The Skeletal System

Overview of the Skeletal System

The skeletal system, or skeleton, is a framework composed of bones, cartilages, joints, and ligaments. It provides structural support, protection, and facilitates movement.

• Bones: Rigid organs forming the skeleton's main structure.

• Cartilage: Flexible tissue found in isolated areas, such as joints.

• Ligaments: Connect bones and reinforce joints.

• The skeleton accounts for approximately 20% of total body mass.

Divisions of the Skeleton

• Axial Skeleton: Forms the long axis of the body, including the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Comprises the limbs and girdles.

Axial Skeleton

Major Regions

• Skull

• Vertebral column

• Thoracic cage (ribs)

The Skull

The skull is the most complex bony structure in the body, formed by two sets of bones:

• Cranial bones (cranium): Enclose the brain in the cranial cavity and provide sites of attachment for head and neck muscles.

• Facial bones: Form the framework of the face, provide cavities for special sense organs (sight, taste, smell), openings for air and food passage, secure teeth, and anchor facial muscles for expression.

Most skull bones are flat and firmly locked together except for the mandible (lower jaw), which is movable. Joints between skull bones are called sutures and have a serrated, saw-tooth appearance.

Structures of the Cranium

• Coronal suture

• Squamous suture

• Lambdoid suture

• Foramen (hole)

Cranial Bones

• The cranium is comprised of 8 cranial bones:

  • Frontal bone

  • Parietal bones (two – left and right)

  • Occipital bone

  • Temporal bones (two – left and right)

  • Sphenoid bone

  • Ethmoid bone

Major Sutures of the Skull

• Coronal suture: Between parietal bone and frontal bone (crown)

• Sagittal suture: Between right and left parietal bones

• Lambdoid suture: Between parietal bones and occipital bone

• Squamous suture: Between parietal and temporal bones on each side of the skull

The Vertebral Column

General Characteristics

• Extends from the skull to the pelvis; also called the spine or spinal column.

• Functions: Transmits weight of trunk to lower limbs, surrounds and protects the spinal cord, provides attachment points for ribs and muscles.

• Flexible structure containing 26 irregular bones called vertebrae, divided into 5 major regions.

Regions and Curvatures

• There are 26 bones in the vertebral column, broken into five major regions:

  • Cervical: 7 vertebrae (C1–C7)

  • Thoracic: 12 vertebrae (T1–T12)

  • Lumbar: 5 vertebrae (L1–L5)

  • Sacrum: 1 bone formed by fusion of several bones, articulates with hip (5 fused vertebrae)

  • Coccyx: 1 bone formed by fusion of several bones (4 fused vertebrae)

• Curvatures: Four main curvatures in the column help increase resilience and flexibility.

  • Cervical and lumbar curvatures: Concave posteriorly

  • Thoracic and sacral curvatures: Convex posteriorly

Regional Vertebrae Characteristics

• Cervical vertebrae (C1–C7): Smallest, lightest vertebrae. C1 (atlas) and C2 (axis) have unique features for head movement.

• Thoracic vertebrae (T1–T12): Increase in size and articulate with ribs. Vertebral foramen is circular; spinous processes are long and point downward.

• Lumbar vertebrae (L1–L5): Massive bodies to support weight; short, thick pedicles and laminae; vertebral foramen is triangular.

• Sacrum: Triangular bone, forms posterior wall of pelvis, articulates with L5 and hip bones.

• Coccyx: Tailbone, very little function, 4 fused vertebrae.

Special Features of Cervical Vertebrae

• Atlas (C1): No body or spinous process; supports the skull; allows nodding "yes" motion.

• Axis (C2): Has a body and processes; dens (odontoid process) projects superiorly and acts as a pivot for rotation of the atlas and skull ("no" motion).

Thoracic Cage

Components and Functions

• Composed of thoracic vertebrae (posteriorly), sternum and costal cartilages (anteriorly), and ribs (laterally).

• Functions:

  • Protects vital organs of the thoracic cavity (heart, lungs).

  • Supports shoulder girdles and upper limbs.

  • Provides attachment sites for muscles of the neck, back, chest, and shoulders.

Ribs

• 12 pairs form the sides of the thoracic cage.

• All attach posteriorly to bodies and transverse processes of thoracic vertebrae.

• Types of ribs:

  • True ribs (1–7): Attach directly to the sternum via costal cartilages.

  • False ribs (8–10): Attach indirectly to the sternum by joining the costal cartilage of the rib above.

  • Floating ribs (11–12): No attachment to the sternum.

Main Parts of a Rib

• Head: Articulates with facets (demifacets) on bodies of two adjacent vertebrae.

• Neck: Constricted portion beyond the head.

• Tubercle: Knoblike structure lateral to the neck; articulates with the transverse costal facet of the same-numbered thoracic vertebra.

• Shaft: Flat bone that makes up most of the rib.

• Costal groove: Houses nerves and vessels.

Summary Table: Vertebral Column Regions

Key Terms and Definitions

• Suture: Immovable joint between skull bones.

• Foramen: Opening or hole in a bone for passage of nerves and blood vessels.

• Process: Projection or outgrowth of bone for muscle or ligament attachment.

• Facet: Smooth, flat surface for articulation with another bone.

Additional info:

• Vertebral foramen: The canal through which the spinal cord passes.

• Transverse foramen: Found only in cervical vertebrae, allows passage of vertebral arteries.

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