The Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and provides support and protection for the brain, spinal cord, and thoracic organs. It consists of the skull, vertebral column, ribs, and sternum.

• Major Components: Skull, vertebral column, thoracic cage (ribs and sternum), and hyoid bone.

• Functions: Protection of vital organs, support for the body, and attachment points for muscles.

General Vertebrae Anatomy

Basic Structure of a Vertebra

Vertebrae are the repeating bony units of the vertebral column, each with characteristic features that allow for protection of the spinal cord and flexibility of the spine.

• Intervertebral Disc: Cartilaginous pad between vertebrae that absorbs shock and allows movement.

• Body: The thick, disc-shaped anterior portion that bears weight.

• Lamina: Part of the vertebral arch, forming the roof of the vertebral foramen.

• Spinous Process: Posterior projection for muscle and ligament attachment.

• Transverse Process: Lateral projections for muscle and ligament attachment.

• Vertebral Foramen: Opening for the spinal cord.

• Inferior and Superior Articular Processes: Projections that form joints with adjacent vertebrae.

Specific Vertebrae Types

The vertebral column is divided into regions, each with vertebrae of distinct structure and function.

• Cervical Vertebrae (7): Smallest vertebrae, found in the neck. Notable features include the transverse foramen.

• Atlas (C1): Supports the skull; allows nodding motion. Lacks a body and spinous process.

• Axis (C2): Has the dens (odontoid process) for rotation of the head.

• Thoracic Vertebrae (12): Articulate with ribs; have long, downward-pointing spinous processes.

• Lumbar Vertebrae (5): Largest vertebrae; support most body weight.

• Sacrum (5 fused): Forms the posterior wall of the pelvis.

• Coccyx (4 fused): Tailbone; vestigial structure.

Sternum

Structure and Function

The sternum is a flat bone located in the center of the chest, forming the anterior portion of the thoracic cage.

• Manubrium: Superior portion; articulates with clavicles and first ribs.

• Body: Middle and largest part; articulates with costal cartilages of ribs 2-7.

• Xiphoid Process: Inferior, small, cartilaginous in youth, ossifies in adulthood.

• Jugular Notch: Superior indentation on the manubrium.

Ribs

Classification and Features

The ribs form the protective cage around the thoracic organs and are classified based on their attachment to the sternum.

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

• False Ribs (8-10): Attach indirectly to the sternum via cartilage of the rib above.

• Floating Ribs (11-12): Do not attach to the sternum.

• Head: Articulates with vertebral bodies.

• Tubercle: Articulates with the transverse process of vertebrae.

Hyoid Bone

Structure and Function

The hyoid bone is a unique, U-shaped bone in the neck that does not articulate with any other bone. It serves as an attachment site for tongue and neck muscles.

• Location: Between the mandible and larynx.

• Function: Supports the tongue and assists in swallowing and speech.

Skull: Cranial and Facial Bones

Cranial Sutures

Sutures are immovable joints that connect the bones of the skull.

• Coronal Suture: Between frontal and parietal bones.

• Lambdoid Suture: Between parietal and occipital bones.

• Sagittal Suture: Between the two parietal bones.

• Squamous Suture: Between parietal and temporal bones.

Cranial Bones

The cranium consists of eight bones that protect the brain.

• Frontal Bone: Forehead region; contains the supraorbital foramen.

• Parietal Bones (2): Form the sides and roof of the cranium.

• Temporal Bones (2): Contain the external acoustic meatus, mastoid process, styloid process, and zygomatic process.

• Occipital Bone: Contains the foramen magnum and occipital condyles.

• Sphenoid Bone: Forms part of the base of the skull; contains the sella turcica and optic canal.

• Ethmoid Bone: Contains the cribriform plate, perpendicular plate, and middle nasal conchae.

Facial Bones

The facial skeleton consists of 14 bones that form the structure of the face.

• Mandible: Lower jaw; only movable bone of the skull. Features include the mandibular condyle, coronoid process, and mental foramen.

• Maxilla (2): Upper jaw; contains infraorbital foramen.

• Palatine Bones (2): Form the posterior part of the hard palate.

• Zygomatic Bones (2): Cheekbones; form part of the orbit.

• Lacrimal Bones (2): Small bones forming part of the medial orbit wall.

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

• Inferior Nasal Conchae (2): Form part of the lateral walls of the nasal cavity.

• Vomer: Forms part of the nasal septum.

The Fetal Skeleton

Fontanelles

Fontanelles are soft, membranous gaps between the cranial bones in infants, allowing for growth of the brain and skull.

• Anterior Fontanelle: Largest, located at the junction of the frontal and parietal bones.

• Posterior Fontanelle: At the junction of the parietal and occipital bones.

• Sphenoidal Fontanelle: At the junction of the frontal, parietal, temporal, and sphenoid bones.

• Mastoid Fontanelle: At the junction of the parietal, occipital, and temporal bones.

Compact Bone Model

Structure of Compact Bone

Compact bone is the dense, outer layer of bone that provides strength and protection. Its structure is organized into osteons or Haversian systems.

• Central Canal: Contains blood vessels and nerves.

• Canaliculi: Small channels connecting osteocytes.

• Circumferential Lamellae: Layers of bone matrix around the entire bone.

• Osteon: Structural unit of compact bone.

• Osteocyte: Mature bone cell.

• Periosteum: Outer fibrous covering of bone.

• Endosteum: Lines the medullary cavity.

• Perforating (Volkmann's) Canals: Connect central canals of adjacent osteons.

• Perforating Fibers: Secure periosteum to underlying bone.

Table: Classification of Ribs

Key Terms and Definitions

• Articular Cartilage: Smooth tissue covering the ends of bones in joints.

• Epiphyseal Line: Remnant of the growth plate in adult bones.

• Epiphysis: End part of a long bone.

• Diaphysis: Shaft of a long bone.

• Medullary Cavity: Central cavity of bone shafts where marrow is stored.

• Spongy Bone: Porous bone found at the ends of long bones and inside flat bones.