Axial Skeleton: The Skull, Vertebral Column, and Thoracic Cage

Axial Skeleton – Part I: The Skull

Overview of the Skull

The skull is a complex structure composed of multiple bones that protect the brain and support the facial structures. It is divided into the cranial bones, which encase the brain, and the facial bones, which form the structure of the face.

• Cranial Bones: Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid

• Facial Bones: Zygomatic, Nasal, Maxillae, Vomer, Palatine, Mandible, Lacrimal, Inferior Nasal Conchae, Hyoid

Key Bones and Bone Markings

• Frontal Bone:

  • Supra-orbital margin: The superior border of the eye socket.

  • Supra-orbital foramen/notch: Opening above the orbit for nerves and vessels.

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

• Parietal Bones:

  • Temporal lines: Ridges for muscle attachment.

  • Sagittal suture: Joint between the two parietal bones.

  • Coronal suture: Joint between frontal and parietal bones.

• Temporal Bones:

  • Squamosal suture: Joint with parietal bone.

  • Mastoid process: Projection for muscle attachment.

  • External auditory meatus: Ear canal opening.

  • Mastoid cells: Air spaces in mastoid process.

  • Zygomatic process: Forms part of zygomatic arch.

  • Zygomatic arch: Cheekbone structure.

  • Styloid process: Slender projection for ligament attachment.

  • Mandibular (glenoid) fossa: Articulates with mandible.

  • Malleus, Incus, Stapes: Auditory ossicles in middle ear.

• Occipital Bone:

  • Occipital condyles: Articulate with the atlas (first cervical vertebra).

  • Foramen magnum: Large opening for spinal cord.

  • Lambdoidal suture: Joint with parietal bones.

• Sphenoid Bone:

  • Body: Central portion.

  • Sella turcica: Depression for pituitary gland.

  • Lesser and greater wings: Lateral projections.

  • Optic foramen: Passage for optic nerve.

  • Sphenoid sinuses: Air-filled cavities.

• Ethmoid Bone:

  • Cribriform plate: Perforated for olfactory nerves.

  • Olfactory foramina: Openings for olfactory nerves.

  • Crista galli: Attachment for brain membranes.

  • Perpendicular plate: Forms part of nasal septum.

  • Nasal conchae (turbinates): Superior and middle; increase surface area in nasal cavity.

  • Ethmoid sinuses: Air-filled spaces.

• Zygomatic (Malar) Bones:

  • Temporal process: Articulates with temporal bone.

  • Zygomatic arch: Cheekbone.

• Nasal Bones: Form bridge of nose.

• Maxillae:

  • Maxillary sinuses: Largest paranasal sinuses.

  • Alveolar process: Contains teeth sockets.

  • Infraorbital foramen: Passage for nerves and vessels.

  • Palatine process: Forms anterior hard palate.

• Vomer: Forms part of nasal septum.

• Palatine Bones: Form posterior hard palate.

• Mandible:

  • Condylar process: Articulates with temporal bone.

  • Coronoid process: Attachment for muscles.

  • Alveolar process: Contains lower teeth sockets.

  • Body, Angle, Ramus: Structural parts of mandible.

• Lacrimal Bones: Form part of medial orbit wall.

• Inferior Nasal Conchae: Increase surface area in nasal cavity.

• Hyoid Bone: Not attached to other bones; supports tongue.

Imaging Views

• Lateral, anterior, and posterior views are important for identifying sinuses and bone relationships.

Key Questions and Concepts

• Bones of the Nasal Septum: Vomer and perpendicular plate of ethmoid.

• Superficial Facial Bones: Zygomatic, nasal, maxillae, mandible, lacrimal, inferior nasal conchae.

• Deep Facial Bones: Palatine, vomer.

• Paranasal Sinuses: Frontal, sphenoid, ethmoid, maxillary (maxillary is largest). Sinuses lighten the skull and aid in resonance of voice.

• "Keystone" Bone of Cranial Floor: Sphenoid bone.

• "Keystone" Bone of Face: Maxilla.

