Which Of The Following Is Included In The Axial Skeleton

The axial skeleton, the central pillar of our body, is more than just a structural support; it's a protector of vital organs and a key player in movement. Understanding which bones comprise this critical framework is essential for anyone studying anatomy, physiology, or related fields. Let's delve into the components of the axial skeleton, exploring each bone's function and significance.

Components of the Axial Skeleton

The axial skeleton is primarily composed of the following bones:

• Skull: Encases and protects the brain.
• Vertebral Column: Provides support, flexibility, and protects the spinal cord.
• Rib Cage: Protects the heart, lungs, and other vital organs in the thorax.

Each of these major components is further divided into specific bones, each with unique functions. Let's examine them in detail.

The Skull: A Protective Vault

The skull, the most complex part of the axial skeleton, consists of 22 bones divided into two main categories: cranial bones and facial bones.

1. Cranial Bones: These eight bones form the cranium, the bony enclosure that protects the brain. They are:

• Frontal Bone: Forms the anterior part of the cranium, including the forehead and the roof of the orbits (eye sockets).
• Parietal Bones (2): Form the majority of the sides and roof 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 sides of the cranium, they house the structures of the inner ear and articulate with the mandible (lower jaw). Key features include the external acoustic meatus (ear canal), the mastoid process (a bony projection behind the ear), and the zygomatic process (which articulates with the zygomatic bone).
• Occipital Bone: Forms the posterior part of the cranium and the base of the skull. It features the foramen magnum, a large opening through which the spinal cord passes. The occipital condyles articulate with the first cervical vertebra (atlas).
• Sphenoid Bone: A complex, butterfly-shaped bone located in the middle of the skull. It articulates with all other cranial bones and contributes to the floor of the cranium, the orbits, and the nasal cavity. The sella turcica, a saddle-shaped depression on the sphenoid bone, houses the pituitary gland.
• Ethmoid Bone: Located anterior to the sphenoid bone, it contributes to the floor of the cranium, the medial walls of the orbits, the roof of the nasal cavity, and the nasal septum. It contains the cribriform plate, a perforated area through which olfactory nerves pass.

2. Facial Bones: These 14 bones form the face, provide attachment points for facial muscles, and contribute to the formation of the nasal cavity and orbits. They are:

• Nasal Bones (2): Form the bridge of the nose.
• Maxillae (2): Form the upper jaw, part of the hard palate, the floor of the orbits, and the sides of the nasal cavity. They contain the alveolar processes, which hold the upper teeth.
• Zygomatic Bones (2): Form the cheekbones and contribute to the lateral walls of the orbits.
• Mandible: The lower jawbone, the only movable bone in the skull. It contains the alveolar processes for the lower teeth and articulates with the temporal bones at the temporomandibular joints (TMJ).
• Lacrimal Bones (2): Small bones located in the medial walls of the orbits, containing the lacrimal fossa for the lacrimal sac (part of the tear drainage system).
• Palatine Bones (2): Form the posterior part of the hard palate and contribute to the floor and lateral walls of the nasal cavity.
• Inferior Nasal Conchae (2): Scroll-shaped bones located in the nasal cavity, increasing the surface area for humidifying and filtering inhaled air.
• Vomer: A single bone that forms the inferior part of the nasal septum.

Hyoid Bone: While not directly articulating with other skull bones, the hyoid bone is often considered part of the axial skeleton due to its location in the anterior neck and its association with the skull through ligaments and muscles. It supports the tongue and provides attachment points for muscles involved in swallowing and speech.

The Vertebral Column: Support, Flexibility, and Protection

The vertebral column, also known as the spine or backbone, is a flexible, segmented structure that provides support for the head and trunk, protects the spinal cord, and allows for movement. It consists of 24 individual vertebrae, plus the sacrum and coccyx.

Regions of the Vertebral Column:

• Cervical Vertebrae (7): Located in the neck, these vertebrae are the smallest and most mobile. The first cervical vertebra (C1) is called the atlas, and it articulates with the occipital condyles of the skull, allowing for nodding movements. The second cervical vertebra (C2) is called the axis, and it has a bony projection called the dens (odontoid process) that articulates with the atlas, allowing for rotational movements of the head.
• Thoracic Vertebrae (12): Located in the upper back, these vertebrae articulate with the ribs. They have facets (small, smooth areas) on their bodies and transverse processes for rib articulation.
• Lumbar Vertebrae (5): Located in the lower back, these vertebrae are the largest and strongest, designed to bear the weight of the upper body.
• Sacrum: A triangular bone formed by the fusion of five sacral vertebrae. It articulates with the hip bones to form the sacroiliac joints.
• Coccyx: The tailbone, formed by the fusion of three to five coccygeal vertebrae.

