Review Sheet Exercise 9 The Axial Skeleton

The axial skeleton, the central pillar of our body, forms the longitudinal axis that supports and protects vital organs. It’s composed of the skull, vertebral column, and thoracic cage, each playing a crucial role in movement, protection, and overall body structure. Let’s delve deeper into understanding its components through a detailed review.

The Skull: A Bony Fortress

The skull, the most complex bony structure in the body, is divided into two main parts: the cranium and the facial bones.

Cranium: Protecting the Brain

The cranium, or braincase, encloses and protects the delicate brain. It's composed of eight bones:

• Frontal Bone: Forms the anterior part of the cranium, including the forehead and the roof of the orbits (eye sockets). Key features include the supraorbital foramen (or notch), which allows the supraorbital nerve and artery to pass through.
• Parietal Bones (2): These 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.
• Temporal Bones (2): Located on the lateral sides of the skull, they house the middle and inner ear structures. Notable features include:
  • External Acoustic Meatus: The opening of the ear canal.
  • Mandibular Fossa: Articulates with the mandible (lower jaw).
  • 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: Projects anteriorly to articulate with the zygomatic bone, forming part of the cheekbone.
• Occipital Bone: Forms the posterior part of the cranium and the base of the skull. The foramen magnum, a large opening, allows the spinal cord to connect with the brain. The occipital condyles articulate with the first vertebra (atlas).
• Sphenoid Bone: A complex, bat-shaped bone that spans the width 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: Form part of the lateral walls of the skull and the orbits.
  • Lesser Wings: Smaller, triangular projections superior to the greater wings.
  • Optic Canal: Allows the optic nerve to pass through to the eye.
• Ethmoid Bone: Located between the orbits, it forms part of the nasal cavity and the orbits. Key features include:
  • Crista Galli: A superior projection that serves as an attachment point for the falx cerebri, a membrane that separates the two cerebral hemispheres.
  • Cribriform Plate: A perforated plate that allows olfactory nerves (sense of smell) to pass through.
  • Perpendicular Plate: 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, helping to warm and humidify inhaled air.

Facial Bones: Shaping Our Face

The facial bones form the framework of the face and provide attachment points for facial muscles. There are 14 facial bones:

• Mandible: The lower jawbone, the only movable bone in the skull. It articulates with the temporal bone at the temporomandibular joint (TMJ).
  • Body: The horizontal part of the mandible.
  • Ramus: The vertical part of the mandible.
  • Mandibular Condyle: Articulates with the mandibular fossa of the temporal bone.
  • Coronoid Process: An anterior projection that serves as an attachment point for the temporalis muscle (involved in chewing).
  • Mental Foramen: An opening on the anterior surface of the mandible that allows the mental nerve and artery to pass through.
• Maxillae (2): The upper jawbones, which form part of the hard palate, the orbits, and the nasal cavity.
  • Alveolar Processes: Contain sockets for the upper teeth.
  • Infraorbital Foramen: An opening below the orbit that allows the infraorbital nerve and artery to pass through.
• Zygomatic Bones (2): The cheekbones, which articulate with the temporal, frontal, and maxillary bones. They contribute to the lateral wall of the orbit.
• Nasal Bones (2): Form the bridge of the nose.
• Lacrimal Bones (2): Small bones located in the medial wall of the orbit, containing a groove for the lacrimal sac (part of the tear drainage system).
• Palatine Bones (2): Form the posterior part of the hard palate and part of the nasal cavity.
• Inferior Nasal Conchae (2): Scroll-like bones located in the nasal cavity, inferior to the middle nasal conchae of the ethmoid bone.
• Vomer: A single bone that forms the inferior part of the nasal septum.

