The Skeletal System

Overview of the Skeletal System

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

• Composed of: Bones, cartilage, joints, ligaments

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

• Major Divisions:

  • Axial skeleton

  • Appendicular skeleton

Fun Fact: Infants have about 270 bones, while adults have 206 bones due to bone fusion during development. The number of ribs, vertebrae, and digits can vary among individuals.

Major Divisions of the Skeleton

Axial Skeleton

The axial skeleton forms the central axis of the body and consists of 80 bones. It is divided into three major regions:

• Skull

• Vertebral column

• Thoracic cage

The Skull

Structure and Function

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

  • Provide sites of attachment for head and neck muscles

• Facial bones:

  • Form the framework of the face

  • Contain cavities for special sense organs (sight, taste, smell)

  • Provide openings for air and food passage

  • Secure teeth

  • Anchor facial muscles used for facial expression

Overview of Skull Geography

The cranium contains several important cavities and features:

• Middle and internal ear cavities

• Nasal cavity

• Orbits that house the eyeballs

• Air-filled sinuses that lighten the skull

• The skull has 85 named openings (foramina, canals, fissures) that provide passageways for the spinal cord, major blood vessels, and the 12 cranial nerves.

Cranial Bones

The cranium is comprised of eight cranial bones:

• Frontal bone

• Parietal bones (two—left and right)

• Occipital bone

• Temporal bones (two—left and right)

• Sphenoid bone

• Ethmoid bone

Key Terms and Concepts

• Foramen (plural: foramina): An opening or hole in a bone for the passage of nerves and blood vessels.

• Sinus: An air-filled cavity within a bone, especially those in the skull.

• Orbit: The bony cavity that contains the eyeball.

Example: Skull Openings

• The foramen magnum is a large opening at the base of the skull through which the spinal cord passes.

• The optic canal allows passage of the optic nerve from the eye to the brain.

Additional info:

• The axial skeleton provides protection for the central nervous system and vital organs, as well as support for the body’s posture.

• Bone fusion during development reduces the number of bones from infancy to adulthood.