Axial skeleton

The axial skeleton forms the central axis of the human body, providing structural support and protecting vital organs. It serves as the foundation for posture, movement, and attachment of muscles and ligaments. This article explores the anatomy, development, function, and clinical relevance of the axial skeleton.

Anatomy of the Axial Skeleton

Definition

The axial skeleton includes bones that form the central axis of the body, comprising the skull, vertebral column, rib cage, and the hyoid bone. It supports the head, neck, and trunk while protecting the brain, spinal cord, and thoracic organs.

Components

The axial skeleton consists of several major components:

• Skull: Protects the brain and forms the framework of the face
• Vertebral column: Encloses and protects the spinal cord and supports the body
• Rib cage: Includes the ribs and sternum, protecting the heart and lungs
• Hyoid bone: Supports the tongue and serves as an attachment for neck muscles

Skull

Cranial Bones

The cranial bones form the protective case around the brain and provide attachment points for muscles of the head and neck:

• Frontal bone
• Parietal bones
• Temporal bones
• Occipital bone
• Sphenoid and ethmoid bones

Facial Bones

The facial bones create the structure of the face and support sensory organs:

• Maxilla and mandible
• Zygomatic bones
• Nasal bones
• Lacrimal, palatine, vomer, and inferior nasal conchae

Sutures and Foramina

The cranial sutures are immovable joints connecting the cranial bones, while foramina allow passage of nerves and blood vessels to and from the brain. Key sutures include the coronal, sagittal, lambdoid, and squamous sutures. Important foramina include the optic canal, jugular foramen, and foramen magnum.

Vertebral Column

Regions

The vertebral column, also known as the spine, consists of a series of vertebrae that protect the spinal cord and support the body. It is divided into five main regions:

• Cervical vertebrae: Seven vertebrae (C1–C7) forming the neck region, allowing head movement
• Thoracic vertebrae: Twelve vertebrae (T1–T12) attached to the ribs, providing thoracic stability
• Lumbar vertebrae: Five vertebrae (L1–L5) bearing most of the body’s weight and providing flexibility
• Sacrum: Five fused vertebrae forming the posterior pelvic wall
• Coccyx: Four fused vertebrae forming the tailbone, providing attachment for ligaments and muscles

Curvatures

The vertebral column exhibits natural curvatures that enhance flexibility and shock absorption:

• Cervical lordosis: Concave posterior curve in the neck
• Thoracic kyphosis: Convex posterior curve in the upper back
• Lumbar lordosis: Concave posterior curve in the lower back
• Sacral kyphosis: Convex posterior curve in the sacral region

Intervertebral Discs and Ligaments

Intervertebral discs lie between vertebrae and act as cushions, absorbing shock and allowing movement. Each disc has an outer fibrous ring called the annulus fibrosus and a central gelatinous core called the nucleus pulposus. Ligaments, including the anterior and posterior longitudinal ligaments, ligamentum flavum, and interspinous ligaments, provide stability to the vertebral column.

Rib Cage

Ribs

The rib cage protects thoracic organs such as the heart and lungs and provides attachment points for muscles involved in respiration. The ribs are categorized as follows:

• True ribs: The first seven pairs attach directly to the sternum via costal cartilages
• False ribs: Ribs 8–10 attach indirectly to the sternum through the cartilage of the rib above
• Floating ribs: Ribs 11 and 12 do not attach to the sternum and provide some protection to the kidneys

Sternum

The sternum, or breastbone, is a flat bone in the anterior thorax that serves as the central point of rib attachment:

• Manubrium: The upper portion that articulates with the clavicles and first pair of ribs
• Body of sternum: The central elongated portion, articulating with ribs 2–7
• Xiphoid process: The small, cartilaginous lower tip that ossifies with age

Costal Cartilages and Articulations

Costal cartilages connect the ribs to the sternum, allowing flexibility and movement during respiration. The costovertebral and costotransverse joints articulate each rib with the thoracic vertebrae, enabling rib elevation and expansion of the thoracic cavity during breathing.