The Vertebral Column: A Flexible Support

The vertebral column, also known as the spine, is a flexible, curved structure that supports the trunk, protects the spinal cord, and allows for movement. It is composed of 33 individual bones called vertebrae, which are divided into five regions:

• Cervical Vertebrae (7): Located in the neck region. They are the smallest and most mobile vertebrae.
  • Atlas (C1): The first cervical vertebra, which articulates with the occipital condyles of the skull. It allows for nodding movements ("yes"). It lacks a body and spinous process.
  • Axis (C2): The second cervical vertebra, which has a superior projection called the dens (odontoid process). The atlas rotates around the dens, allowing for shaking the head ("no").
  • Transverse Foramina: Unique to cervical vertebrae, these openings in the transverse processes allow the vertebral arteries to pass through to the brain.
  • Bifid Spinous Processes: The spinous processes of C2-C6 are typically forked (bifid).
• Thoracic Vertebrae (12): Located in the chest region. They articulate with the ribs.
  • Costal Facets: Facets on the vertebral bodies and transverse processes that articulate with the ribs.
  • Long, Downward-Pointing Spinous Processes: Provide attachment for back muscles.
• Lumbar Vertebrae (5): Located in the lower back region. They are the largest and strongest vertebrae, designed to bear the weight of the upper body.
  • Large Bodies: Designed to support weight.
  • Short, Blunt Spinous Processes: Project posteriorly.
• Sacrum (5 fused vertebrae): A triangular bone located at the base of the spine. It articulates with the hip bones to form the sacroiliac joints.
  • Sacral Promontory: The anterior border of the first sacral vertebra.
  • Sacral Foramina: Openings that allow sacral nerves and arteries to pass through.
• Coccyx (4 fused vertebrae): The tailbone, the terminal part of the vertebral column.

General Structure of a Vertebra

While each region of the vertebral column has unique features, all vertebrae share a common basic structure:

• Body: The main weight-bearing portion of the vertebra.
• Vertebral Arch: Formed by the pedicles and laminae. It encloses the vertebral foramen.
• 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: Projections that articulate with the vertebrae above and below, forming facet joints.
• Intervertebral Foramina: Openings formed between adjacent vertebrae, allowing spinal nerves to exit the vertebral column.

Spinal Curvatures

The vertebral column exhibits four normal curvatures:

• Cervical Curvature: A concave curvature in the neck region.
• Thoracic Curvature: A convex curvature in the chest region.
• Lumbar Curvature: A concave curvature in the lower back region.
• Sacral Curvature: A convex curvature in the sacral region.

These curvatures increase the strength and flexibility of the vertebral column and help to distribute weight.

The Thoracic Cage: Protecting the Heart and Lungs

The thoracic cage, or rib cage, protects the vital organs of the chest, including the heart and lungs. It is composed of the ribs, the sternum, and the thoracic vertebrae.

Ribs

There are 12 pairs of ribs, which are classified into three groups:

• True Ribs (1-7): Attach directly to the sternum via their own costal cartilage.
• False Ribs (8-10): Attach indirectly to the sternum via the costal cartilage of the rib above.
• Floating Ribs (11-12): Do not attach to the sternum at all.

Sternum

The sternum, or breastbone, is a flat bone located in the midline of the chest. It is composed of three parts:

• Manubrium: The superior part of the sternum, which articulates with the clavicles (collarbones) and the first pair of ribs.
  • Jugular Notch: A shallow indentation on the superior border of the manubrium.
• Body: The middle and largest part of the sternum, which articulates with ribs 2-7.
• Xiphoid Process: The inferior and smallest part of the sternum. It is cartilaginous in youth but ossifies with age.

Review Sheet Exercises: Testing Your Knowledge

Let's apply our knowledge to typical review sheet exercises related to the axial skeleton:

Exercise 1: Identifying Bones and Features

• Objective: To identify the bones of the axial skeleton and their key features.
• Example Questions:
  • "Identify the bone that contains the foramen magnum." (Answer: Occipital bone)
  • "Name the bone that articulates with all other cranial bones." (Answer: Sphenoid bone)
  • "What are the names of the first two cervical vertebrae?" (Answer: Atlas and Axis)
  • "Identify the part of the sternum that articulates with the clavicles." (Answer: Manubrium)
• Tips: Use anatomical models, diagrams, and online resources to visually identify the bones and their features. Practice labeling diagrams and quizzes.

Exercise 2: Understanding Articulations

• Objective: To understand how the bones of the axial skeleton articulate with each other.
• Example Questions:
  • "Describe the articulation between the occipital bone and the atlas." (Answer: Occipital condyles of the occipital bone articulate with the superior articular facets of the atlas.)
  • "What bones articulate at the temporomandibular joint (TMJ)?" (Answer: Mandible and temporal bone)
  • "How do the ribs articulate with the thoracic vertebrae?" (Answer: Costal facets on the vertebral bodies and transverse processes articulate with the heads and tubercles of the ribs.)
• Tips: Focus on the names of the bony landmarks involved in the articulations. Visualize the movement allowed at each joint.

