Axial Skeleton and Skull: Structure and Key Bones

Introduction to the Axial Skeleton

The axial skeleton forms the central axis of the human body and provides support and protection for the brain, spinal cord, and thoracic organs. It consists of 80 bones in the adult human skeleton, which is made up of a total of 206 bones.

• Skull: 22 bones

• Associated bones: 7 bones (including auditory ossicles and hyoid bone)

• Vertebral column: 26 bones

• Thoracic cage: 25 bones

The axial skeleton is distinct from the appendicular skeleton, which includes the limbs and girdles.

Bones of the Skull

The skull is a complex structure composed of 22 bones, which are categorized based on their relationship to the brain:

• Facial bones: 14 bones that do not contact the brain

• Cranial bones: 8 bones that form the cranium and have direct contact with the brain

The cranial bones include the frontal, parietal (2), temporal (2), occipital, sphenoid, and ethmoid bones.

Bones of the Skull: Frontal Bone

The frontal bone forms the forehead and the roof of the orbits (eye sockets). It is a single bone located at the anterior part of the skull.

• Supraorbital margin: The superior rim of the eye socket.

• Supraorbital foramen (notch): An opening above the orbit for nerves and blood vessels.

• Superciliary arches (brow ridges): Bony ridges above the orbits.

• Glabella: The smooth area between the eyebrows.

• Squamous part: The flat, vertical part of the frontal bone.

• Frontal sinuses: Air-filled cavities within the frontal bone.

Example: The frontal bone contributes to the formation of the forehead and the upper part of the eye sockets, providing protection for the frontal lobes of the brain.

Bones of the Skull: Parietal Bones

The parietal bones are paired bones that form the superior and lateral walls and roof of the cranium.

• Superior and inferior temporal lines: Ridges for muscle attachment.

• Parietal eminence: The rounded prominence on each side of the skull, marking the widest part of the cranium.

Example: The parietal bones articulate with the frontal, temporal, and occipital bones at major sutures.

Bones of the Skull: Temporal Bones

The temporal bones are paired bones that form the lateral walls and part of the floor of the cranium. They house structures essential for hearing and balance.

• Squamous part: Flat, thin portion forming part of the cranial wall.

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

• Mastoid process: A projection serving as an attachment point for neck muscles.

• Styloid process: A slender projection serving as an attachment point for muscles and ligaments of the tongue and hyoid bone.

Example: The temporal bone contains the structures of the middle and inner ear, including the cochlea and semicircular canals.

Table: Major Bones of the Axial Skeleton

Region	Number of Bones	Main Components
Skull	22	Cranial and facial bones
Associated bones	7	Auditory ossicles, hyoid bone
Vertebral column	26	Cervical, thoracic, lumbar vertebrae, sacrum, coccyx
Thoracic cage	25	Sternum, ribs

Additional info: The axial skeleton provides the main support for the body and protects vital organs such as the brain, spinal cord, heart, and lungs. The bones of the skull are joined by immovable joints called sutures, which provide strength and stability.