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

The Skeletal System

Overview and Divisions

The skeletal system is a vital organ system composed of bones, cartilage, and ligaments. It provides structural support, protection, and facilitates movement. The skeleton accounts for approximately 20% of total body mass and is divided into two main regions:

• Axial skeleton: Forms the longitudinal axis of the body.

• Appendicular skeleton: Includes the limbs and girdles that attach them to the axial skeleton.

Axial Skeleton

Main Regions and Functions

The axial skeleton consists of 80 bones divided into three major regions:

• Skull

• Vertebral column

• Thoracic cage

Functions of the axial skeleton include:

• Forming the longitudinal axis of the body

• Supporting the head, neck, and trunk

• Protecting the brain, spinal cord, and thoracic organs

The Skull

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

• Cranium: Encloses the brain in the cranial cavity and provides 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, 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.

Cranial Bones

There are eight cranial bones:

• Parietal bones (two—left and right)

• Temporal bones (two—left and right)

• Other bones include the frontal, occipital, sphenoid, and ethmoid bones.

Key features include:

• Articulations of parietal bones with frontal, occipital, and temporal bones

• Internal ear cavities and orbits for eyeballs

Foramina and Openings

The skull contains numerous foramina (holes), canals, and fissures that serve as passageways for the spinal cord, major blood vessels, and the 12 cranial nerves.

Frontal Bone

The frontal bone forms the anterior portion of the cranium, creating the forehead. It features:

• Supraorbital margins: Area under the eyebrows

• Frontal sinuses: Located just lateral to the glabella

Appendicular Skeleton

Main Regions and Functions

The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment.

• Pectoral girdle: Attaches upper limbs to the body trunk

• Pelvic girdle: Attaches lower limbs to the body trunk

• Upper and lower limb bones: Facilitate locomotion and manipulation

Key Terms and Definitions

• Bone marking: Surface features of bones that serve as sites of muscle, ligament, and tendon attachment, or as passageways for nerves and blood vessels.

• Suture: An immovable joint between skull bones.

• Fontanel: Soft spot on a baby's skull where bones have not yet fused.

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

Summary Table: Divisions of the Human Skeleton

Division	Main Components	Functions
Axial Skeleton	Skull, vertebral column, thoracic cage	Support, protection of central nervous system and thoracic organs
Appendicular Skeleton	Pectoral girdle, pelvic girdle, upper and lower limbs	Movement, manipulation of environment

Examples and Applications

• Clinical relevance: Knowledge of bone markings is essential for identifying sites for injections, understanding fractures, and interpreting radiographs.

• Developmental context: Fontanels allow for growth of the brain and skull in infants.

Additional info: The notes above expand on the brief points in the source material, providing definitions, context, and examples for key anatomical structures and terms relevant to college-level Anatomy & Physiology.