components
Skull
The skull forms the uppermost portion of the axial skeleton, consisting of 22 bones that are divided into the cranium and the facial skeleton. The cranium comprises 8 bones that enclose and protect the brain, while the facial skeleton includes 14 bones that support the facial structures and sensory organs. In addition to these 22 bones, the skull region incorporates 6 auditory ossicles—three per middle ear (malleus, incus, and stapes)—and the hyoid bone, resulting in a total of 29 bones associated with the head.[3][20][21] The cranial bones include the frontal bone (1), parietal bones (2), temporal bones (2), occipital bone (1), sphenoid bone (1), and ethmoid bone (1). These bones are joined by immovable fibrous joints known as sutures, which provide stability while allowing slight movement during birth or growth. Major sutures include the coronal suture (between the frontal and parietal bones), the sagittal suture (between the two parietal bones), and the lambdoid suture (between the parietal and occipital bones).[3][22] The facial bones consist of the nasal bones (2), maxillae (2), zygomatic bones (2), mandible (1), lacrimal bones (2), palatine bones (2), inferior nasal conchae (2), and vomer (1). These bones articulate to form the nasal cavity, orbits, and oral cavity, contributing to the structural framework of the face.[3][20] Key features of the skull include numerous foramina that permit the passage of neurovascular structures; for example, the foramen magnum in the occipital bone serves as the conduit for the spinal cord connecting to the brainstem. The paranasal sinuses—air-filled cavities within the frontal, ethmoid, sphenoid, and maxillary bones—reduce the skull's weight, humidify inhaled air, produce mucus, and enhance vocal resonance. The skull also houses critical sensory organs, such as the eyes within the orbits, the nasal structures for olfaction, and the ears for hearing, integrating protection with sensory function. As part of the axial skeleton, the skull primarily protects the brain and encephalic structures.[23][24][25][20] The hyoid bone is a unique, U-shaped structure located in the anterior neck, suspended by ligaments and muscles without direct bony articulation to the skull or other bones. It supports the tongue and serves as an attachment point for muscles involved in swallowing and speech, as well as the larynx. The hyoid consists of a central body, two greater horns posteriorly, and two lesser horns superiorly.[26]Vertebral Column
The vertebral column, or spine, forms the central axis of the axial skeleton, providing structural support, flexibility, and protection for the spinal cord. In adults, it comprises 26 bones derived from 33 individual vertebrae that develop embryonically but undergo fusion in certain regions: 7 cervical vertebrae (C1–C7), 12 thoracic vertebrae (T1–T12), 5 lumbar vertebrae (L1–L5), 5 sacral vertebrae fused into a single sacrum, and typically 4 coccygeal vertebrae (varying from 3 to 5) fused into the coccyx.[3][27] These segments enable a range of motions, including the specialized head rotation facilitated by the atlas (C1) and axis (C2) in the cervical region.[28] A typical vertebra consists of a thick, weight-supporting anterior body and a posterior vertebral arch formed by paired pedicles and laminae, which enclose the vertebral foramen for the spinal cord. Projecting from this structure are the spinous process (posterior midline for muscle attachment), transverse processes (lateral extensions), and paired articular processes that form synovial joints with adjacent vertebrae for stability and movement. Between most vertebrae, intervertebral discs provide cushioning and allow limited motion; each disc features a central nucleus pulposus—a gel-like, hydrated core of proteoglycans and collagen that absorbs compressive forces—and an outer annulus fibrosus, a tough, concentric ring of fibrocartilage that contains the nucleus and resists torsion.[27][29] The vertebral column exhibits natural curvatures that enhance balance, distribute weight, and absorb shock during movement: lordosis (concave posteriorly) in the cervical and lumbar regions, and kyphosis (convex posteriorly) in the thoracic and sacral regions. These S-shaped curves develop progressively, with primary thoracic and sacral kyphoses present at birth and secondary cervical and lumbar lordoses forming as the infant assumes upright posture.[27]| Region | Number of Vertebrae | Key Features |
|---|---|---|
| Cervical (C1–C7) | 7 | Transverse foramina in transverse processes for passage of vertebral arteries and veins; C1 (atlas) lacks a body and has large superior articular facets to support the skull; C2 (axis) features a dens (odontoid process) for pivotal head rotation; small bodies overall for neck mobility.[30][28] |
| Thoracic (T1–T12) | 12 | Costal facets on vertebral bodies (superior and inferior demifacets) and transverse processes for rib articulation; heart-shaped bodies and longer spinous processes contributing to the region's relative rigidity.[31] |
| Lumbar (L1–L5) | 5 | Robust, kidney-shaped bodies and thick pedicles adapted for primary weight-bearing; large vertebral foramina but no transverse foramina; short, broad spinous processes.[32] |
| Sacral | 5 (fused into 1 sacrum) | Triangular bone with anterior concavity; superior articular facets articulate with L5; forms posterior wall of pelvis.[27] |
| Coccygeal | 3–5 (fused into 1 coccyx) | Small, rudimentary tailbone; provides muscle attachments for pelvic floor.[27] |