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

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

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and consists of the skull, vertebral column, and thoracic cage. It provides support, protection for vital organs, and attachment points for muscles.

• Major Components: Skull, vertebral column, ribs, and sternum.

• Functions: Protects the brain, spinal cord, and thoracic organs; supports the body; enables movement.

The Skull

General Structure and Views

The skull is a complex bony structure that houses and protects the brain and forms the framework of the face. It can be studied from various views: all views, cranial cavity, and orbit.

• Cranial Bones: Enclose the brain.

• Facial Bones: Form the structure of the face.

Major Bones and Markings of the Skull

• Frontal Bone:

  • Supra-orbital margin: Superior rim 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 the mastoid process.

  • Zygomatic process: Forms part of the zygomatic arch.

  • Zygomatic arch: Cheekbone structure.

  • Styloid process: Slender projection for ligament/muscle attachment.

  • Mandibular (glenoid) fossa: Depression for mandible articulation.

  • Malleus, incus, stapes: Auditory ossicles in the middle ear.

• Occipital Bone:

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

  • Foramen magnum: Large opening for the spinal cord.

  • Lambdoidal suture: Joint with parietal bones.

• Sphenoid Bone:

  • Body: Central part of the bone.

  • Sella turcica: Depression housing the pituitary gland.

  • Lesser and greater wings: Lateral projections.

  • Optic foramen: Passage for optic nerve.

  • Sphenoid sinuses: Air-filled spaces within the bone.

• Ethmoid Bone:

  • Cribriform plate: Roof of nasal cavity with olfactory foramina.

  • Olfactory foramina: Openings for olfactory nerves.

  • Crista galli: Vertical projection for membrane attachment.

  • Perpendicular plate: Forms part of nasal septum.

  • Nasal conchae (superior and middle): Increase surface area in nasal cavity.

  • Ethmoid sinuses: Air spaces within the ethmoid bone.

• Zygomatic (Malar) Bones:

  • Temporal process: Articulates with zygomatic process of temporal bone.

  • Zygomatic arch: Cheekbone structure.

• Nasal Bones: Form the bridge of the nose.

• Maxillae:

  • Maxillary sinuses: Largest paranasal sinuses.

  • Alveolar process: Contains tooth sockets.

  • Infraorbital foramen: Opening below the orbit.

  • Palatine process: Forms anterior hard palate.

• Vomer: Forms part of the nasal septum.

• Palatine Bones: Form posterior hard palate and part of nasal cavity.

• Mandible:

  • Condylar process: Articulates with temporal bone.

  • Coronoid process: Insertion for temporalis muscle.

  • Alveolar process: Contains lower teeth sockets.

  • Body, angle, ramus: Main parts of the mandible.

• Lacrimal Bones: Small bones forming part of the medial orbit wall.

• Inferior Nasal Conchae: Separate bones in the nasal cavity.

• Hyoid Bone: U-shaped bone in the neck, not directly attached to other bones.

Imaging and Views

• Head and Neck Imaging: Lateral, anterior, and posterior views are useful for studying sinuses and bone relationships.

Key Questions and Concepts

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

• Superficial Facial Bones: Maxillae, zygomatic, nasal, mandible, lacrimal, and inferior nasal conchae.

• Deep Facial Bones: Palatine, vomer, and parts of the maxillae and ethmoid.

• Paranasal Sinuses: Frontal, maxillary, sphenoid, and ethmoid sinuses. The maxillary sinus is the largest.

• Importance of Paranasal Sinuses: Lighten the skull, warm and moisten air, enhance voice resonance.

• "Keystone" Bone of the Cranial Floor: Sphenoid bone (articulates with all other cranial bones).

• "Keystone" Bone of the Face: Maxilla (articulates with all other facial bones except mandible).

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

Fetal Skull and Fontanels

The fetal skull contains soft spots called fontanels that allow for growth and flexibility during birth.

• Anterior (Frontal) Fontanel

• Posterior (Occipital) Fontanel

• Anterolateral (Sphenoidal) Fontanel

• Posterolateral (Mastoid) Fontanel

Vertebral Column

Regions and Structure

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles. It is divided into regions:

• Cervical (7 vertebrae): Neck region.

• Thoracic (12 vertebrae): Upper back, articulates with ribs.

• Lumbar (5 vertebrae): Lower back.

• Sacral (5 fused vertebrae): Forms the sacrum.

• Coccygeal (4 fused vertebrae): Forms the coccyx.

Total vertebrae: 33 in embryo, 26 in adult (due to fusion of sacral and coccygeal regions).

General Vertebra Structure

• Body: Weight-bearing portion.

• Intervertebral disk: Fibrocartilage pad between vertebrae.

• Intervertebral foramina: Openings for spinal nerves.

• Neural (vertebral) arch: Encloses the spinal cord.

• Pedicles and laminae: Parts of the vertebral arch.

• Superior and inferior articulating processes: Form joints with adjacent vertebrae.

• Facets: Articular surfaces on processes.

• Transverse processes: Lateral projections.

• Spinous process: Posterior projection.

• Vertebral (spinal) foramen: Central opening for spinal cord.

• Vertebral (spinal) canal: Series of vertebral foramina.

• Inferior notch: Forms part of intervertebral foramen.

Special Cervical Vertebrae

• Atlas (C1): Supports the skull, allows nodding motion.

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

• Transverse foramen: Passage for vertebral arteries (unique to cervical vertebrae).

Identifying Vertebrae

• Cervical: Small body, bifid spinous process, transverse foramina.

• Thoracic: Heart-shaped body, facets for rib articulation, long spinous process.

• Lumbar: Large, kidney-shaped body, short blunt spinous process.

Sacrum and Coccyx

• Sacral promontory: Anterior border of the sacrum.

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

• Posterior (dorsal) sacral foramina: Openings on the dorsal surface.

• Median sacral crest: Fused spinous processes.

• Lateral sacral crest: Fused transverse processes.

• Sacral canal: Continuation of vertebral canal.

Thoracic Cage

Structure and Function

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

Sternum

• Manubrium: Superior portion, articulates with clavicles and first ribs.

• Body: Main, elongated part.

• Xiphoid process: Inferior, cartilaginous in youth, ossifies in adulthood.

Ribs

• Vertebrosternal (true) ribs (1-7): Attach directly to sternum via costal cartilage.

• Vertebrochondral (false) ribs (8-10): Attach indirectly to sternum via cartilage of rib 7.

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

• Head: Articulates with vertebral bodies.

• Neck: Narrow region next to head.

• Costal cartilage: Hyaline cartilage connecting ribs to sternum.

Classification of Ribs

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

Summary Table: Vertebral Column Regions

Region	Number of Vertebrae (Adult)	Main Features
Cervical	7	Neck, transverse foramina
Thoracic	12	Articulate with ribs
Lumbar	5	Large bodies, lower back
Sacral	5 (fused)	Forms sacrum
Coccygeal	4 (fused)	Forms coccyx

Additional info:

• The axial skeleton is essential for protecting the central nervous system and vital organs, as well as providing structural support and facilitating movement.

• Fontanels in the fetal skull allow for brain growth and passage through the birth canal.

• Paranasal sinuses are lined with mucous membranes and are involved in humidifying and warming inhaled air.