• Cleft Palate: Failure of palatine processes of maxillae to fuse, resulting in an opening in the roof of the mouth.

Fetal Skull Fontanels

Fontanels are soft spots on the fetal skull that allow for growth and passage through the birth canal.

• Anterior (Frontal) Fontanel

• Posterior (Occipital) Fontanel

• Anterolateral (Sphenoidal) Fontanel

• Posterolateral (Mastoid) Fontanel

Axial Skeleton – Part II: Vertebrae, Ribs, and Sternum

Overview of the Vertebral Column

The vertebral column is a flexible structure that supports the body and protects the spinal cord. It is divided into regions, each with characteristic vertebrae.

• Cervical: 7 vertebrae (neck)

• Thoracic: 12 vertebrae (upper back)

• Lumbar: 5 vertebrae (lower back)

• Sacral: 5 fused vertebrae (sacrum)

• Coccygeal: 3-5 fused vertebrae (coccyx)

Total vertebrae: Embryo: 33; Adult: 26 (due to fusion of sacral and coccygeal regions).

Vertebral Anatomy

• Body: Main weight-bearing portion.

• Intervertebral disk: Fibrocartilage between vertebrae for cushioning.

• Intervertebral foramina: Openings for spinal nerves.

• Neural (vertebral) arch: Encloses spinal cord.

• Pedicles and Laminae: Parts of vertebral arch.

• Articulating processes (superior/inferior): Form joints between vertebrae.

• Facets: Smooth surfaces for articulation.

• Transverse and Spinous Processes: Projections for muscle attachment.

• Vertebral (spinal) foramen: Canal for spinal cord.

• Inferior notch: Forms part of intervertebral foramen.

Cervical Vertebrae Special Features

• Atlas (C1): Supports the skull; lacks a body.

• Axis (C2): Has odontoid process (dens) for rotation.

• Transverse foramen: Passage for vertebral arteries.

Sacrum Anatomy

• Sacral promontory: Anterior projection.

• Anterior (pelvic) and posterior (dorsal) sacral foramina: Openings for nerves.

• Median and lateral sacral crests: Fused spinous and transverse processes.

• Sacral canal: Continuation of vertebral canal.

Identifying Vertebrae

• Cervical: Small, transverse foramen, bifid spinous process.

• Thoracic: Medium, facets for ribs, long spinous process.

• Lumbar: Large, thick body, short spinous process.

Thoracic Cage: Sternum and Ribs

The thoracic cage protects vital organs and supports respiration. It consists of the sternum and ribs.

• Sternum:

  • Manubrium: Superior portion.

  • Body: Main portion.

  • Xiphoid process: Inferior tip.

• Ribs:

  • Vertebrosternal (true) ribs: 1-7; attach directly to sternum.

  • Vertebrochondral (false) ribs: 8-10; attach indirectly via cartilage.

  • Vertebral (floating) ribs: 11-12; do not attach to sternum.

  • Head and neck: Articulate with vertebrae.

  • Costal cartilage: Connects ribs to sternum.

Classification of Ribs

Type	Numbers	Attachment
Vertebrosternal (True)	1-7	Directly to sternum
Vertebrochondral (False)	8-10	Indirectly via cartilage
Vertebral (Floating)	11-12	No attachment to sternum

Summary Table: Vertebral Regions

Region	Number of Vertebrae (Adult)	Key Features
Cervical	7	Transverse foramen, bifid spinous process
Thoracic	12	Facets for ribs, long spinous process
Lumbar	5	Large body, short spinous process
Sacral	5 (fused)	Sacral canal, crests
Coccygeal	3-5 (fused)	Small, fused

Example: Identifying a Vertebra

• A vertebra with a transverse foramen and bifid spinous process is cervical.

• A vertebra with facets for rib articulation is thoracic.

• A vertebra with a large, thick body and short spinous process is lumbar.

Additional info:

• The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage.

• Paranasal sinuses are lined with mucous membranes and help humidify and warm inhaled air.

• The hyoid bone is unique in that it does not articulate directly with any other bone.