The Axial Skeleton Review Sheet 8

Let's delve into the intricate world of the axial skeleton, a fundamental framework that supports our body, protects vital organs, and enables essential movements. This comprehensive review will explore the components of the axial skeleton, their functions, and key anatomical features, ensuring a solid understanding of this critical system.

The Axial Skeleton: An Overview

The axial skeleton, comprising approximately 80 bones, forms the central axis of the body. Unlike the appendicular skeleton, which consists of the limbs and their attachments, the axial skeleton is primarily responsible for:

• Support: Providing a rigid framework to maintain posture and bear the weight of the body.
• Protection: Enclosing and safeguarding delicate organs like the brain, spinal cord, heart, and lungs.
• Movement: Providing attachment points for muscles, enabling movements of the head, neck, and trunk.

The axial skeleton is divided into three major regions: the skull, the vertebral column, and the thoracic cage. Each region possesses unique structural characteristics and plays a distinct role in overall body function.

The Skull: A Bony Fortress

The skull, the most complex bony structure in the body, protects the brain and houses sensory organs. It is formed by two sets of bones: the cranial bones and the facial bones.

Cranial Bones

These eight bones enclose and protect the brain within the cranial cavity. They include:

1. Frontal Bone: Forms the anterior part of the cranium, including the forehead and the roof of the orbits (eye sockets). Key features include:

   • Supraorbital margin: The bony ridge above each orbit.
   • Glabella: The smooth area between the eyebrows.
   • Frontal sinuses: Air-filled spaces within the bone, contributing to voice resonance and reducing skull weight.

2. Parietal Bones (2): Form the superior and lateral walls of the cranium. They articulate with each other at the sagittal suture and with the frontal bone at the coronal suture.

3. Temporal Bones (2): Located on the lateral sides of the cranium, inferior to the parietal bones. They house the inner ear and articulate with the mandible (lower jaw). Key features include:

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

4. Occipital Bone: Forms the posterior part of the cranium and the base of the skull. 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, articulating with the atlas (first cervical vertebra).
   • External occipital protuberance: A prominent bump on the posterior surface, serving as an attachment point for neck muscles.

5. Sphenoid Bone: A complex, bat-shaped bone that forms part of the base of the skull and articulates with all other cranial bones. Key features include:

   • Sella turcica: A saddle-shaped depression that houses the pituitary gland.
   • Greater wings: Lateral extensions that form part of the orbits and the middle cranial fossa.
   • Lesser wings: Superior extensions that form part of the orbits and the anterior cranial fossa.
   • Pterygoid processes: Inferior projections that serve as attachment points for jaw muscles.
   • Sphenoidal sinuses: Air-filled spaces within the bone.

6. Ethmoid Bone: Located anterior to the sphenoid bone, forming part of the nasal cavity and the orbits. Key features include:

   • Cribriform plate: A perforated plate that allows olfactory nerves to pass from the nasal cavity to the brain.
   • Crista galli: A superior projection that serves as an attachment point for the falx cerebri, a membrane that separates the cerebral hemispheres.
   • Perpendicular plate: Forms the superior part of the nasal septum.
   • Ethmoidal sinuses: Air-filled spaces within the bone.
   • Superior and middle nasal conchae: Scroll-like projections that increase the surface area of the nasal cavity, humidifying and filtering inhaled air.

Facial Bones

These 14 bones form the face, providing structure and support for the eyes, nose, and mouth. They include:

1. Mandible: The lower jawbone, the only movable bone in the skull. Key features include:

   • Body: The horizontal portion that forms the chin.
   • Ramus: The vertical portion that ascends towards the temporal bone.
   • Condylar process: Articulates with the mandibular fossa of the temporal bone to form the TMJ.
   • Coronoid process: A projection that serves as an attachment point for jaw muscles.
   • Alveolar processes: Sockets that hold the teeth.
   • Mental foramen: An opening on the anterior surface for the passage of nerves and blood vessels.

2. Maxillae (2): Form the upper jaw and contribute to the floor of the orbits, the sides of the nasal cavity, and the anterior part of the hard palate. Key features include:

   • Alveolar processes: Sockets that hold the upper teeth.
   • Palatine processes: Horizontal extensions that form the anterior part of the hard palate.
   • Maxillary sinuses: Air-filled spaces within the bones.
   • Infraorbital foramen: An opening below the orbit for the passage of nerves and blood vessels.

3. Zygomatic Bones (2): Form the cheekbones and contribute to the lateral walls of the orbits. They articulate with the temporal, maxillary, and frontal bones.

4. Nasal Bones (2): Form the bridge of the nose.

5. Lacrimal Bones (2): Small bones located in the medial walls of the orbits, containing a groove that forms part of the nasolacrimal canal (tear duct).

6. Palatine Bones (2): Form the posterior part of the hard palate and contribute to the floor of the nasal cavity.

7. Inferior Nasal Conchae (2): Scroll-like bones located in the nasal cavity, inferior to the middle nasal conchae of the ethmoid bone. They increase the surface area of the nasal cavity.

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

Sutures of the Skull

The bones of the skull are joined together by immovable joints called sutures. These sutures are fibrous joints that interlock the bones, providing stability and protection. The major sutures of the skull include:

• Coronal suture: Joins the frontal bone to the parietal bones.
• Sagittal suture: Joins the two parietal bones.
• Lambdoid suture: Joins the parietal bones to the occipital bone.
• Squamous sutures (2): Join the temporal bones to the parietal bones.

