Chapter 7: The Skeleton – Structure and Function of the Human Skeletal System

Chapter 7: The Skeleton

Overview of the Skeletal System

The human skeletal system provides the framework for the body, supporting and protecting organs, enabling movement, and serving as a reservoir for minerals. It is composed of bones, cartilage, joints, and ligaments, and accounts for approximately 20% of total body mass.

• Skeleton: The entire framework of bones and their associated cartilages and ligaments.

• Key Functions:

  • Support and protection of vital organs

  • Facilitation of movement

  • Mineral storage (e.g., calcium, phosphate)

  • Blood cell formation (hematopoiesis)

Main Divisions of the Skeleton

The skeleton is divided into two major regions, each with distinct roles and components:

• Axial Skeleton: Forms the longitudinal axis of the body and consists of 80 bones.

• Appendicular Skeleton: Comprises the bones of the limbs and girdles, facilitating movement and manipulation of the environment.

Table: Major Divisions of the Skeleton

Division	Main Components	Primary Functions
Axial Skeleton	Skull, vertebral column, thoracic cage (ribs and sternum)	Support, protection of brain, spinal cord, and thoracic organs
Appendicular Skeleton	Pectoral girdles, upper limbs, pelvic girdle, lower limbs	Movement, manipulation, locomotion

Axial Skeleton

The axial skeleton consists of the skull, vertebral column, and thoracic cage. It provides the main support for the body and protects the central nervous system and thoracic organs.

• Skull: Protects the brain and forms the structure of the face.

• Vertebral Column: Supports the trunk, protects the spinal cord, and provides attachment points for ribs and muscles.

• Thoracic Cage: Protects the heart and lungs, supports the shoulder girdles and upper limbs.

Skull: Structure and Function

The skull is the most complex bony structure in the body, composed of two main parts:

• Cranium: Encloses and protects the brain in the cranial cavity; provides attachment for head and neck muscles.

• Facial Bones: Form the framework of the face, house special sense organs, provide openings for air and food, and anchor facial muscles.

Most skull bones are flat and firmly locked together by immovable joints called sutures, which have a serrated, saw-tooth appearance. The mandible (lower jaw) is the only movable bone of the skull.

Major Cranial Bones

• There are eight cranial bones:

  • Parietal bones (2 – left and right)

  • Temporal bones (2 – left and right)

  • Frontal bone

  • Occipital bone

  • Sphenoid bone

  • Ethmoid bone

• These bones form the cranial cavity, which houses the brain, and provide attachment points for head and neck muscles.

Major Openings and Markings

• Foramina, canals, and fissures: Openings that allow passage of nerves and blood vessels.

• Orbits: Cavities for the eyeballs.

• Nasal cavity: Houses structures for smell and air passage.

• Paranasal sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

Selected Cranial Bone Features

• Frontal bone: Forms the forehead and superior part of the orbits (eye sockets).

• Supraorbital margins: Thickened superior margins of the orbits, under the eyebrows.

• Frontal sinuses: Located just lateral to the glabella (smooth area between the eyes).

• Occipital bone: Forms the posterior part of the skull and base of the cranium; contains the foramen magnum (passage for the spinal cord).

• Occipital condyles: Articulate with the first cervical vertebra (atlas).

• Temporal bones: Form the sides and base of the cranium; house structures of the ears.

Table: Major Openings of the Skull (Foramina)

Foramen	Location	Structures Passing Through
Foramen magnum	Occipital bone	Spinal cord, vertebral arteries
Optic canal	Sphenoid bone	Optic nerve (CN II)
Jugular foramen	Between temporal and occipital bones	Internal jugular vein, cranial nerves IX, X, XI
Other foramina	Various	Other cranial nerves and blood vessels

Additional info: Students are expected to recognize the main foramina and their significance, but not memorize every detail unless specified by the instructor.

Summary Table: Functions of the Axial Skeleton

Region	Main Functions
Skull	Protects brain, supports sensory organs, forms facial structure
Vertebral Column	Supports trunk, protects spinal cord, provides attachment for ribs and muscles
Thoracic Cage	Protects thoracic organs, supports shoulder girdles and upper limbs, aids in breathing

Key Terms and Definitions

• Axial skeleton: The part of the skeleton that includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: The part of the skeleton that includes the limbs and girdles.

• Suture: An immovable joint between skull bones.

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

• Sinus: A cavity within a bone, usually filled with air and lined with mucous membrane.

Examples and Applications

• Clinical Example: Damage to the foramen magnum can result in life-threatening injury due to its role as the passageway for the spinal cord.

• Application: Knowledge of the locations of foramina is essential for understanding the pathways of cranial nerves and blood vessels in clinical practice.