Review Sheet 9 The Axial Skeleton

The axial skeleton, the central framework of our bodies, provides support, protection, and a foundation for movement. It encompasses the bones of the skull, vertebral column, ribs, and sternum. Understanding the axial skeleton is crucial for anyone studying anatomy, physiology, or related healthcare fields. This comprehensive review sheet aims to solidify your knowledge of this essential body system.

The Skull: A Protective Vault

The skull, a complex bony structure, encases and protects the delicate brain. It's composed of two main sets of bones: the cranial bones and the facial bones.

Cranial Bones: Forming the Cranium

These eight bones form the superior, lateral, and posterior aspects of the skull, collectively known as the cranium.

• Frontal Bone: Forms the forehead and the superior part of the orbits (eye sockets). Key features include:

  • Supraorbital foramen/notch: A small opening or indentation above each orbit, allowing passage of the supraorbital nerve and vessels.
  • Glabella: The smooth area between the eyebrows.

• Parietal Bones (paired): These bones form the majority of the cranial roof and lateral walls. They articulate with each other at the sagittal suture and with the frontal bone at the coronal suture.

• Temporal Bones (paired): Located on the lateral sides of the skull, inferior to the parietal bones. They house the structures of the inner ear and articulate with the mandible (lower jaw). Important features include:

  • External acoustic meatus: The opening of the ear canal.
  • Mandibular fossa: A depression that articulates with the mandibular condyle, forming the temporomandibular joint (TMJ).
  • Mastoid process: A prominent projection posterior to the ear, serving as an attachment point for neck muscles.
  • Styloid process: A slender, pointed projection inferior to the ear, serving as an attachment point for ligaments and muscles.
  • Zygomatic process: A projection that articulates with the zygomatic bone, forming part of the zygomatic arch (cheekbone).

• Occipital Bone: Forms the posterior part of the skull and the base of the cranium. Key features include:

  • Foramen magnum: A large opening through which the spinal cord passes.
  • Occipital condyles: Oval processes on either side of the foramen magnum that articulate with the atlas (first cervical vertebra).
  • External occipital protuberance: A prominent projection on the posterior surface of the occipital bone.

• Sphenoid Bone: A complex, bat-shaped bone that spans the width of the cranial floor. It articulates with all other cranial bones. Important features include:

  • Sella turcica: A saddle-shaped depression that houses the pituitary gland.
  • Greater wings: Large, lateral projections that form part of the middle cranial fossa and contribute to the orbits.
  • Lesser wings: Smaller, superior projections that also contribute to the orbits.
  • Optic canal: A canal that allows passage of the optic nerve and ophthalmic artery.
  • Pterygoid processes: Inferior projections that serve as attachment points for jaw muscles.

• Ethmoid Bone: Located between the orbits, forming part of the nasal cavity and the medial walls of the orbits. Key features include:

  • Cribriform plate: A horizontal plate with numerous foramina (olfactory foramina) that allow passage of olfactory nerves.
  • Crista galli: A superior projection that serves as an attachment point for the falx cerebri, a dural membrane that separates the cerebral hemispheres.
  • Perpendicular plate: A vertical plate that forms the superior part of the nasal septum.
  • Superior and middle nasal conchae: Scroll-like projections that increase the surface area of the nasal cavity, aiding in warming and humidifying inhaled air.

Facial Bones: Shaping the Face

These 14 bones form the framework of the face, providing attachment points for facial muscles and housing the sensory organs of sight, smell, and taste.

• Maxillae (paired): These bones form the upper jaw and the central part of the facial skeleton. They articulate with all other facial bones except the mandible. Key features include:

  • Alveolar processes: Sockets that hold the upper teeth.
  • Infraorbital foramen: An opening below the orbit that allows passage of the infraorbital nerve and vessels.
  • Palatine processes: Horizontal extensions that form the anterior part of the hard palate.

• Mandible: The lower jawbone, the only movable bone in the skull. Important features include:

  • Body: The horizontal portion of the mandible.
  • Ramus (pl. rami): The vertical portion of the mandible.
  • Mandibular condyle: A rounded projection that articulates with the mandibular fossa of the temporal bone, forming the TMJ.
  • Coronoid process: A pointed projection anterior to the mandibular condyle, serving as an attachment point for jaw muscles.
  • Alveolar process: Sockets that hold the lower teeth.
  • Mental foramen: An opening on the anterior surface of the body that allows passage of the mental nerve and vessels.

• Zygomatic Bones (paired): These bones form the cheekbones and contribute to the lateral walls of the orbits. They articulate with the temporal bone (zygomatic process) to form the zygomatic arch.

• Nasal Bones (paired): These small bones form the bridge of the nose.

• Lacrimal Bones (paired): These small bones are located in the medial walls of the orbits, contributing to the nasolacrimal groove, which houses the lacrimal sac (part of the tear drainage system).

• Palatine Bones (paired): These bones form the posterior part of the hard palate and contribute to the floor of the nasal cavity.

• Inferior Nasal Conchae (paired): These scroll-like bones are located in the nasal cavity, inferior to the middle nasal conchae of the ethmoid bone. They increase the surface area of the nasal cavity.

