1. Overview

This unit covers the anatomy of three cranial bones: Ethmoid, Sphenoid, and Temporal bones. These bones are located at the base and sides of the skull, contributing to the cranial structure and housing crucial foramina and processes for nerves, vessels, and muscle attachments. They form parts of the neurocranium and viscerocranium, impacting the orbit, nasal cavity, and temporal fossa. Key ideas include the localization, anatomical parts, articulations, foramina, and functional relevance of each bone.

2. Core Concepts & Key Elements

Ethmoid Bone

Unpaired, anterior base of skull, part of neurocranium and viscerocranium.
Articulates anteriorly with frontal and nasal bones; posteriorly with sphenoid bone.
Forms part of the base of skull, medial orbital walls, superior nasal septum, and superior lateral nasal cavity wall.
Anatomical parts:
- A. Cribriform plate: horizontal, roof of nasal cavity, has olfactory foramina, superior surface divided by crista galli (olfactory nerve CN I).
- B. Perpendicular plate: vertical, part of nasal septum, divided by cribriform plate into crista galli (falx cerebri attachment) and perpendicular plate (medial nasal walls, grooves).
- C. Lateral masses (ethmoid labyrinth): contain ethmoidal cells (3-18 air spaces forming ethmoidal sinuses), part of medial orbit and lateral nasal walls.
  - Scroll-shaped conchae: superior and middle nasal conchae fixed superiorly to lateral masses; meatuses (superior and middle) below conchae enhance air warming and humidification.
  - Lamina papyracea: smooth plate forming medial orbital wall.

Sphenoid Bone

Located medially at skull base; articulates with all neurocranial bones: frontal, ethmoid (anterior), occipital, temporal (posterior), parietal (superior/lateral).
Anatomical parts:
- I. Body: cube-shaped with two sphenoidal air sinuses separated by a septum.
  - Superior surface: jugum sphenoidale (with ethmoidal spine anteriorly, articulates with ethmoid cribriform plate & crista galli), chiasmatic groove (optic chiasm), optic foramina, tuberculum sellae, sella turcica (pituitary fossa), middle clinoid processes, dorsum sellae (posterior sella), posterior clinoid processes, clivus joint.
  - Anterior surface: forms supero-posterior nasal vault; anterior sphenoidal crest (articulates with ethmoid perpendicular plate), openings for sphenoidal sinuses.
  - Inferior surface: inferior sphenoidal crest, sphenoidal rostrum, vaginal process, articulations.
  - Posterior surface: joins basilar part of occipital bone (sphenobasilar synchondrosis).
  - Lateral surfaces lead to lesser and greater wings and superior orbital fissure.
- II.a. Lesser wings: two triangular plates with upper (jugum continuation) and lower roots forming the optic canal for optic nerve & ophthalmic artery, ends as anterior clinoid processes.
- II.a. Greater wings: posteroinferior, anterolateral cranial floor; features sphenoidal spine, foramen lacerum, 3 foramina on cerebral surface (rotundum, ovale, spinosum), temporalis muscle attachment on temporal surface; infratemporal surface forms infratemporal fossa.
- II.b. Pterygoid processes: arise from inferior sphenoid body via medial (body) and lateral (greater wing) roots join to form pterygoid canal; lateral and medial pterygoid plates separated by pterygoid notch, forming pterygoid fossa.
  - Articulations: anterior with palatine bone perpendicular plate and pyramidal process; medial surface forms posterior nasal cavity lateral wall (choanae); medial pterygoid hamulus anchors pterygomandibular ligament.
  - Muscle attachments: medial plate (medial pterygoid muscle), lateral plate (lateral pterygoid muscle).