Typical Vertebra Structure:

A typical vertebra consists of the following parts:

• Body: The main weight-bearing part of the vertebra, located anteriorly.
• Vertebral Arch: Formed by the pedicles (short, bony processes that connect the body to the transverse processes) and the laminae (flat, bony plates that extend from the transverse processes to the spinous process).
• Vertebral Foramen: The opening formed by the vertebral body and the vertebral arch, through which the spinal cord passes.
• Spinous Process: A posterior projection from the vertebral arch, serving as an attachment point for muscles and ligaments.
• Transverse Processes: Lateral projections from the vertebral arch, serving as attachment points for muscles and ligaments.
• Articular Processes: Superior and inferior projections that articulate with adjacent vertebrae, forming facet joints (also known as zygapophyseal joints).

Intervertebral Discs:

Located between the vertebral bodies, intervertebral discs are fibrocartilaginous structures that act as shock absorbers and allow for movement of the vertebral column. Each disc consists of an annulus fibrosus (outer ring of tough fibrous tissue) and a nucleus pulposus (inner, gel-like core).

The Rib Cage: Protecting Vital Organs

The rib cage, also known as the thoracic cage, is a bony structure that protects the heart, lungs, and other vital organs in the thorax. It consists of the following:

• Ribs (12 pairs): Long, curved bones that articulate with the thoracic vertebrae posteriorly and with the sternum anteriorly (except for the floating ribs).
  • True Ribs (1-7): Attach directly to the sternum via their own costal cartilages.
  • False Ribs (8-10): Attach indirectly to the sternum via the costal cartilage of the seventh rib.
  • Floating Ribs (11-12): Do not attach to the sternum at all.
• 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, articulating with the clavicles (collarbones) and the first rib.
  • Body: The middle and largest part of the sternum, articulating with ribs 2-7.
  • Xiphoid Process: The inferior, cartilaginous part of the sternum, which ossifies with age.

Functions of the Axial Skeleton

The axial skeleton plays several crucial roles in the body:

• Support: Provides a central axis of support for the body, allowing for upright posture.
• Protection: Protects vital organs, such as the brain, spinal cord, heart, and lungs.
• Movement: Provides attachment points for muscles, enabling movement of the head, neck, and trunk.
• Respiration: The rib cage expands and contracts during breathing, facilitating the movement of air in and out of the lungs.
• Hematopoiesis: Red bone marrow, found in some bones of the axial skeleton (e.g., ribs, vertebrae), is responsible for producing blood cells.

Common Conditions Affecting the Axial Skeleton

Several conditions can affect the health and function of the axial skeleton:

• Fractures: Breaks in the bones of the skull, vertebral column, or rib cage.
• Osteoporosis: A condition characterized by decreased bone density, increasing the risk of fractures, particularly in the vertebrae.
• Scoliosis: An abnormal curvature of the spine.
• Herniated Disc: Protrusion of the nucleus pulposus through the annulus fibrosus of an intervertebral disc, causing pain and nerve compression.
• Arthritis: Inflammation of the joints, including the facet joints of the vertebral column.
• Spinal Stenosis: Narrowing of the spinal canal, which can compress the spinal cord and nerves.
• Temporomandibular Joint (TMJ) Disorders: Conditions affecting the temporomandibular joint, causing pain, clicking, and limited jaw movement.

Frequently Asked Questions (FAQ)

• Is the pelvis part of the axial skeleton? No, the pelvis is part of the appendicular skeleton, which includes the bones of the limbs and their girdles (the bones that attach the limbs to the axial skeleton).
• Is the clavicle part of the axial skeleton? No, the clavicle (collarbone) is also part of the appendicular skeleton. It connects the upper limb to the axial skeleton at the sternoclavicular joint.
• What is the difference between the axial and appendicular skeleton? The axial skeleton forms the central axis of the body, providing support and protection for vital organs. The appendicular skeleton includes the bones of the limbs and their girdles, allowing for movement and interaction with the environment.
• How many bones are in the axial skeleton? The axial skeleton typically consists of 80 bones: 22 in the skull, 26 in the vertebral column (including the sacrum and coccyx), and 25 in the rib cage (including the sternum).
• What is the function of the hyoid bone? The hyoid bone supports the tongue and provides attachment points for muscles involved in swallowing and speech.
• What are the fontanelles in an infant's skull? Fontanelles are soft spots in an infant's skull where the cranial bones have not yet fused. They allow for growth of the brain and skull during infancy and early childhood.

Conclusion

The axial skeleton is a fundamental framework that supports, protects, and enables movement. From the intricate architecture of the skull to the flexible strength of the vertebral column and the protective embrace of the rib cage, each component plays a vital role in maintaining our health and function. Understanding the bones included in the axial skeleton, their individual features, and their collective functions is crucial for anyone seeking a deeper understanding of human anatomy and physiology. Whether you are a student, healthcare professional, or simply curious about the human body, knowledge of the axial skeleton provides a valuable foundation for appreciating the complexity and resilience of our musculoskeletal system.