• Vomer: A single bone that forms the inferior part of the nasal septum.

Sutures of the Skull: Immovable Joints

Sutures are fibrous joints that connect the bones of the skull. In adults, these sutures are generally immovable. The major sutures include:

• Coronal Suture: Connects the frontal bone to the parietal bones.
• Sagittal Suture: Connects the two parietal bones along the midline.
• Lambdoid Suture: Connects the occipital bone to the parietal bones.
• Squamous Suture: Connects the temporal bone to the parietal bone.

Special Features: Hyoid Bone

The hyoid bone, although not directly part of the skull, is closely associated with it. It's a small, U-shaped bone located in the neck, inferior to the mandible. The hyoid bone does not articulate with any other bone; instead, it's suspended by ligaments and muscles. It serves as an attachment point for tongue and neck muscles, playing a crucial role in swallowing and speech.

The Vertebral Column: A Flexible Support Structure

The vertebral column, or spine, is a flexible, segmented column that provides support for the head and trunk, protects the spinal cord, and allows for movement. It consists of 26 bones in adults: 24 individual vertebrae, the sacrum (formed by the fusion of five vertebrae), and the coccyx (formed by the fusion of three to five vertebrae).

Regions of the Vertebral Column

The vertebral column is divided into five regions:

• Cervical Vertebrae (7): Located in the neck. They are the smallest and most mobile vertebrae.
• Thoracic Vertebrae (12): Located in the chest region. They articulate with the ribs.
• Lumbar Vertebrae (5): Located in the lower back. They are the largest and strongest vertebrae.
• Sacrum (1): A triangular bone formed by the fusion of five sacral vertebrae. It articulates with the hip bones to form the sacroiliac joints.
• Coccyx (1): The tailbone, formed by the fusion of three to five coccygeal vertebrae.

General Structure of a Vertebra

Although vertebrae vary in size and shape depending on their location, they share a common basic structure:

• Body: The weight-bearing, cylindrical portion of the vertebra.

• Vertebral Arch: Forms the posterior part of the vertebra. It consists of:

  • Pedicles: Two short, bony processes that connect the vertebral arch to the body.
  • Laminae: Two flat plates that extend from the pedicles and fuse in the midline to form the spinous process.

• Vertebral Foramen: The opening formed by the vertebral body and vertebral arch. The spinal cord passes through the vertebral foramen.

• Spinous Process: A posterior projection that serves as an attachment point for muscles and ligaments.

• Transverse Processes: Lateral projections that serve as attachment points for muscles and ligaments.

• Superior and Inferior Articular Processes: Paired projections that articulate with the vertebrae above and below, forming intervertebral joints. Each articular process has a smooth, cartilage-covered articular facet.

• Intervertebral Foramina: Openings formed between adjacent vertebrae when they articulate. Spinal nerves exit the vertebral column through these foramina.

Regional Differences in Vertebrae

Each region of the vertebral column has unique characteristics:

• Cervical Vertebrae:

  • Smallest vertebral bodies.
  • Vertebral foramen is relatively large and triangular.
  • Transverse processes have transverse foramina, which allow passage of the vertebral arteries.
  • Spinous processes are typically bifid (split) in C2-C6.
  • C1 (Atlas): Lacks a body and spinous process. It articulates with the occipital condyles of the skull, allowing for nodding movements.
  • C2 (Axis): Has a prominent dens (odontoid process) that projects superiorly and articulates with the atlas, allowing for rotational movements.
  • C7 (Vertebra Prominens): Has a long, prominent spinous process that is easily palpable.

• Thoracic Vertebrae:

  • Larger vertebral bodies than cervical vertebrae.
  • Vertebral foramen is smaller and circular.
  • Transverse processes have facets for articulation with the ribs (transverse costal facets).
  • Bodies have facets for articulation with the heads of the ribs (superior and inferior costal facets).
  • Spinous processes are long and point inferiorly.

• Lumbar Vertebrae:

  • Largest vertebral bodies.
  • Vertebral foramen is triangular.
  • Transverse processes are short and blunt.
  • Spinous processes are short, thick, and hatchet-shaped, projecting posteriorly.

• Sacrum:

  • Triangular bone formed by the fusion of five sacral vertebrae.
  • Sacral promontory: The anterior projection of the first sacral vertebra.
  • Sacral foramina: Openings that allow passage of sacral spinal nerves and vessels.
  • Median sacral crest: A ridge formed by the fused spinous processes of the sacral vertebrae.
  • Lateral sacral crests: Ridges formed by the fused transverse processes of the sacral vertebrae.
  • Auricular surface: Articulates with the ilium of the hip bone to form the sacroiliac joint.

• Coccyx:

  • Tailbone, formed by the fusion of three to five coccygeal vertebrae.
  • Provides limited support for pelvic floor muscles.

Intervertebral Discs: Cushions Between Vertebrae

Intervertebral discs are fibrocartilaginous pads located between the bodies of adjacent vertebrae. They act as shock absorbers and allow for movement of the vertebral column. Each disc consists of:

• Anulus Fibrosus: The outer ring of the disc, composed of tough fibrocartilage.
• Nucleus Pulposus: The inner, gelatinous core of the disc.

