The Axial Skeleton: Bones, Markings, and Key Features

Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central supporting axis of the body and consists of the skull, vertebral column, and thoracic cage. It provides protection for the brain, spinal cord, and thoracic organs, and serves as an attachment point for muscles.

• Major Components: Skull, vertebral column, thoracic cage (ribs and sternum)

• Functions: Protection, support, muscle attachment, and hematopoiesis (in some bones)

Cranium (Flat Bones)

Main Cranial Bones and Markings

The cranium consists of eight flat bones that protect the brain. Each bone has distinct markings that serve as passageways for nerves and blood vessels or as articulation points.

• Occipital (1): foramen magnum, occipital condyle, hypoglossal foramen/canal

• Frontal (1): supraorbital foramen

• Parietal (2): paired bones forming the sides and roof of the cranium

• Ethmoid (1): cribriform plate, olfactory foramina, perpendicular plate, crista galli

• Temporal (2): external acoustic (auditory) meatus, styloid process, mastoid process, zygomatic process, internal acoustic meatus, carotid canal, jugular foramen

• Sphenoid (1): sella turcica (houses pituitary gland), inferior orbital fissure, superior orbital fissure, optic canal

Sutures: Immovable joints between cranial bones. Major sutures include coronal, sagittal, lambdoid, and squamous.

Fontanelles: Soft spots in the fetal skull (anterior and posterior fontanelles) that allow for growth of the brain and skull during infancy.

Facial Bones

Main Facial Bones and Markings

The facial skeleton consists of 14 bones that form the structure of the face, house the teeth, and provide attachment points for facial muscles.

• Zygomatic (2): temporal process

• Maxilla (2): infraorbital foramen, alveolar process, inferior nasal conchae

• Palatine (2): form the posterior part of the hard palate

• Lacrimal (2): smallest facial bones, form part of the medial orbit wall

• Nasal (2): form the bridge of the nose

• Vomer (1): forms the inferior part of the nasal septum

• Mandible (1): body, ramus, condylar process, coronoid process, mental foramen, alveoli (tooth sockets)

Paranasal Sinuses

Location and Function of Paranasal Sinuses

Paranasal sinuses are air-filled spaces within certain skull bones. They lighten the skull, produce mucus, and contribute to voice resonance.

• Frontal Sinus

• Maxillary Sinus

• Sphenoidal Sinus

• Ethmoidal Sinus

Vertebral Column

Structure and Regions of the Vertebral Column

The vertebral column (spine) supports the head and trunk, protects the spinal cord, and provides attachment points for ribs and muscles. It is composed of 33 vertebrae in five regions.

• Typical Vertebra: spinous process, facets, body, transverse process, transverse foramina (cervical only), vertebral foramen, lamina, pedicle

• Cervical Vertebrae (7): Atlas (C1, allows nodding "yes"), Axis (C2, allows rotation "no", contains dens)

• Thoracic Vertebrae (12): Articulate with 12 pairs of ribs

• Lumbar Vertebrae (5): Largest vertebrae, support most body weight

• Sacrum (1): Formed by fusion of 5 vertebrae, features ala and sacral promontory

• Coccyx (1): Formed by fusion of 3-5 small vertebrae

Thoracic Cage

Components and Functions of the Thoracic Cage

The thoracic cage protects the heart and lungs and supports the shoulder girdles and upper limbs. It consists of the ribs, sternum, and thoracic vertebrae.

• Ribs (12 pairs):

  • True ribs (1-7): Attach directly to the sternum via costal cartilage

  • False ribs (8-12): Attach indirectly or not at all to the sternum

  • Floating ribs (11-12): Do not attach to the sternum

• Sternum (1): Manubrium, body, xiphoid process

Summary Table: Major Bones and Markings of the Axial Skeleton

Bone	Key Markings/Features	Function/Notes
Occipital	Foramen magnum, occipital condyle, hypoglossal canal	Passage for spinal cord, articulation with atlas
Frontal	Supraorbital foramen	Passage for nerves and vessels
Ethmoid	Cribriform plate, olfactory foramina, perpendicular plate, crista galli	Supports nasal cavity, olfaction
Temporal	External/internal acoustic meatus, styloid/mastoid/zygomatic processes, carotid canal, jugular foramen	Houses ear structures, passage for vessels/nerves
Sphenoid	Sella turcica, optic canal, orbital fissures	Houses pituitary gland, passage for optic nerve
Mandible	Body, ramus, condylar/coronoid process, mental foramen, alveoli	Lower jaw, supports teeth
Vertebrae	Spinous/transverse process, body, vertebral foramen	Protects spinal cord, supports body
Ribs	True, false, floating	Protect thoracic organs
Sternum	Manubrium, body, xiphoid process	Attachment for ribs and muscles

Example: The atlas (C1 vertebra) articulates with the occipital condyles of the skull, allowing the head to nod up and down. The axis (C2 vertebra) contains the dens, which acts as a pivot for rotation of the atlas and skull.

Additional info: The mnemonic "of pets" helps recall the cranial bones: Occipital, Frontal, Parietal, Ethmoid, Temporal, Sphenoid.