UNIT 6: Cranial and Facial Bones I - Ethmoid, Sphenoid, Temporal Bones

1. Overview

• Focus on the ethmoid, sphenoid, and temporal bones: their location, anatomy, and functional significance.
• These bones are key components of the neurocranium and viscerocranium.
• They form critical structures: the base of the skull, orbits, nasal septum, and middle cranial fossa.
• The chapter covers their localization, main parts, articulations, and specific features.

2. Core Concepts & Key Elements

Ethmoid Bone

• Localization:

  • Unpaired, in anterior skull base.
  • Connects with frontal, nasal, sphenoid bones posteriorly.
  • Forms:
    • Part of anterior skull base.
    • Medial wall of orbits.
    • Superior nasal septum.
    • Superior lateral nasal wall.

• Anatomical Parts:

  • Cribriform Plate:
    • Horizontal, roof of nasal cavity.
    • Olfactory foramina for CN I.
    • Divided medially by crista galli.
  • Perpendicular Plate:
    • Vertical, medial nasal septum.
    • Divides nasal cavities.
    • Contains grooves for nerves/vessels.
  • Lateral Masses / Ethmoidal Labyrinth:
    • Located lateral to cribriform plate.
    • Contains 3-18 ethmoidal air cells (sinuses).
    • Articulates with orbit, nasal cavity.
    • Scroll-shaped projections: superior lo, middle conchae.
    • Meatuses: superior, middle (inside nasal cavities).
    • Lamina papyracea: orbital medial wall.

Sphenoid Bone

• Localization:

  • Central base of skull.
  • Joins with frontal, ethmoid (anterior); occipital, temporal (posterior); parietals (lateral).

• Anatomical Parts:

  • Body:
    • Central, cuboid shape.
    • Contains sphenoidal air sinuses.
    • Surfaces:
      • Superior: jugum sphenoidale, chiasmatic groove, tuberculum sellae, sella turcica (hypophyseal fossa), dorsum sellae, posterior clinoid processes.
      • Anterior: anterior sphenoidal crest, openings for sinuses.
      • Inferior: sphenoidal rostrum, pterygoid processes.
      • Posterior: joins basilar occipital (clivus).
  • Lateral surfaces:
    • Lesser wings:
      • Triangular, articulate with frontal, ethmoid.
      • Contain optic canals, anterior clinoid processes.
      • Surfaces for dura mater and orbit.
    • Greater wings:
      • Form part of middle cranial fossa, lateral skull wall.
      • Contain foramina rotundum, ovale, spinosum.
      • Surfaces for temporal lobe, infratemporal fossa, orbit.
      • Sphenoid spine and foramen lacerum.
  • Pterygoid processes:
    • Anterior projections with medial and lateral plates.
    • Pterygoid notch, pterygoid fossa.
    • Articulate with palatine, pterygopalatine fossa, nasal cavity.
    • Surfaces/attachments for muscles and ligaments.

Temporal Bone

• Localization:

  • Lateral skull sides, base.
  • Protects hearing and balance organs.
  • Contains structures for the middle and inner ear, neurovascular passage.

• Main parts:

  • Squama:
    • Thin, convex, attachment for temporalis muscle.
    • Features: zygomatic process, parietal notch, groove for posterior deep temporal artery.
  • Mastoid portion:
    • Posterior-inferior.
    • Contains mastoid process with mastoid cells.
    • Features: digastric and sigmoid sulci, mastoid foramen.
  • Petrous portion:
    • Pyramid-shaped, in base of skull.
    • Houses cochlea, vestibular system.
    • Related to carotid canal, jugular fossa, styloid process, internal acoustic meatus.
    • Surfaces:
      • Superior-anterior
      • Superior-posterior
      • Inferior-anterior
      • Inferior-posterior

• Important features:

  • Articular eminence (temporomandibular joint).
  • Styloid process (muscle/ligament attachment).
  • Carotid canal (internal carotid artery entry).
  • Jugular fossa (jugular vein exit).

3. High-Yield Facts

• Ethmoid's olfactory foramina transmit CN I.
• Crista galli: attachment point for falx cerebri.
• Perpendicular plate: forms the superior part of the nasal septum.
• Sphenoid sella turcica: houses the pituitary gland.
• Foramina in sphenoid wings:
  • Rotundum (V2)
  • Ovale (V3)
  • Spinosum (middle meningeal artery).
• Temporal bone mastoid process: contains air cells.
• Carotid canal: admits internal carotid artery.
• Jugular fossa: origin of the internal jugular vein.
• Pterygoid processes: attachments for muscles of mastication.

4. Summary Table

5. Mini-Schema (ASCII)

Skull Bones
 ├─ Ethmoid
 │   ├─ Cribriform Plate
 │   ├─ Perpendicular Plate
 │   └─ Lateral Masses (Ethmoidal Cells, Conchae)
 ├─ Sphenoid
 │   ├─ Body (sinuses, sella turcica)
 │   ├─ Lesser Wings (optic canal, anterior clinoid)
 │   └─ Greater Wings (foramina, temporal and orbital surfaces)
 └─ Temporal
     ├─ Squama (muscle attachment)
     ├─ Mastoid process (air cells)
     └─ Petrous part (ear cavity, carotid canal, internal auditory meatus)

6. Rapid-Review Bullets

• Ethmoid forms part of nasal septum and orbit medial wall.
• Olfactory foramina transmit CN I.
• Crista galli is attachment for falx cerebri.
• Ethmoidal air cells communicate with nasal cavity.
• Sphenoid sella turcica holds the pituitary gland.
• Foramina rotundum, ovale, spinosum are in sphenoid greater wing.
• Temporal squama attaches temporalis muscle.
• Mastoid process contains mastoid air cells.
• Petrous portion houses cochlear, vestibular structures.
• Carotid canal transmits internal carotid artery.
• Jugular fossa is for the jugular vein.
• Pterygoid processes serve as muscle attachments.
• Temporal bone protects hearing and balance organs.
• Middle meningeal vessels pass through foramen spinosum.
• Stylopoid process: ligament/muscle attachment site.
• Foramen lacerum: articulates with carotid canal (filled with cartilage in life).

End of Summary