Axial Skeleton and Skull: Structure and Function

Axial and Appendicular Skeletons

Overview of Human Skeleton

• The human skeleton consists of 206 bones, making up approximately 20% of the human body mass.

• Bones are grouped into two main divisions: axial skeleton and appendicular skeleton.

Axial Skeleton

• Composed of bones of the skull, vertebral column, and rib cage.

• Contains 80 bones.

Appendicular Skeleton

• Includes bones of the upper and lower limbs.

• Includes pectoral (shoulder) and pelvic girdles, which attach limbs to the axial skeleton.

Axial Skeleton

Skull

The skull is the most complex bony structure in the body, protecting the brain and supporting sensory organs.

• Consists of two sets of bones: cranial and facial bones, totaling 22 bones.

• Most skull bones are flat bones (except the mandible) and are united by sutures.

• Facial bones form the anterior part of the skull, while cranial bones form the rest.

• The skull contains orbits (eye sockets) and paranasal sinuses, and houses organs for sight, smell, and taste.

Structure of the Skull

• Vault: Forms the superior, lateral, and posterior aspects of the skull and 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)

  • Middle cranial fossa

  • Posterior cranial fossa (lowest)

Facial Bones

• Form the framework of the face.

• Contain cavities for sensory organs of sight, smell, and taste.

• Provide openings for passage of air and food.

• Secure the teeth.

• Anchor facial muscles used for expression.

Cranial Bones

i) Frontal Bone

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

• Articulates with paired parietal bones posteriorly.

• Features:

  • Supraorbital margin

  • Supraorbital foramen

  • Glabella: Area between the eyebrows; lateral to glabella are the left and right frontal sinuses.

ii) Parietal Bones

• Paired bones forming the superior and lateral aspects of the skull.

• Form the bulk of the cranial vault.

iii) Occipital Bone

• Forms the walls of the posterior cranial fossa.

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

• Features:

  • Foramen magnum: Large hole at the base for passage of the spinal cord.

  • Occipital condyles: Articulation with the first cervical vertebra.

  • External occipital protuberance: Projection at the back of the skull, more prominent in males.

iv) Temporal Bones

• Paired bones forming the inferior and lateral aspects of the skull and parts of the cranial floor.

• Located just below the parietal bones; have three distinct regions:

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

  • Tympanic part: Surrounds the external acoustic meatus.

  • Petrous part: Contains important foramina (jugular foramen, carotid canal, internal acoustic meatus).

  • Styloid process: Attachment area for muscles of the tongue and some neck muscles.

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.

• Contributes to the base of the anterior cranial fossa.

• Central body contains sphenoid sinuses.

• Three 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 between greater and lesser wings for cranial nerves controlling eye movement.

Summary Table: Major Cranial Bones and Their Features

Bone	Main Features	Function/Articulation
Frontal	Supraorbital margin, glabella, frontal sinuses	Forms forehead, roof of orbits, anterior cranial fossa
Parietal	Paired, form cranial vault	Superior and lateral aspects of skull
Occipital	Foramen magnum, occipital condyles, external protuberance	Posterior cranial fossa, articulation with vertebral column
Temporal	Squamous, tympanic, petrous parts; styloid process	Inferior/lateral skull, cranial floor, muscle attachment
Sphenoid	Greater/lesser wings, pterygoid processes, optic foramina	Base of cranial fossae, articulates with all cranial bones

Key Terms and Definitions

• Suture: Immovable joint between skull bones.

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

• Fossa: Shallow depression or hollow in a bone.

• Sinus: Air-filled cavity within a bone.

Additional info:

• The notes provide a foundational overview of the axial skeleton, focusing on the skull and its major bones, their features, and functions. This is essential for understanding the protection and support of the brain and sensory organs in Anatomy & Physiology.