1. Overview

This unit covers the anatomy of three cranial bones: the Ethmoid, Sphenoid, and Temporal bones. These bones are located at the base and sides of the skull, contributing to the neurocranium and viscerocranium. The Ethmoid bone is unpaired and situated anteriorly, forming parts of the skull base, nasal septum, and orbit walls. The Sphenoid bone lies medially at the skull base, articulating with all neurocranial bones with multiple processes and sinuses. The Temporal bone is lateral, protecting auditory and vestibular organs and featuring important foramina and fossae. Understanding their anatomical parts, articulations, surfaces, and neurovascular relationships is essential.

2. Core Concepts & Key Elements

• Ethmoid Bone

  • Unpaired, anterior skull base
  • Belongs to neurocranium and viscerocranium
  • Articulates anteriorly with frontal and nasal bones, posteriorly with sphenoid
  • Forms anterior skull base, medial orbit walls, superior nasal septum, lateral nasal cavity
  • Anatomical parts:
    • Cribriform Plate (horizontal) with olfactory foramina and crista galli (olfactory nerve CN I passage, falx cerebri attachment)
    • Perpendicular Plate (vertical) separated into crista galli and perpendicular plate forming medial nasal walls
    • Lateral Masses (ethmoid labyrinth): medial orbit wall, lateral nasal cavity walls, ethmoidal cells
    • Thin scroll-shaped superior and middle nasal conchae attached superiorly to lateral masses
    • Meatus spaces inferior and lateral to conchae (superior and middle meatus) increase mucosal surface
    • Lamina papyracea forms large part of medial orbit wall

• Sphenoid Bone

  • Medial skull base location
  • Articulates with all neurocranial bones: frontal, ethmoid (anterior), occipital, temporal (posterior), parietal (superior and lateral)
  • Anatomical parts:
    • Body (cube-shaped) with sphenoidal air sinuses separated by septum
    • Superior surface: jugum sphenoidale (anterior with ethmoidal spine articulations, posterior with transverse crest)
    • Chiasmatic groove with optic foramen in lesser wing base
    • Sella turcica lodging pituitary fossa
    • Clinoid processes (middle, anterior, posterior) and dorsum sellae continuous with clivus of occipital bone
    • Anterior surface: anterior sphenoidal crest articulates with ethmoid perpendicular plate; openings for sphenoidal sinuses
    • Inferior surface: inferior sphenoidal crest, sphenoidal rostrum (continuous with anterior sphenoidal crest), vaginal process
    • Posterior surface: quadrilateral, joined to occipital bone (sphenobasilar synchondrosis)
    • Lateral surfaces: lesser wings (superior/anterior), greater wings (inferior/posterior), separated by superior orbital fissure
  • Lesser Wings
    • Triangular plates from body through two roots (upper root continuous with jugum, lower from body) delimiting optic canal (for optic nerve, ophthalmic artery)
    • Posterior corners are anterior clinoid processes
  • Greater Wings
    • Contribute to anterolateral cranial floor
    • Feature sphenoid spine (attachment spheno-mandibular ligament) and foramina: rotundum, ovale, spinosum
    • Surfaces: cerebral (medial cranial fossa, concave), lateral (temporal surface for temporalis muscle attachment, infratemporal surface for lateral pterygoid muscle attachment), orbital (lateral orbit surface)
  • Pterygoid Processes
    • Two roots (medial/internal from sphenoid body, lateral/external from greater wing) join to form pterygoid canal
    • Form medial (medial pterygoid plate) and lateral (lateral pterygoid plate) plates separated inferiorly by pterygoid notch
    • Pterygoid fossa (space between plates) - insertion for medial pterygoid muscle
    • Anterior surface articulates with perpendicular plate and pyramidal process of palatine bone
    • Medial pterygoid plate ends inferiorly at pterygoid hamulus (insertion pterygomandibular ligament)
    • Medial plate forms lateral wall of choanae (nasal cavity posterior lateral wall)

