Ethmoid & Sphenoid Bones

8 décembre 2025

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1. Overview

  • Focus on cranial bones: Ethmoid, Sphenoid, and Temporal bones
  • Location within skull: anterior, medial, lateral, base regions
  • These bones form critical structural, respiratory, and neurovascular pathways
  • Roles include forming the skull base, orbit walls, nasal cavity, and protecting sensory organs
  • Key course ideas: anatomical parts, articulations, foramina, processes, and ossification

2. Core Concepts & Key Elements

  • Ethmoid Bone:

    • Unpaired, anterior skull base, medial orbit wall, nasal septum, superior lateral nasal wall
    • Parts: Cribriform plate (ethmoidal roof), perpendicular plate (nasal septum), lateral masses (ethmoidal labyrinth)
    • Cribriform plate: olfactory foramina, hosts CN I, divided by crista Galli (attachment for falx cerebri)
    • Perpendicular plate: vertical, medial wall of nasal cavities, divided into crista Galli (triangular process) and perpendicular lamina
    • Lateral masses/ethmoidal labyrinth: contain ethmoidal cells (sinuses), superior/middle conchae, spaces (meatuses)
    • Lamina papyracea: orbital medial wall

  • Sphenoid Bone:

    • Located centrally at skull base, connects anterior (frontal, ethmoid) and posterior (occipital, temporal)
    • Parts:
      • Body: contains sphenoidal sinuses, sulcus chiasmaticus, sella turcica (pituitary fossa), dorsum sellae, posterior clinoid processes
      • Lesser wings: form anterior boundary of optic canal, articulate with frontal bone
      • Greater wings: contribute to skull base, form lateral orbital wall, contain foramina rotundum, ovale, spinosum
      • Pterygoid processes: medial and lateral plates, form pterygoid fossa, articulate with palatine and pyramidal process, attachment for pterygomandibular ligament
      • Pterygoid notch and pterygoid hamulus
      • Optic canal: passes CN II and ophthalmic artery
      • Foramina: rotundum (maxillary nerve), ovale (mandibular nerve), spinosum (middle meningeal artery)
      • Surfaces: cerebral (concave), lateral (convex), orbital (smooth, with zygomatic crest)
      • Pterygoid processes’ joints: connections with palatine and nasal bones, articulation with muscles

  • Temporal Bone:

    • Located on skull sides, protects hearing and balance organs, contains middle and inner ear
    • Parts:
      • Squama: large, convex, attachment for temporalis muscle, contains parietal notch, groove for posterior deep temporal artery
      • Zygomatic process: articulates with zygomatic bone; forms part of the zygomatic arch
      • Mastoid portion: contains mastoid cells, attachment for SCM; features mastoid foramen, sigmoid sulcus
      • Petrous portion: pyramid-shaped, houses inner ear, contains internal acoustic meatus, carotid canal, stylomastoid foramen, jugular foramen, and styloid process
      • Tympanic part: forms external acoustic meatus, separates ear canal from middle ear
      • Structures: digastric fossa, mastoid process, styloid process, jugular fossa
      • Bone ossifies endochondrally

3. High-Yield Facts

  • Ethmoid:
    • Cribriform plate: transmits olfactory nerves (CN I)
    • Perpendicular plate: forms nasal septum
    • Ethmoidal labyrinths: contain ethmoidal cells (air sinuses)
    • Crista Galli: attachment point for falx cerebri
  • Sphenoid:
    • Body contains sphenoidal sinuses, sella turcica
    • Foramina: rotundum (maxillary nerve), ovale (mandibular nerve), spinosum (middle meningeal artery)
    • Pterygoid processes articulate with palatine, pterygoid muscles
    • Optic canal: CN II and ophthalmic artery pass
  • Temporal:
    • Squamous part: origin of temporalis muscle
    • Petrous part: houses cochlea and vestibular apparatus
    • Mastoid process: attachment for SCM, contains pneumatic cells
    • Stylomastoid foramen: facial nerve exits
    • Jugular foramen: passage for CN IX, X, XI and internal jugular vein

4. Summary Table

ConceptKey PointsNotes
Ethmoid boneUnpaired, anterior skull base; Cribriform plate (olfactory nerves), perpendicular plate (nasal septum); Lateral masses with ethmoidal cells and conchaeForms part of nasal septum, medial orbit wall
Sphenoid boneCentral skull base; contains sella turcica, sphenoidal sinuses, wings, pterygoid processesarticulates with all cranial bones; features foramina rotundum, ovale, spinosum
Temporal boneLateral skull sides; protects ear structures, contains auditory and vestibular organsParts: squama, mastoid, petrous, tympanic; features styloid process, jugular foramen

5. Mini-Schema

Cranial Bones
 ├─ Ethmoid Bone
 │   ├─ Cribriform plate: olfactory foramina
 │   ├─ Perpendicular plate: nasal septum
 │   └─ Lateral masses: ethmoidal cells, conchae
 ├─ Sphenoid Bone
 │   ├─ Body: sphenoidal sinuses, sella turcica
 │   ├─ Lesser wings: optic canal boundary
 │   ├─ Greater wings: foramen rotundum, oval, spinosum
 │   └─ Pterygoid processes: muscles, pterygoid notch
 └─ Temporal Bone
     ├─ Squama: temporal muscle origin
     ├─ Mastoid process: pneumatic, SCM attachment
     ├─ Petrous part: houses inner ear, carotid canal
     └─ Styloid process: muscle and ligament attachment

6. Rapid-Review Bullets

  • Ethmoid bone forms nasal septum and medial orbit wall
  • Cribriform plate transmits olfactory nerves (CN I)
  • Sphenoid bone centrally located; contains sella turcica
  • Foramina: rotundum, ovale, spinosum (cranial nerves and vessels)
  • Pterygoid processes: attach muscles, form pterygoid fossa
  • Temporal bone: protects hearing/balance organs
  • Squamous portion: attachment for temporalis muscle
  • Mastoid process: pneumatic, attachment point for SCM
  • Petrous part: contains cochlea, semicircular canals
  • Styloid process: attachment for muscles and ligaments
  • Jugular foramen: passage of CN IX, X, XI and internal jugular vein