Chapter 7: The Axial Skeleton – Structure, Function, and Development

Axial Skeleton Overview

Functions of the Skeletal System

The skeletal system provides the structural framework for the human body and serves several essential functions.

• Support: The skeleton supports body tissues and organs, maintaining body shape.

• Protection: Bones protect vital organs such as the brain (skull), heart and lungs (thoracic cage).

• Movement: Bones act as levers for muscles, enabling movement and locomotion.

• Mineral Storage: Bones store minerals, primarily calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Production: Bone marrow produces red and white blood cells (hematopoiesis).

• Fat Storage: Yellow bone marrow stores fat as an energy reserve.

Divisions of the Skeletal System

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the central axis of the body and includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Includes the limbs and girdles (shoulder and pelvic), enabling movement and manipulation of the environment.

Bone Count: The adult human skeleton typically consists of 206 bones.

Axial Skeleton Components

Skull

The skull protects the brain and forms the structure of the face. It consists of 22 bones: 8 cranial bones and 14 facial bones.

• Cranial Bones: Frontal, parietal (2), occipital, temporal (2), sphenoid, ethmoid.

• Facial Bones: Maxilla (2), palatine (2), zygomatic (2), nasal (2), lacrimal (2), inferior nasal conchae (2), vomer, mandible.

• Mandible: The only moveable bone of the skull.

Major Suture Lines: Joints between skull bones, including coronal, sagittal, lambdoid, and squamosal sutures.

Cranial Fossae and Fossa

The cranial cavity is divided into three fossae that accommodate different parts of the brain:

• Anterior Cranial Fossa: Houses the frontal lobes; bounded by the frontal bone and lesser wing of the sphenoid.

• Middle Cranial Fossa: Contains the temporal lobes; bounded by the sphenoid and temporal bones.

• Posterior Cranial Fossa: Contains the cerebellum; bounded by the petrous ridge of the temporal bone and occipital bone.

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within certain skull bones that lighten the skull and add resonance to the voice.

• Frontal Sinus: Located above the eyebrows.

• Maxillary Sinus: Located below the orbits.

• Sphenoidal Sinus: Located beneath the sella turcica.

• Ethmoidal Sinus: Located between the nasal cavity and orbit.

Orbit and Nasal Structures

The orbit is the bony cavity that houses the eye, formed by seven bones: frontal, zygomatic, maxilla, sphenoid, lacrimal, ethmoid, and palatine.

• Nasal Septum: Formed by the perpendicular plate of the ethmoid, vomer, and cartilage.

• Nasal Conchae: Superior and middle conchae (ethmoid), inferior concha (independent bone); function to warm, humidify, and filter air.

Hyoid Bone

The hyoid bone is located in the upper neck and does not articulate with other bones. It serves as an attachment site for muscles involved in swallowing and speech.

Bony Openings of the Skull

Openings (foramina) in the skull allow passage of nerves and blood vessels. Examples include:

• Foramen Magnum: Passage for the spinal cord.

• Optic Canal: Passage for the optic nerve.

• Stylomastoid Foramen: Passage for the facial nerve.

• Mental Foramen: Passage for nerves and vessels to the chin.

The Vertebral Column

Structure and Regions

The vertebral column (spine) consists of 33 vertebrae separated by intervertebral discs. It supports the head, neck, and trunk, protects the spinal cord, and allows flexible movement.

• Cervical Vertebrae: 7 (C1–C7)

• Thoracic Vertebrae: 12 (T1–T12)

• Lumbar Vertebrae: 5 (L1–L5)

• Sacral Vertebrae: 5 (fused to form sacrum)

• Coccygeal Vertebrae: 4 (fused to form coccyx)

Curvatures of the Spine

The vertebral column has four main curvatures:

• Primary Curvatures: Thoracic and sacral (present at birth)

• Secondary Curvatures: Cervical and lumbar (develop postnatally)

• Function: Curvatures increase strength and shock absorption.

Abnormal Curvatures: Scoliosis (lateral curvature), kyphosis (excessive thoracic curvature), lordosis (excessive lumbar curvature).

Structure of a Typical Vertebra

Each vertebra has several key features:

• Body: Weight-bearing portion.

• Processes: Transverse, spinous, and articular processes for muscle and ligament attachment.

• Vertebral Foramen: Canal for the spinal cord.

• Intervertebral Foramen: Openings for spinal nerves.

Regional Vertebral Differences

• Cervical Vertebrae: Small bodies, bifid spinous processes, transverse foramina (for vertebral arteries). C1 (atlas) and C2 (axis) are specialized for head movement.

• Thoracic Vertebrae: Heart-shaped bodies, long downward-sloping spinous processes, facets for rib articulation.