Hyoid Bone

Anatomical Features

The hyoid bone is a U-shaped bone located in the anterior neck at the level of the third cervical vertebra. It consists of a central body and two pairs of horns: greater and lesser cornua. It does not articulate directly with other bones, making it unique in the skeleton.

Functional Significance

The hyoid bone serves as an anchor for muscles of the tongue, pharynx, and larynx, facilitating swallowing, speech, and airway protection. Its position and mobility are essential for proper function of the upper respiratory and digestive tracts.

Development and Ossification

Embryological Development

The axial skeleton develops from mesenchymal cells derived from the paraxial mesoderm, which form somites along the neural tube. These somites differentiate into sclerotomes, giving rise to vertebrae, ribs, and portions of the skull. The hyoid bone originates from the second and third pharyngeal arches.

Ossification Centers and Timeline

Ossification of the axial skeleton occurs in multiple centers throughout development. Most cranial bones undergo intramembranous ossification, whereas vertebrae and ribs develop via endochondral ossification. The sternum and hyoid bone also ossify from multiple centers, with fusion occurring progressively from childhood to adulthood.

Function

Support and Posture

The axial skeleton provides the main structural framework of the body, supporting the head, neck, and trunk. It maintains upright posture and allows controlled movements of the body segments.

Protection of Vital Organs

The axial skeleton protects critical organs:

• The skull encases and protects the brain
• The vertebral column safeguards the spinal cord
• The rib cage shields the heart and lungs

Attachment for Muscles

Numerous muscles attach to the axial skeleton, including muscles of the head, neck, back, and thorax. These attachments facilitate movement, respiration, and stability of the body.

Respiration Assistance

The rib cage and associated muscles contribute to breathing by expanding and contracting the thoracic cavity. This allows the lungs to inflate and deflate efficiently during respiration.

Clinical Significance

Congenital Abnormalities

Congenital malformations of the axial skeleton can lead to functional impairments and structural deformities:

• Spina bifida: Incomplete closure of the vertebral arches, which may cause neurological deficits
• Craniosynostosis: Premature fusion of cranial sutures, resulting in abnormal skull shape and potential intracranial pressure increase

Fractures and Trauma

Trauma to the axial skeleton can compromise structural integrity and organ protection:

• Skull fractures, which may lead to brain injury or hemorrhage
• Vertebral fractures, potentially causing spinal cord injury
• Rib fractures, which can impair respiration and injure thoracic organs

Degenerative Disorders

Degenerative changes affect the axial skeleton over time, leading to pain and functional limitations:

• Osteoarthritis, causing joint degeneration in the vertebral facets and costovertebral joints
• Osteoporosis, increasing the risk of vertebral compression fractures

Infections and Tumors

Pathological conditions such as infections and tumors can affect axial bones:

• Osteomyelitis, an infection of vertebrae or ribs
• Primary bone tumors or metastatic lesions affecting the vertebral column, skull, or ribs

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
3. Rohen JW, Yokochi C, Lütjen-Drecoll E. Color Atlas of Anatomy: A Photographic Study of the Human Body. 8th ed. Philadelphia: Wolters Kluwer; 2016.
4. Behrman RE, Kliegman RM, Jenson HB. Nelson Textbook of Pediatrics. 21st ed. Philadelphia: Elsevier; 2020.
5. Schuenke M, Schulte E, Schumacher U. Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System. 3rd ed. Stuttgart: Thieme; 2019.
6. Fick RS, Standring S. Clinical anatomy of the spine and spinal cord. In: Tubbs RS, editor. Neuroanatomy: An Atlas of Structures, Sections, and Systems. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2016. p. 321-350.
7. Bridwell KH, DeWald RL, eds. The Spine. 5th ed. Philadelphia: Elsevier; 2019.
8. Snell RS. Clinical Anatomy by Regions. 10th ed. Philadelphia: Wolters Kluwer; 2019.
9. Standring S, Healy JC, Gray H. Skull and Cranial Sutures: Anatomy and Clinical Relevance. J Anat. 2016;229(4):407-417.
10. Rogers LF. Radiology of the Spine and Spinal Cord. 2nd ed. Philadelphia: Saunders; 2017.