Exercise 3: Clinical Applications

• Objective: To understand the clinical significance of the axial skeleton.
• Example Questions:
  • "What is scoliosis, and which part of the axial skeleton is affected?" (Answer: Scoliosis is an abnormal lateral curvature of the vertebral column.)
  • "What is a herniated disc, and how does it affect the spinal cord or nerves?" (Answer: A herniated disc occurs when the nucleus pulposus protrudes through the annulus fibrosus, potentially compressing the spinal cord or spinal nerves.)
  • "What are the potential consequences of a fracture to the ribs?" (Answer: Potential damage to the lungs, heart, or other internal organs.)
  • "Describe the condition known as 'whiplash' and what part of the axial skeleton is primarily affected." (Answer: Whiplash is a neck injury due to sudden forceful back-and-forth movement of the neck, primarily affecting the cervical vertebrae and surrounding soft tissues.)
• Tips: Research common conditions and injuries related to the axial skeleton. Understand how anatomical structures are affected in these conditions.

Exercise 4: Identifying Foramina and Their Contents

• Objective: To identify important foramina (openings) in the skull and vertebrae, and the structures that pass through them.
• Example Questions:
  • "What structure passes through the foramen magnum?" (Answer: Spinal cord)
  • "What nerve passes through the optic canal?" (Answer: Optic nerve)
  • "What structures pass through the transverse foramina of the cervical vertebrae?" (Answer: Vertebral arteries and veins)
  • "What structures pass through the mental foramen of the mandible?" (Answer: Mental nerve and vessels)
• Tips: Create a table listing each foramen and its contents. Use diagrams to visualize the pathways of nerves and vessels.

Exercise 5: Understanding Bone Development and Age-Related Changes

• Objective: To understand how the bones of the axial skeleton develop and how they change with age.
• Example Questions:
  • "Describe the process of intramembranous ossification, which is involved in the formation of many skull bones."
  • "How does the fontanelle (soft spots) of an infant's skull contribute to childbirth and brain growth?"
  • "What is osteoporosis, and how does it affect the vertebrae?" (Answer: Osteoporosis is a condition characterized by decreased bone density, making the vertebrae more susceptible to fractures.)
  • "How does the curvature of the vertebral column change from infancy to adulthood?"
• Tips: Review the basics of bone development (ossification). Research age-related changes in bone density and structure.

Common Mistakes and How to Avoid Them

• Confusing Left and Right: When identifying bones, especially paired bones like the parietal or temporal bones, carefully consider their orientation.
• Misidentifying Foramina: Many foramina look similar. Pay close attention to their location and the structures that pass through them.
• Forgetting Regional Variations: Remember that vertebrae in different regions have unique characteristics.
• Ignoring Articular Surfaces: Pay attention to the articular surfaces and facets, as they indicate how bones connect.
• Rushing Through the Material: Take your time to study the bones and features of the axial skeleton. Use multiple resources and practice regularly.

Mnemonics and Memory Aids

• Cervical Vertebrae Breakfast Time: "Eat breakfast at 7, lunch at 12, and dinner at 5" corresponds to 7 Cervical, 12 Thoracic, and 5 Lumbar vertebrae.
• Facial Bones Mnemonic: "Virgil Can Not Make My Pet Zebra Laugh" (Vomer, Conchae (Inferior Nasal), Nasal, Maxilla, Mandible, Palatine, Zygomatic, Lacrimal). Remember this only covers SOME of the facial bones!
• STAR - Sternum Top to Bottom: Sternum, Manubrium, Body, process (Xiphoid).

Conclusion: Mastering the Axial Skeleton

The axial skeleton is a foundational component of the human body, providing support, protection, and enabling movement. By understanding its intricate structure and key features, you can build a strong foundation for further study in anatomy and physiology. Thoroughly review the bones, landmarks, articulations, and clinical significance. By doing so, you will master this vital part of human anatomy and be well-prepared for any review sheet exercise or examination. Remember to use all available resources, including anatomical models, diagrams, and online tools, to reinforce your learning. With consistent effort and a systematic approach, you can achieve a comprehensive understanding of the axial skeleton and its critical role in the human body. Good luck with your studies!