Spinal Curvatures: Providing Resilience and Balance

The vertebral column exhibits natural curvatures that increase its resilience and help maintain balance. These curvatures include:

• Cervical Curvature: Concave posteriorly (lordosis).
• Thoracic Curvature: Convex posteriorly (kyphosis).
• Lumbar Curvature: Concave posteriorly (lordosis).
• Sacral Curvature: Convex posteriorly (kyphosis).

The Thoracic Cage: Protecting Vital Organs

The thoracic cage, or rib cage, protects the vital organs of the thorax, including the heart, lungs, and major blood vessels. It also provides support for the shoulder girdle and upper limbs, and plays a role in breathing.

Components of the Thoracic Cage

The thoracic cage consists of:

• Sternum: A flat bone located in the midline of the anterior chest wall. It consists of three parts:

  • Manubrium: The superior part of the sternum. It articulates with the clavicles and the first pair of ribs. The jugular notch (suprasternal notch) is a shallow indentation on the superior border of the manubrium.
  • Body: The middle part of the sternum. It articulates with the second through seventh pairs of ribs.
  • Xiphoid Process: The inferior, cartilaginous part of the sternum. It gradually ossifies during adulthood.

• Ribs: Twelve pairs of bones that form the lateral walls of the thoracic cage.

  • True Ribs (1-7): Articulate directly with the sternum via their own costal cartilages.
  • False Ribs (8-12): Articulate indirectly with the sternum via the costal cartilage of the seventh rib (ribs 8-10) or do not articulate with the sternum at all (ribs 11-12, floating ribs).

• Thoracic Vertebrae: As discussed earlier, the thoracic vertebrae form the posterior part of the thoracic cage.

Features of a Typical Rib

A typical rib consists of:

• Head: Articulates with the vertebral body.
• Neck: Connects the head to the body.
• Tubercle: Articulates with the transverse process of the thoracic vertebra.
• Body (Shaft): The main portion of the rib.
• Costal Groove: A groove on the inferior surface of the rib that houses intercostal nerves and vessels.

Articulations of the Thoracic Cage

The thoracic cage involves several important articulations:

• Sternoclavicular Joints: Connect the clavicles to the manubrium of the sternum.
• Sternocostal Joints: Connect the ribs to the sternum via costal cartilages.
• Costochondral Joints: Connect the ribs to their costal cartilages.
• Costovertebral Joints: Connect the heads of the ribs to the vertebral bodies.
• Costotransverse Joints: Connect the tubercles of the ribs to the transverse processes of the thoracic vertebrae.

Clinical Considerations: Common Axial Skeleton Conditions

Understanding the anatomy of the axial skeleton is critical for diagnosing and treating various clinical conditions. Some common examples include:

• Fractures: Fractures of the skull, vertebrae, or ribs can result from trauma.
• Scoliosis: An abnormal lateral curvature of the spine.
• Kyphosis: An exaggerated thoracic curvature (hunchback).
• Lordosis: An exaggerated lumbar curvature (swayback).
• Herniated Disc: Protrusion of the nucleus pulposus through the anulus fibrosus of an intervertebral disc.
• Osteoporosis: A condition characterized by decreased bone density, increasing the risk of fractures, particularly in the vertebrae and hip.
• Arthritis: Inflammation of the joints, which can affect the intervertebral joints and the sacroiliac joints.
• Temporomandibular Joint (TMJ) Disorders: A group of conditions that cause pain and dysfunction in the TMJ.

Frequently Asked Questions (FAQ)

• What is the function of the axial skeleton? The axial skeleton provides support, protection, and a foundation for movement. It supports the head, neck, and trunk; protects the brain, spinal cord, and thoracic organs; and provides attachment points for muscles that move the head, neck, trunk, and limbs.

• How many bones are in the axial skeleton? The axial skeleton typically has 80 bones.

• What are the main divisions of the axial skeleton? The main divisions are the skull (cranial and facial bones), vertebral column, and thoracic cage.

• What is the significance of the foramen magnum? The foramen magnum is the large opening in the occipital bone through which the spinal cord passes to connect with the brain.

• What is the purpose of the intervertebral discs? Intervertebral discs act as shock absorbers and allow for movement of the vertebral column.

• What is the difference between true ribs, false ribs, and floating ribs? True ribs (1-7) articulate directly with the sternum via their own costal cartilages. False ribs (8-12) articulate indirectly with the sternum via the costal cartilage of the seventh rib (ribs 8-10) or do not articulate with the sternum at all (ribs 11-12, floating ribs).

Conclusion

The axial skeleton is a complex and vital part of the human body. A thorough understanding of its components, structure, and function is essential for anyone studying anatomy, physiology, or related healthcare fields. This review sheet has provided a comprehensive overview of the axial skeleton, covering the skull, vertebral column, and thoracic cage. By mastering this information, you'll be well-equipped to tackle more advanced topics in anatomy and physiology and to understand the clinical implications of axial skeleton disorders. Remember to supplement this review with visual aids, such as diagrams and models, to further enhance your learning. Good luck!