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

The Skeletal System

Main Divisions of the Skeleton

The skeletal system is a complex framework composed of bones, cartilage, joints, and ligaments. It accounts for approximately 20% of total body mass and provides structural support, protection, and movement for the human body. The skeleton is divided into two major regions:

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

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

Axial Skeleton

Overview and Functions

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

• Skull

• Vertebral Column

• Thoracic Cage

Functions of the axial skeleton include:

• Supporting the head, neck, and trunk

• Protecting the brain, spinal cord, and thoracic organs

• Providing attachment points for muscles

The Skull

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

• Cranial Bones: Enclose and protect 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; anchor facial muscles used for expression.

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 (2 – left and right)

• Temporal Bones (2 – left and right)

• Frontal Bone

• Occipital Bone

• Sphenoid Bone

• Ethmoid Bone

Key features include:

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

• Internal ear cavities and orbital cavities for the eyeballs

Skull Openings

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

• Foramen: A round or oval opening through a bone.

• Canal: A tunnel-like passage through a bone.

• Fissure: A narrow, slit-like opening.

These openings are essential for the transmission of nerves and blood vessels.

Frontal Bone

The frontal bone forms the anterior portion of the cranium, commonly known as the forehead. Key features include:

• Supraorbital Margins: Area under the eyebrows

• Frontal Sinuses: Located just lateral to the glabella (between the eyebrows)

Appendicular Skeleton

Overview

The appendicular skeleton consists of the bones of the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton. It is responsible for movement and manipulation of objects in the environment.

• Pectoral Girdle: Attaches the upper limbs to the body trunk

• Pelvic Girdle: Attaches the lower limbs to the body trunk

Key Terms and Definitions

• Bone: A rigid organ that constitutes part of the vertebrate skeleton; supports and protects various organs.

• Cartilage: A resilient and smooth elastic tissue; covers and protects the ends of long bones at the joints.

• Ligament: A short band of tough, flexible connective tissue; connects two bones or cartilages or holds together a joint.

• Suture: An immovable joint between two bones, especially in the skull.

• Foramen: An opening, hole, or passage, typically in bone.

Example Table: Major Divisions of the Skeleton

Division	Main Components	Functions
Axial Skeleton	Skull, Vertebral Column, Thoracic Cage	Support, protection of central nervous system and thoracic organs
Appendicular Skeleton	Limb bones, Pectoral and Pelvic Girdles	Movement, manipulation of environment

Additional info:

• Fontanels are soft spots in the skull of infants that allow for growth of the brain and skull during development.

• The axial skeleton provides the main support for the body and protects vital organs, while the appendicular skeleton is primarily involved in movement.