Anatomy of the Skull

Fontanelles

• There are two major fontanelles: frontal (anterior) and occipital (posterior)
• Frontal fontanelle is located at the junction of the coronal and sagittal sutures
• Occipital fontanelle is located at the junction of the sagittal and lambdoid sutures

Frontal Bone

• Forms the anterosuperior aspect of the cranium
• Consists of four parts: squamous, orbital, and nasal
• Squamous part:
  • Largest component of the frontal bone
  • External convex surface forms the shape of the forehead
  • Internal surface features: sagittal sulcus, frontal crest, and frontal sinuses
• Orbital part:
  • Comprised of a triangular-shaped flat plate
  • Forms the roof of the orbit
  • Anterolateral portion consists of the lacrimal fossa
• Nasal part:
  • Small area that projects inferiorly between the supraorbital margins
  • Articulates with the nasal bones and frontal processes of the maxilla
• Articulations:
  • Unpaired: ethmoid, sphenoid
  • Paired: nasal, maxilla, lacrimal, parietal, zygomatic

Parietal Bone

• Has four borders that articulate with surrounding skull bones via sutures
• Sagittal border: superiorly, articulates with the other parietal bone to form the sagittal suture
• Squamosal border: inferiorly, articulates with the greater wing of the sphenoid bone and the squamous and petrous parts of the temporal bone
• Frontal border: anteriorly, articulates with the frontal bone
• Occipital border: posteriorly, articulates with the occipital bone to form the lambdoid suture

Sphenoid Bone

• 'Butterfly-shaped'
• Comprised of:
  • Body
  • Greater wings
  • Lesser wings
  • Two pterygoid processes
• Body:
  • Contains the sphenoidal sinuses
  • Articulates with the ethmoid bone anteriorly
  • Superior surface features: sella turcica, chiasmatic groove

Ethmoid Bone

• Comprised of:
  • Cribriform plate
  • Perpendicular plate
  • Ethmoidal labyrinth
• Cribriform plate:
  • Forms the roof of the nasal cavity
  • Pierced by olfactory nerve fibers
• Perpendicular plate:
  • Forms the superior two-thirds of the nasal septum
• Ethmoidal labyrinth:
  • Contains the ethmoidal air cells (sinuses)
  • Comprised of an orbital plate and a medial sheet
• Articulations:
  • Unpaired: frontal, vomer, and sphenoid bones
  • Paired: nasal bones, maxillae, lacrimal bones, palatine bones, and inferior conchae

Temporal Bone

• Contributes to the lower lateral walls of the skull
• Contains the middle and inner portions of the ear
• Articulates with the mandible to form the temporomandibular joint
• Squamous part:
  • Articulates with the sphenoid bone and the parietal bone
  • Zygomatic process articulates with the zygomatic bone to form the zygomatic arch

Occipital Bone

• Unpaired bone that covers the back of the head (occiput)
• Comprised of three parts: squamous, condylar, and basilar
• Squamous part:
  • Largest and most posterior portion
  • Borders the parietal and temporal bones
  • Internal surface features: grooves for dural venous sinuses, cruciform eminence, and internal occipital protuberance
  • External surface features: nuchal lines
• Condylar part:
  • Forms the lateral walls of the foramen magnum
  • Articulates with the atlas (at the atlanto-occipital joint)
  • Contains the hypoglossal canal
• Basilar part:
  • Forms the anterior part of the foramen magnum
  • Articulates with the temporal and sphenoid bones
• Foramen magnum:
  • Allows passage of structures to and from the cranial cavity
  • Structures that pass through: medulla oblongata, accessory nerves, vertebral arteries
• Articulations:
  • Parietal bones superiorly to form the lambdoid suture
  • Temporal bones laterally to form the occipitomastoid structure
  • Sphenoid bone anteriorly
  • Atlas (C1) inferiorly at the atlanto-occipital joint