• Lumbar Vertebrae: Largest bodies, short spinous processes, articular facets oriented medially/laterally for stability.

• Sacrum: Fused vertebrae forming the posterior pelvis.

• Coccyx: Fused vertebrae forming the vestigial tailbone.

Intervertebral Discs

Intervertebral discs separate vertebral bodies and provide shock absorption and flexibility.

• Annulus Fibrosus: Tough outer ring.

• Nucleus Pulposus: Gel-like center.

• Function: Discs contribute to height and can compress spinal nerves if herniated.

Disc Herniation: Occurs when the nucleus pulposus protrudes through the annulus fibrosus, potentially compressing spinal nerves.

Ligaments of the Spine

Ligaments stabilize the vertebral column:

• Anterior Longitudinal Ligament: Resists hyperextension.

• Posterior Longitudinal Ligament: Supports the vertebral bodies.

• Ligamentum Flavum: Elastic, connects laminae.

• Supraspinous Ligament: Connects spinous processes.

• Nuchal Ligament: Supports the head and neck.

The Thoracic Cage

Components of the Thoracic Cage

The thoracic cage protects vital organs and supports breathing.

• Sternum: Consists of the manubrium (superior), body (middle), and xiphoid process (inferior).

• Ribs: 12 pairs, attached posteriorly to thoracic vertebrae.

• Costal Cartilages: Hyaline cartilage connecting ribs to sternum.

Parts of the Sternum

• Manubrium: Superior portion, articulates with clavicles and first ribs.

• Body: Middle portion, articulates with ribs 2–7.

• Xiphoid Process: Inferior tip, ossifies in adulthood.

• Sternal Angle: Junction of manubrium and body; important anatomical landmark.

Rib Structure and Classification

• Head: Articulates with vertebral body.

• Neck: Narrow region lateral to head.

• Tubercle: Articulates with transverse process of vertebra.

• Shaft: Curved main portion.

• Costal Groove: Inferior margin for vessels and nerves.

Rib Classifications:

• True Ribs (1–7): Attach directly to sternum via costal cartilage.

• False Ribs (8–12): Do not attach directly to sternum; ribs 8–10 attach via cartilage of rib 7.

• Floating Ribs (11–12): No anterior attachment; terminate in abdominal musculature.

Functions of the Thoracic Cage

• Protection: Shields thoracic organs from injury.

• Respiration: Facilitates breathing movements via rib elevation and depression.

• Muscle Attachment: Provides sites for muscles of posture and upper limb movement.

Embryonic Development of the Axial Skeleton

Overview of Development

The axial skeleton develops from mesenchyme, notochord, and somites during early embryonic stages. Bone formation continues into adolescence and adulthood.

• Intramembranous Ossification: Direct conversion of mesenchyme into bone; forms most cranial bones.

• Endochondral Ossification: Bone forms from a cartilage model; forms facial bones, cranial base, vertebrae, and ribs.

Intramembranous Ossification

• Process: Mesenchymal cells differentiate directly into bone.

• Examples: Bones of the brain case (frontal, parietal, occipital, temporal).

• Fontanelles: Soft spots in the infant skull that allow deformation during birth and postnatal growth; typically close by age 2.

Endochondral Ossification

• Process: Mesenchymal cells form a cartilage model, which is gradually replaced by bone.

• Examples: Facial bones, cranial base, vertebrae, ribs.

Development of the Vertebral Column and Thoracic Cage

• Vertebral Column: Bone cells gather around the notochord, forming a cartilage model that ossifies into vertebrae. Remnants of the notochord become the nucleus pulposus of intervertebral discs.

• Sternum: Develops from paired cartilage models along the midline; ribs attach during development.

Summary Table: Ossification Types

Ossification Type	Examples	Process
Intramembranous	Brain case bones	Direct from mesenchymal tissue
Endochondral	Facial bones, cranial base, vertebrae, ribs	Intermediate cartilage stage

Key Terms and Definitions

• Axial Skeleton: Central part of the skeleton including skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Limbs and girdles attached to the axial skeleton.

• Fontanelle: Soft spot in the infant skull where bone formation is incomplete.

• Foramen: Opening in bone for passage of nerves and vessels.

• Ossification: Process of bone formation.

Sample Exam Questions

• What does the axial skeleton include?

• How many bones are in the adult skeleton?

• Which bone is the only moveable bone of the skull?

• What is the function of the nasal conchae?

• Which region has 12 vertebrae?

• Which ribs are considered floating ribs?

• A patient is diagnosed with scoliosis. This condition is characterized by a(n):

• Which of the following is a characteristic of a typical thoracic vertebra?

• Which bones form via intramembranous ossification?

• Which of the following statements about fontanelles are true?

• During embryonic development, the notochord is completely replaced by the vertebral column.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and academic context.