Temporal Bone

Located at skull sides and base, lateral to temporal lobes.
Protects auditory (cochlea), vestibular systems (semicircular canals), and neurovascular structures (internal carotid artery, facial nerve, sigmoid sinus).
Contains glenoid fossa for TMJ (mandibular condyle).
Three parts:
- I. Squama: largest, thin, smooth, anterior-superior; attachment for temporalis muscle; features temporal line (boundary with mastoid), parietal notch.
  - Zygomatic process (anterior faces zygomatic bone; posterior root includes external acoustic meatus, anterior root forms mandibular fossa and articular tubercle).
  - Inferior outer surface features articular eminence, mandibular fossa, petrotympanic fissure (chorda tympani exit), tympanomastoid fissure (auricular branch vagus).
  - Medial surface concave with grooves for middle meningeal vessels.
- II. Mastoid portion: posterior-inferior to external acoustic meatus, continues as mastoid process with mastoid cells; muscular attachments (sternocleidomastoid, splenius capitis, digastric, longissimus capitis); digastric fossa for digastric muscle posterior belly; medial surface has sigmoid sulcus (sigmoid sinus groove) and mastoid foramen (emissary veins).
- III. Petrous portion: pyramidal between sphenoid & occipital bones; houses auditory & vestibular apparatus; four surfaces:
  - a. Superior-anterior: middle cranial fossa part, tegmen tympani (separates tympanic & cranial cavities), arcuate eminence (semicircular canals), hiatuses for greater & lesser petrosal nerves, impression of semilunar ganglion (trigeminal).
  - b. Superior-posterior: posterior cranial fossa, internal acoustic meatus, subarcuate fossa.
  - c. Inferior-anterior: vaginal process (continuous with tympanic plate, surrounds styloid root).
  - d. Inferior-posterior: styloid process (muscle/ligament attachments), stylomastoid foramen (facial nerve motor fibers exit), carotid canal (internal carotid artery entrance), jugular fossa & surface (internal jugular vein origin and occipital articulation).

3. High-Yield Facts

Ethmoid bone: located anterior base of skull, unpaired, part of neurocranium & viscerocranium.
Cribriform plate allows olfactory nerve passage; crista galli anchors falx cerebri.
Lateral masses contain ethmoidal sinuses; lamina papyracea forms medial orbit wall.
Sphenoid bone articulates with all neurocranial bones; body houses sphenoidal sinuses.
Optic canal (between lesser wings) transmits optic nerve and ophthalmic artery.
Foramina on greater wings: rotundum (V2), ovale (V3), spinosum (middle meningeal vessels, recurrent mandibular branch).
Pterygoid processes provide muscle attachments and articulate with palatine bone.
Temporal bone's squama forms part of temporal fossa; mastoid portion contains mastoid cells; petrous portion houses inner ear & nerve foramina.
Carotid canal transmits internal carotid artery; stylomastoid foramen transmits facial nerve motor fibers.
Glenoid fossa on temporal bone forms TMJ with mandibular condyle.

4. Summary Table

Concept	Key Points	Notes
Ethmoid Bone	Unpaired, anterior skull base; cribriform plate, perpendicular plate, lateral masses	Olfactory nerve (CN I) passage; ethmoidal sinuses
Sphenoid Bone	Medial skull base; body with sinuses, lesser & greater wings, pterygoid processes	Foramina for trigeminal nerve branches; optic canal
Temporal Bone	Sides and base of skull; squama, mastoid, petrous portions	Houses auditory structures; carotid canal; facial nerve

5. Mini-Schema (ASCII)

Cranial and Facial Bones I
 ├─ Ethmoid Bone
 │    ├─ Cribriform Plate
 │    ├─ Perpendicular Plate
 │    └─ Lateral Masses (Ethmoidal Labyrinth)
 ├─ Sphenoid Bone
 │    ├─ Body
 │    │    ├─ Sphenoidal Sinuses
 │    │    ├─ Jugum sphenoidale
 │    │    ├─ Chiasmatic groove
 │    │    ├─ Sella turcica
 │    │    └─ Clivus articulation
 │    ├─ Lesser Wings (optic canal)
 │    ├─ Greater Wings (foramina, muscle attachments)
 │    └─ Pterygoid Processes (medial & lateral plates, muscles)
 └─ Temporal Bone
      ├─ Squama (temporalis muscle, TMJ fossa)
      ├─ Mastoid Portion (mastoid cells, muscular attachments)
      └─ Petrous Portion (inner ear, foramina, carotid canal)

6. Rapid-Review Bullets

Ethmoid: unpaired, anterior skull base, olfactory nerve via cribriform plate.
Crista galli anchors falx cerebri.
Perpendicular plate forms nasal septum medial wall.
Lateral masses contain ethmoidal air cells (sinuses), form medial orbit wall.
Sphenoid: articulates with all neurocranial bones.
Body contains sphenoidal sinuses, separated by septum.
Optic canal between lesser wings transmits optic nerve & artery.
Greater wing foramina: rotundum, ovale, spinosum for trigeminal nerve branches.
Pterygoid processes form pterygoid canal and muscles attachments.
Temporal squama: largest part, attaches temporalis muscle, contains TMJ mandibular fossa.
Mastoid portion: mastoid process, mastoid cells, muscle attachments.
Petrous portion: pyramid-shaped; houses auditory/vestibular apparatus.
Carotid canal transmits internal carotid artery.
Stylomastoid foramen transmits facial nerve motor fibers.
Glenoid fossa articulates with mandibular condyle (TMJ).