The Vertebral Column: The Body's Backbone

The vertebral column, also known as the spine, is a flexible, S-shaped structure that supports the head, neck, and trunk. It protects the spinal cord and provides attachment points for muscles. The vertebral column is composed of 33 individual bones called vertebrae, which are divided into five regions:

1. Cervical Vertebrae (7): Located in the neck region. They are the smallest and most mobile vertebrae. Key features include:

   • Transverse foramina: Openings in the transverse processes that allow passage of vertebral arteries.
   • Bifid spinous processes: Spinous processes that are split into two.
   • Atlas (C1): The first cervical vertebra, which articulates with the occipital condyles of the skull, allowing for nodding movements. It lacks a body and a spinous process.
   • Axis (C2): The second cervical vertebra, which possesses a prominent projection called the dens (odontoid process) that articulates with the atlas, allowing for rotational movements.

2. Thoracic Vertebrae (12): Located in the chest region. They articulate with the ribs and have characteristic facets for rib attachment. Key features include:

   • Costal facets: Facets on the body and transverse processes for articulation with the ribs.
   • Long, slender spinous processes that point inferiorly.

3. Lumbar Vertebrae (5): Located in the lower back region. They are the largest and strongest vertebrae, designed to bear the weight of the body. Key features include:

   • Large, kidney-shaped bodies.
   • Short, thick spinous processes that point posteriorly.

4. Sacrum: A triangular bone formed by the fusion of five sacral vertebrae. It articulates with the hip bones to form the sacroiliac joints.

5. Coccyx: The tailbone, formed by the fusion of four coccygeal vertebrae.

General Structure of a Vertebra

Although vertebrae vary in size and shape depending on their location in the vertebral column, they share a common structural plan:

• Body: The weight-bearing, anterior portion of the vertebra.
• Vertebral arch: Forms the posterior portion of the vertebra and encloses the vertebral foramen. It is formed by two pedicles and two laminae.
• Vertebral foramen: The opening through which the spinal cord passes.
• 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: Processes that articulate with adjacent vertebrae, forming intervertebral joints.

Intervertebral Discs

Between each vertebra (except for the atlas and axis) lies an intervertebral disc, a cushion-like structure that absorbs shock and allows for movement of the vertebral column. Each disc consists of:

• Anulus fibrosus: A tough, outer ring of fibrocartilage.
• Nucleus pulposus: A gel-like, inner core that provides cushioning.

The Thoracic Cage: A Protective Shield

The thoracic cage, also known as the rib cage, protects the heart, lungs, and major blood vessels. It is formed by the:

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

  • Manubrium: The superior portion, which articulates with the clavicles and the first pair of ribs.
  • Body: The middle portion, which articulates with ribs 2-7.
  • Xiphoid process: The inferior portion, which is a small, cartilaginous projection that ossifies with age.

• Ribs (12 pairs): Curved bones that articulate with the thoracic vertebrae posteriorly and the sternum anteriorly.

  • True ribs (1-7): Attach directly to the sternum via their own costal cartilages.
  • False ribs (8-12): Attach indirectly to the sternum via the costal cartilage of rib 7 or do not attach to the sternum at all.
  • Floating ribs (11-12): Do not attach to the sternum.

Rib Structure

A typical rib consists of:

• Head: Articulates with the vertebral body.
• Neck: Connects the head to the tubercle.
• Tubercle: Articulates with the transverse process of the vertebra.
• Body (shaft): The main portion of the rib.

Functions of the Thoracic Cage

The thoracic cage plays several vital roles:

• Protection: Shields the heart, lungs, and major blood vessels from injury.
• Support: Provides attachment points for muscles of the shoulder girdle, chest, back, and abdomen.
• Respiration: Facilitates breathing by expanding and contracting during inhalation and exhalation.

Clinical Significance

Understanding the anatomy of the axial skeleton is crucial for diagnosing and treating a variety of clinical conditions, including:

• Fractures: Breaks in the bones of the skull, vertebral column, or thoracic cage.
• Dislocations: Displacement of bones at joints, such as the temporomandibular joint (TMJ) or intervertebral joints.
• Scoliosis: Abnormal lateral curvature of the vertebral column.
• Kyphosis: Exaggerated thoracic curvature, resulting in a "hunchback" appearance.
• Lordosis: Exaggerated lumbar curvature, resulting in a "swayback" appearance.
• Herniated disc: Protrusion of the nucleus pulposus through the anulus fibrosus of an intervertebral disc, potentially compressing spinal nerves.
• Osteoporosis: A condition characterized by decreased bone density, increasing the risk of fractures.
• Arthritis: Inflammation of the joints, affecting the vertebrae and ribs.

Review Questions

To solidify your understanding of the axial skeleton, consider the following review questions:

1. What are the three major regions of the axial skeleton?
2. Name the eight cranial bones and describe their key features.
3. Name the 14 facial bones and describe their key features.
4. What are the major sutures of the skull?
5. How many vertebrae are in each region of the vertebral column?
6. Describe the general structure of a vertebra.
7. What are the components of an intervertebral disc?
8. What are the three parts of the sternum?
9. Differentiate between true ribs, false ribs, and floating ribs.
10. What are the functions of the thoracic cage?

Conclusion

The axial skeleton forms the central framework of the body, providing support, protection, and enabling movement. A thorough understanding of its components and functions is essential for healthcare professionals and anyone interested in human anatomy and physiology. This review has provided a comprehensive overview of the axial skeleton, covering the skull, vertebral column, and thoracic cage. By mastering this information, you will gain a deeper appreciation for the intricate design and vital role of this fundamental skeletal system.