• Temporal Bone

  • Located lateral and base of skull, adjacent to temporal lobes
  • Protects auditory and vestibular apparatus, important neurovascular structures
  • Contains mandibular (glenoid) fossa for TMJ articulation
  • Principal parts:
    • Squama (thin, semicircular, convex, largest portion; attachment temporalis muscle)
      • Features groves (posterior deep temporal artery), temporal line (boundary with mastoid part), parietal notch
      • Zygomatic process with anterior (articulation with zygomatic bone) and posterior (external acoustic meatus and mandibular fossa) roots
      • Inferior outer view: articular eminence, mandibular fossa, petrotympanic fissure (for chorda tympani, facial nerve branch), tympanomastoid fissure (auricular branch vagus nerve)
      • Inner surface concave for temporal lobe and middle meningeal vessels
    • Mastoid portion (posteroinferior; posterior to external acoustic meatus)
      • Mastoid process with mastoid cells, attachment for sternocleidomastoid, splenius capitis, digastric posterior belly, longissimus capitis
      • Digastric fossa on inferior surface
      • Medial surface: sigmoid sulcus groove (sigmoid sinus), mastoid foramen (emissary veins)
    • Petrous portion (tympanic and styloid processes)
      • Pyramid shape between sphenoid and occipital bones at skull base
      • Four surfaces:
        • Superior-anterior: tegmen tympani, arcuate eminence (semicircular canals), hiatus for greater and lesser petrosal nerves, trigeminal ganglion impression
        • Superior-posterior: internal acoustic meatus, subarcuate fossa
        • Inferior-anterior: vaginal process (continuous with tympanic plate, surrounds styloid root), styloid process (attachment stylohyoid, stylopharynx, styloglossus muscles, stylomandibular ligament), stylomastoid foramen (exit facial nerve motor fibers), carotid canal (internal carotid artery entrance), jugular fossa (origin internal jugular vein), jugular surface (articulates with occipital jugular process)
        • Inferior-posterior: continuation carotid canal inferior orifice

3. High-Yield Facts

• Ethmoid bone: only unpaired bone of anterior skull base; contains olfactory foramina in cribriform plate for CN I
• Perpendicular plate of ethmoid forms major part of nasal septum
• Lateral ethmoid masses form ethmoidal sinuses (3-18 cells) and orbit medial walls
• Sphenoid body contains large sphenoidal sinuses separated by septum
• Optic canal (~5 mm) is between roots of lesser wings (optic nerve and ophthalmic artery)
• Sella turcica houses pituitary gland
• Major foramina in greater wings: rotundum (V2 maxillary nerve), ovale (V3 mandibular nerve), spinosum (middle meningeal artery)
• Pterygoid hamulus located at medial pterygoid plate, attachment pterygomandibular ligament
• Temporal bone contains the glenoid/mandibular fossa (TMJ), external acoustic meatus, mastoid process, and carotid canal
• Stylomastoid foramen transmits facial nerve motor fibers
• Mastoid process serves as attachment for sternocleidomastoid and other muscles
• Petrous portion contains internal acoustic meatus (cranial nerves VII, VIII)

4. Summary Table

5. Mini-Schema (ASCII)

Unit 6: Cranial and Facial Bones I
 ├─ Ethmoid Bone
 │   ├─ Cribriform Plate (olfactory foramina, crista galli)
 │   ├─ Perpendicular Plate (nasal septum, falx cerebri attachment)
 │   └─ Lateral Masses/Ethmoid Labyrinth (ethmoidal sinuses, conchae)
 ├─ Sphenoid Bone
 │   ├─ Body (sphenoidal sinuses, sella turcica)
 │   ├─ Lesser Wings (optic canal)
 │   ├─ Greater Wings (cranial floor, foramina rotundum/ovale/spinosum)
 │   └─ Pterygoid Processes (medial/lateral plates, pterygoid canal, pterygoid fossa)
 └─ Temporal Bone
     ├─ Squama (temporalis muscle, mandibular fossa)
     ├─ Mastoid Portion (mastoid process, muscle attachments)
     └─ Petrous Portion 
         ├─ Superior-anterior (tegmen tympani, arcuate eminence)
         ├─ Superior-posterior (internal acoustic meatus)
         ├─ Inferior-anterior (styloid process, carotid canal)
         └─ Inferior-posterior (jugular fossa, stylomastoid foramen)

6. Rapid-Review Bullets

• Ethmoid is only unpaired anterior skull base bone
• Cribriform plate contains olfactory nerve foramina (CN I)
• Crista galli is falx cerebri attachment on ethmoid
• Perpendicular plate forms nasal septum
• Ethmoid lateral masses house ethmoidal sinuses and form medial orbit walls
• Sphenoid body contains paired sphenoidal sinuses
• Optic canal formed between lesser wing roots; transmits CN II and ophthalmic artery
• Sella turcica houses pituitary gland
• Greater wing foramina: rotundum (V2), ovale (V3), spinosum (middle meningeal artery)
• Pterygoid processes have medial and lateral plates with pterygoid canal between roots
• Pterygoid hamulus attaches pterygomandibular ligament
• Temporal bone houses ear related structures and carotid canal
• Glenoid fossa articulates with mandibular condyle (TMJ)
• Mastoid process provides muscular attachments (sternocleidomastoid)
• Stylomastoid foramen transmits facial nerve motor fibers
• Tegmen tympani separates middle ear from cranial cavity
• Internal acoustic meatus transmits CN VII and VIII
• Jugular foramen transmits internal jugular vein origin
• Carotid canal transmits internal carotid artery to cranial cavity
• Temporal squama provides attachment for temporalis muscle
• Temporal zygomatic process articulates with zygomatic bone