Cranial Base and Side Bones Revision Sheet

1. 📌 Essentials

Ethmoid: Unpaired, anterior skull base, contributes to nasal septum, orbit, and cranial cavity.
Sphenoid: Central wedge-shaped bone, articulates with all neurocranial bones, contains sphenoidal sinuses.
Temporal: Paired, lateral skull sides, contains structures for hearing, balance, and neurovascular passage.
Foramina: Significant openings (e.g., optic canal, rotundum, ovale, spinosum, carotid canal, stylomastoid foramen).
Crista Galli: Attaches falx cerebri; extends from ethmoid cribriform plate.
Sella Turcica: Houses pituitary gland, present in sphenoid body.
Petrous Part: Houses inner ear structures within temporal bones.
Jugum Sphenoidale: Articulates ethmoid with sphenoid.
Pterygoid Process: Muscular attachment points; form part of the nasal cavity lateral wall.
Temporal Fossa: Formed partly by temporal bone squama, site of temporalis muscle attachment.
Key Relationships: These bones form part of neurocranium, orbit, nasal cavity, middle ear, and contribute to skull base stability.

2. 🧩 Key Structures & Components

Ethmoid Bone

Cribriform Plate: Allows passage of olfactory nerves (CN I).
Crista Galli: Bony projection anchoring falx cerebri.
Perpendicular Plate: Forms superior nasal septum.
Lateral Masses (Labyrinths): Contain ethmoidal air cells, ethmoidal sinuses.
Conchae (Superior & Middle): Scroll-shaped processes on lateral masses.
Lamina Papyracea: Thin orbital medial wall.

Sphenoid Bone

Body: Contains sphenoidal sinuses, sella turcica.
Lesser Wings: Form optic canal, anterior cranial fossa boundary.
Greater Wings: Form middle cranial fossa, contain foramen rotundum, ovale, spinosum.
Pterygoid Processes: Extending downward, sites for muscle attachments, form pterygoid fossa.
Optic Canal: Transmits optic nerve and ophthalmic artery.
Foramina of Cranial Nerve V (V2, V3): Rotundum, ovale, spinosum.

Temporal Bone

Squama: Flat, contributes to temporal fossa; attaches temporalis muscle.
Zygomatic Process: Connects to zygomatic bone.
Stylomastoid Foramen: Facial nerve exit.
Mastoid Process: Contains mastoid air cells.
Mastoid Part: Mastoid air cells, muscular attachments.
Petrous Part: Houses cochlea, vestibular apparatus, internal acoustic meatus, carotid canal.

3. 🔬 Functions, Mechanisms & Relationships

Olfactory Nerve Passage: Cribriform plate of ethmoid allows CN I; crista galli stabilizes meninges.
Nasal Septum Formation: Perpendicular plate (ethmoid) + vomer (not in this scope).
Orbital Walls: Ethmoid lamina papyracea forms medial wall; frontal, maxilla, sphenoid form other walls.
Sphenoid as Central Base: Connects cranial vault to facial bones; contains multiple foramina for cranial nerves and vessels.
Foramina of Sphenoid & Temporal: Transmit nerves, arteries, veins; critical for neurovascular communication.
Muscle Attachments: Pterygoid plates (lateral & medial) anchor muscles of mastication.
Temporal Bone: Protects structures for hearing, balance; provides anchorage for muscles (temporalis via squama).

