Axial Skeleton Summary

Axial Skeleton Overview

• Components of the axial skeleton:

  • Skull: Encloses and protects the brain and supports the structures of the face.

  • Vertebral column: Composed of vertebrae that protect the spinal cord, allowing for flexibility and support to maintain an upright posture.

  • Thoracic cage: Includes ribs and sternum, providing protection for thoracic organs such as the heart and lungs while also playing a role in respiration.

• Appendicular skeleton includes:

  • Upper extremity and pectoral girdle: Responsible for the movement of the arms and shoulders, facilitating a wide range of activities.

  • Lower extremity and pelvic girdle: Supports body weight and enables locomotion and movement, including walking and running.

Learning Objectives

• Identify and describe structures related to the axial skeleton, emphasizing their anatomical positioning and function, including how they connect to adjacent structures.

• Distinguish between axial and appendicular skeleton parts based on their functions, locations, and associated movements.

• Describe the various skull cavities and components of the cranial cavity floor, explaining their significance in housing and protecting neural structures.

• List and understand the functions of different vertebrae, highlighting the differences in shape and function across the cervical, thoracic, lumbar, sacral, and coccygeal regions.

Skull Anatomy

Cranial Bones

• Eight cranial bones:

  • 1 Frontal bone: Forms the forehead, part of the eye sockets, and the anterior cranial fossa.

  • 2 Parietal bones: Make up the sides and roof of the cranium.

  • 2 Temporal bones: House the structures of the ear and contribute to the base of the skull.

  • 1 Occipital bone: Contains the foramen magnum, where the spinal cord passes through.

  • 1 Sphenoid bone: Key part of the cranial base, articulates with all other cranial bones and contains the sella turcica.

  • 1 Ethmoid bone: Contributes to the nasal cavity and forms part of the orbits.

• Cranium divided into:

  • Calvaria (roof): Contains frontal, parietal, and occipital bones that encase the brain.

  • Base: Contains anterior, middle, and posterior cranial fossae, which accommodate different parts of the brain.

Skull Cavities

• Main cavities include:

  • Cranial cavity: Houses the brain, cerebrospinal fluid, and meninges.

  • Orbits: Bony cavities that contain the eyes, extrinsic eye muscles, and associated structures.

  • Nasal cavity: Air-filled space contributing to respiration and olfaction.

  • Buccal cavity: Contains structures involved in digestion and sound production.

  • Middle and inner ear cavities: House the auditory ossicles and structures involved in balance.

• Types of sinuses:

  • Named for the bone in which they occur (e.g., frontal, ethmoid, sphenoid, maxillary) and serve to lighten the skull and improve voice resonance.

Facial Bones

• Key facial bones:

  • 2 Maxillae: Form the upper jaw and part of the orbits.

  • 2 Zygomatic bones: Form the cheekbones and contribute to the orbit.

  • 2 Lacrimal bones: Smallest bones of the face, located in the medial wall of the orbit, involved in tear drainage.

  • 2 Nasal bones: Form the bridge of the nose.

  • 2 Palatine bones: Contribute to the hard palate and part of the orbit.

  • 1 Vomer: Forms part of the nasal septum.

  • 1 Mandible: The lower jaw, which is the only movable facial bone.

Vertebral Column

• Comprises 33 vertebrae:

  • 7 Cervical: Support the head; C1 (Atlas) allows for nodding, while C2 (Axis) allows for rotation of the head.

  • 12 Thoracic: Articulate with the ribs, providing stability and a point of attachment.

  • 5 Lumbar: Largest vertebrae, designed to bear weight and manage the stress of lifting.

  • 5 Sacral (fused): Form the back of the pelvis and are essential for weight distribution.

  • 4 Coccygeal (fused): Make up the tailbone, providing attachment for ligaments and muscles.

• Function:

  • Provides structural support for the trunk and head, facilitating upright posture.

  • Facilitates movement: flexion, extension, rotation, and lateral movement of the torso.

  • Provides shock absorption through intervertebral discs, which are fibrocartilaginous structures between vertebrae.

General Structure of Vertebrae

• Composed of:

  • Body (centrum): Weight-bearing part, absorbing loads during daily activities.

  • Vertebral foramen: Canal for the spinal cord, protecting it from injury.

  • Spinous process and transverse processes: Sites for muscle attachment and leverage for movement.

  • Intervertebral foramen: Allow nerves to exit the vertebral column and innervate the body.

Specific Vertebral Types

• Cervical Vertebrae:

  • Lightest, characterized by transverse foramen allowing passage of vertebral arteries.

  • C1 (Atlas): Supports the head; articulates with the occipital condyles.

  • C2 (Axis): Allows head rotation due to the dens (odontoid process).

• Thoracic Vertebrae:

  • Associated with ribs; body heart-shaped; provide attachment points for ribs and have costal facets.

• Lumbar Vertebrae:

  • Thick bodies with blunt spinous processes for weight-bearing, allowing for significant load during lifting or walking.

Thoracic Cage

• Functions:

  • Protects thoracic organs (heart, lungs, great vessels).

  • Provides attachment for limbs and stabilizes posture while breathing.

• Structures:

  • Sternum: Composed of three parts (manubrium, gladiolus, xiphoid process) which articulate with ribs and provide protection to underlying structures.

  • Twelve pairs of ribs:

    • True ribs (1-7): Attach directly to sternum via costal cartilage.

    • False ribs (8-12): Indirectly connect to the sternum or lack anterior attachment (floating ribs - 11-12).

Conclusion

• The axial skeleton is critical for structural support, protection of vital organs, and efficient movement.

• Knowledge of bones, their positioning, and functions is essential in anatomy and physiology, as it lays the foundation for understanding the human body's mechanics.

Key Reading

• Marieb EN, Hoehn K: Anatomy & Physiology, Seventh Edition. Pearson-Benjamin Cummings, 2020.