Axial Skeleton: Bones, Joints, and Functional Classifications

Axial Skeleton: Overview and Classification

Objectives

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage. Understanding its structure and function is essential for studying human anatomy and physiology.

• Classify bones as part of the axial or appendicular skeleton.

• Identify bones and bone features from individual bones, articulated skeletons, and images.

• Classify joints by structure and function, and describe their movement capabilities.

Divisions of the Skeleton

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage (ribs and sternum).

• Appendicular Skeleton: Composed of the bones of the appendages (limbs) and the girdles that attach them to the axial skeleton.

As you study, note the protrusions and depressions on bones that give each bone its characteristic shape and texture. These features are important for learning the anatomy and for muscle attachment.

Joints: Classification and Function

Functional Classifications

Joints are classified by the amount of movement they allow:

• Synarthrosis: Allows no movement.

• Amphiarthrosis: Allows very limited movement.

• Diarthrosis: Freely movable joint.

Structural Classifications

Joints are also classified by their structure:

• Fibrous: Bones joined by dense regular connective tissue.

  • Sutures: Occur between bones of the skull.

  • Syndesmoses: Occur between the tibia and fibula, and the radius and ulna.

  • Gomphoses: Occur between teeth and the maxilla or mandible.

• Cartilaginous: Bones are connected by cartilage.

  • Synchondroses: Bones are joined by hyaline cartilage (e.g., costal cartilage).

  • Symphyses: Bones are connected by fibrocartilage (e.g., intervertebral discs).

• Synovial: Bones are separated by a synovial cavity, lined by a synovial membrane and filled with synovial fluid. Types include:

  • Plane: Nonaxial movement (gliding).

  • Hinge: Uniaxial movement (flexion/extension).

  • Pivot: Uniaxial movement (rotation).

  • Condylar: Biaxial movement (flexion/extension, abduction/adduction).

  • Saddle: Biaxial movement (opposing surfaces).

  • Ball and Socket: Triaxial movement (movement around three axes).

Skull: Bones and Features

Cranial Skull

The cranial skull houses and protects the brain. It is composed of the following bones:

• Frontal

• Parietal (2)

• Temporal (2)

• Occipital

• Sphenoid

• Ethmoid

Key features include sutures (joints between skull bones):

• Coronal suture: Between frontal and parietal bones.

• Sagittal suture: Between two parietal bones.

• Lambdoid suture: Between parietal and occipital bones.

• Squamous suture: Between parietal and temporal bones.

Facial Skull

The facial skull provides the socket for the eyes, openings for air and food, and attachment points for facial muscles. It is composed of:

• Maxilla (2)

• Mandible

• Zygomatic (2)

• Nasal (2)

• Lacrimal (2)

• Vomer

• Palatine (2)

• Inferior nasal conchae (2)

Vertebral Column

Structure and Regions

The vertebral column is made up of 24 individual vertebrae, divided into regions:

• Cervical vertebrae: 7

• Thoracic vertebrae: 12

• Lumbar vertebrae: 5

• Sacrum and Coccyx (fused bones)

Key features of vertebrae include the body, spinous process, transverse process, and vertebral foramen.

Special Vertebrae

• Atlas (C1): Supports the skull, allows nodding "yes".

• Axis (C2): Has the dens (odontoid process), allows rotation "no".

Thoracic Cage

Sternum and Ribs

The thoracic cage includes the sternum and ribs, protecting vital organs and supporting respiration.

• Sternum: Manubrium, body, xiphoid process.

• Ribs: 12 pairs total

  • True ribs: 7 pairs, directly attached to sternum.

  • False ribs: 3 pairs, attached indirectly via costal cartilage.

  • Floating ribs: 2 pairs, not attached to sternum.

Tables: Bone and Joint Classification

Major Bones of the Axial Skeleton

Cranial Skull	Vertebral Column
Frontal Parietal (2) Temporal (2) Occipital Sphenoid Ethmoid	Vertebrae (24) Cervical vertebrae (7) Thoracic vertebrae (12) Lumbar vertebrae (5) Sacrum Coccyx
Facial Skull	Thorax
Maxilla (2) Mandible Zygomatic (2) Nasal (2) Lacrimal (2) Vomer Palatine (2) Inferior nasal conchae (2)	Sternum Ribs (True, False, Floating)

Joints of the Axial Skeleton

Joint	Classification
Atlanto-occipital	Diarthrotic, synovial, condylar
Atlantoaxial	Diarthrotic, synovial, pivot
Costochondral	Synarthrotic, cartilaginous, synchondrosis
Intervertebral – adjacent articular processes	Diarthrotic, synovial, gliding
Intervertebral – adjacent bodies	Amphiarthrotic, cartilaginous, symphysis
Sutures (sagittal, squamous, lambdoid, coronal)	Synarthrotic, fibrous, suture
Temporomandibular	Diarthrotic, synovial, hinge

Key Terms and Definitions

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

• Appendicular Skeleton: The limbs and girdles that attach them to the axial skeleton.

• Suture: A fibrous joint between skull bones.

• Synovial Joint: A freely movable joint with a synovial cavity.

• Symphysis: A joint where bones are connected by fibrocartilage.

• Condylar Joint: A synovial joint allowing movement in two planes.

Formulas and Equations

There are no direct mathematical formulas for bone classification, but joint movement can be described by axes of rotation:

• Uniaxial movement: $\text{Movement occurs around one axis}$

• Biaxial movement: $\text{Movement occurs around two axes}$

• Triaxial movement: $\text{Movement occurs around three axes}$

Summary Table: Joint Types and Movements

Joint Type	Movement	Example
Hinge	Uniaxial (flexion/extension)	Elbow, knee
Pivot	Uniaxial (rotation)	Atlantoaxial joint
Condylar	Biaxial	Wrist
Ball and Socket	Triaxial	Shoulder, hip
Plane	Nonaxial (gliding)	Intervertebral articular processes

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables have been reconstructed and summarized for study purposes.