Skull Base & Side Bones
 ├─ Ethmoid
 │    ├─ Cribriform Plate → Olfactory Nerve (CN I)
 │    ├─ Crista Galli → Falx cerebri attachment
 │    ├─ Perpendicular Plate → Nasal septum
 │    └─ Ethmoidal Labyrinths → Sinuses
 ├─ Sphenoid
 │    ├─ Body → Sphenoidal sinuses, sella turcica
 │    ├─ Lesser Wings → Optic canal
 │    ├─ Greater Wings → Foramina rotundum, ovale, spinosum
 │    └─ Pterygoid Process → Muscle attachment, lateral nasal wall
 └─ Temporal
      ├─ Squama → Temporal fossa, temporalis attachment
      ├─ Mastoid Process → Mastoid air cells
      └─ Petrous Part → Inner ear, carotid canal

4. 📊 Comparative Table

Feature	Ethmoid	Sphenoid	Temporal
Location	Anterior skull base, medial wall	Central skull base, wedge-shaped	Sides and base of skull
Main contribution	Nasal septum, orbit, anterior cranial	Cranial cavity floor, orbit, sella turcica	Houses sensory organs (ear), mastoid air cells
Articulations	Frontal, maxilla, ethmoid, sphenoid	All cranial bones, vomer (not shown)	Parietal, occipital, sphenoid, zygomatic
Major foramina	Cribriform (CN I), anterior/posterior ethmoidal	Optic canal, rotundum, ovale, spinosum, foramen lacerum	Stylomastoid, jugular, carotid canal
Sinuses	Ethmoidal air cells	Sphenoidal in body	None

5. 🗂️ Hierarchical Diagram (ASCII)

Cranial Bones & Structures
 ├─ Ethmoid
 │    ├─ Cribriform Plate
 │    ├─ Perpendicular Plate
 │    └─ Ethmoidal Labyrinths (Sinuses)
 ├─ Sphenoid
 │    ├─ Body
 │    ├─ Lesser Wings
 │    ├─ Greater Wings
 │    └─ Pterygoid Processes
 └─ Temporal
      ├─ Squamous Part
      ├─ Mastoid Part
      └─ Petrous Part

6. ⚠️ High-Yield Pitfalls & Confusions

Confusing Ethmoid and Vomer: Vomer is a separate nasal (not in scope); ethmoid's perpendicular plate forms part of nasal septum.
Mistaking sphenoid wings: Lesser wings (optic canal) vs. greater wings (middle cranial fossa foramina).
Foramina confusion: Ovale (V3), rotundum (V2), spinosum (middle meningeal vessels); know their nerve contents.
Petrous part’s importance: Houses inner ear and internal acoustic meatus (facial nerve entry).
Temporal squama: distinguishes temporal from parietal bones.
Mastoid air cells: often confused with sinus cavities, but mediate middle ear functions.

7. ✅ Final Exam Checklist

Know the unpaired nature and locations of ethmoid.
Identify the cribriform plate and crista galli’s roles.
Recognize key parts of sphenoid: body, wings, pterygoid processes.
Understand the foramina of the sphenoid and their contents.
Know the features and attachments of temporal bone components.
Memorize the bones' articulations with neighboring bones.
Be familiar with cranial nerve passages through foramina.
Understand the relationship between the pterygoid plates and muscles.
Recognize the importance of the petrous part in housing ear structures.
Be aware of the pathways for arteries such as the internal carotid.
Differentiate between the skull base bones and their functions.
Know the major landmarks for clinical relevance: optic canal, stylomastoid foramen, carotid canal.
Remember the significance of foramina for neurovascular transmission.
Understand the bones' roles in forming the orbit, nasal cavity, and cranial cavity.

End of Revision Sheet

Cranial Bone Structures and Functional Insights

Crée tes propres fiches en 30 secondes

1. Overview

2. Core Concepts & Key Elements

Ethmoid Bone

Sphenoid Bone

Temporal Bone

3. High-Yield Facts

4. Summary Table

5. Mini-Schema (ASCII)

6. Rapid-Review Bullets

Cranial Bone Structures and Functional Insights

Crée tes propres fiches en 30 secondes

Cranial Base and Side Bones Revision Sheet

1. 📌 Essentials

2. 🧩 Key Structures & Components

Ethmoid Bone

Sphenoid Bone

Temporal Bone

3. 🔬 Functions, Mechanisms & Relationships

4. 📊 Comparative Table

5. 🗂️ Hierarchical Diagram (ASCII)

6. ⚠️ High-Yield Pitfalls & Confusions

7. ✅ Final Exam Checklist

Cranial Bone Structures and Functional Insights

Cranial Bone Structures and Functional Insights

Which nerve passes through the stylomastoid foramen?

Cranial Bone Structures and Functional Insights

Progression globale

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Introduction au système

Les différents types

Structure axiale

Structure appendiculaire

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