Overview of the Human Skeleton: Axial and Appendicular Divisions, Skull Anatomy, and Cranial Bones

Human Skeleton: Organization and Overview

General Structure

The human skeleton consists of 206 bones, making up approximately 20% of the human body mass. It is divided into two major parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Includes bones of the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes bones of the upper and lower limbs, as well as the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

Example: The pectoral girdle (shoulder) connects the arms to the trunk, while the pelvic girdle connects the legs to the spine.

Axial Skeleton

Components and Functions

The axial skeleton is composed of 80 bones and forms the central axis of the body. Its primary functions are protection, support, and housing sensory organs.

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

• Vertebral Column: Supports the body and protects the spinal cord.

• Rib Cage: Protects the heart and lungs.

Skull Anatomy

Overview

The skull is the most complex bony structure in the body, consisting of 22 bones grouped into two sets:

• Cranial Bones: 8 bones that encase and protect the brain.

• Facial Bones: 14 bones that form the framework of the face.

Most skull bones are flat bones (except the mandible) and are united by immovable joints called sutures.

Functions of the Skull

• Encloses and protects the brain.

• Houses organs of sight, smell, taste, and hearing.

• Forms the cavities for sensory organs and provides openings for air and food passage.

• Secures the teeth and anchors facial muscles.

Cranial Bones

Vault and Base

• Vault: Forms the superior, lateral, and posterior aspects of the skull, as well as the forehead.

• Base: Forms the inferior aspect of the skull.

The cranium surrounds and protects the brain and organs of hearing and balance. Internally, three bony ridges divide the cranial base into three fossae:

• Anterior Cranial Fossa: Highest and most anterior.

• Middle Cranial Fossa

• Posterior Cranial Fossa: Lowest and most posterior.

Facial Bones

• Form the framework of the face.

• Contain cavities for sensory organs (sight, smell, taste).

• Provide openings for air and food passage.

• Secure the teeth.

• Anchor facial muscles for expression.

Individual Cranial Bones

i) Frontal Bone

The frontal bone is dome-shaped and forms the roof of the orbits and the anterior cranial fossa.

• Articulates with paired parietal bones posteriorly.

• Features: Supraorbital margin, supraorbital foramen, and glabella (smooth area between the eyebrows).

• Lateral to the glabella are the left and right frontal sinuses.

ii) Parietal Bones

The parietal bones are paired and form the superior and lateral aspects of the skull, making up the bulk of the cranial vault.

iii) Occipital Bone

• Forms the wall of the posterior cranial fossa.

• Attaches anteriorly to the parietal and temporal bones, and to the sphenoid.

• Features: Foramen magnum (large hole for the spinal cord), occipital condyles (articulation with the first cervical vertebra), and external occipital protuberance (projection at the back of the skull, more prominent in males).

iv) Temporal Bones

• Paired; form the inferior and lateral aspects of the skull and parts of the cranial floor.

• Located just below the parietal bones.

• Three regions:

  • Squamous Part: Flattened, forms the zygomatic process (cheek bones).

  • Tympanic Part: Surrounds the external acoustic meatus (ear canal).

  • Petrous Part: Houses structures of the inner ear.

• Features: Styloid process (attachment for tongue and neck muscles), jugular foramen, carotid canal, internal acoustic meatus.

v) Sphenoid Bone

The sphenoid bone is a complex bone that articulates with all other cranial bones and forms the base of the middle cranial fossa and part of the anterior cranial fossa.

• Central body contains sphenoid sinuses.

• Projections:

  • Greater and lesser wings: Form parts of the orbits and cranial fossae.

  • Pterygoid processes: Muscles for mastication attach here.

  • Optic foramina: Canals for optic nerves.

  • Superior orbital fissure: Passage for cranial nerves controlling eye movement.

Summary Table: Major Cranial Bones and Their Features

Bone	Location	Main Features	Function
Frontal	Forehead, roof of orbits	Supraorbital margin, glabella, frontal sinuses	Protects brain, forms face
Parietal	Superior & lateral skull	Bulk of cranial vault	Protects brain
Occipital	Posterior skull	Foramen magnum, occipital condyles	Protects brain, connects to spine
Temporal	Inferior & lateral skull	Squamous, tympanic, petrous parts; styloid process	Protects brain, houses ear structures
Sphenoid	Base of skull	Greater/lesser wings, pterygoid processes, optic foramina	Connects cranial bones, supports brain

Key Terms and Definitions

• Axial Skeleton: Central part of the skeleton including skull, vertebral column, and rib cage.

• Appendicular Skeleton: Limbs and girdles attaching them to the axial skeleton.

• Sutures: Immovable joints between skull bones.

• Foramen Magnum: Large opening in the occipital bone for the spinal cord.

• Sinus: Air-filled cavity within a bone.

Additional info:

• The cranial fossae (anterior, middle, posterior) are important for housing different parts of the brain and associated structures.

• Facial bones not only provide structure but also play a role in speech, eating, and facial expressions.