1. Overview

This unit covers three essential cranial bones: ethmoid, sphenoid, and temporal bones. These bones are located at the base and sides of the skull and are critical for forming the skull base, orbit walls, nasal cavity structures, and accommodating neurovascular structures. The chapter follows the anatomical localization, parts, connections, and clinical relevance of each bone sequentially: Ethmoid bone (anterior base, orbit and nasal cavity), Sphenoid bone (central base, multiple processes, sinus cavities), and Temporal bone (side/base of skull with auditory and vestibular functions). Key insights include identification of anatomical landmarks, foramina for nerves and vessels, muscle attachment sites, and articulations with neighboring bones.

2. Core Concepts & Key Elements

Ethmoid Bone

• Localization: Unpaired, anterior skull base, part of neurocranium and viscerocranium
• Articulations: Anterior with frontal and nasal bones; posterior with sphenoid bone
• Functions: Contributes to anterior skull base, medial orbit wall, superior nasal septum, lateral nasal cavity wall
• Anatomical parts:
  • Cribriform plate: horizontal, roof of nasal cavity, olfactory foramina for CN I, divided by crista galli
  • Perpendicular plate: vertical, forms superior nasal septum, crista galli serves as falx cerebri attachment
  • Lateral masses (ethmoidal labyrinth): bilateral, medial orbit wall, lateral nasal cavity wall, contains ethmoidal air cells (sinuses)
  • Ethmoidal conchae: superior and middle nasal conchae, increase mucous membrane surface
  • Meatuses: superior and middle below respective conchae for airflow
  • Lamina papyracea: thin plate forming most of medial orbital wall

Sphenoid Bone

• Localization: Medial skull base, articulates with all neurocranium bones (frontal, ethmoid, occipital, temporal, parietal)
• Anatomical parts:
  • Body: cube-shaped, contains sphenoidal air sinuses separated by septum
  • Superior surface features:
    • Jugum sphenoidale (flat anterior surface) articulates with ethmoid cribriform plate and crista galli
    • Chiasmatic groove (transverse, optic chiasm location), ends laterally at optic foramen
    • Tuberculum sellae, sella turcica (pituitary fossa), dorsum sellae and clivus
  • Anterior surface: posterior vault of nasal cavity, anterior sphenoidal crest articulates with ethmoid perpendicular plate
  • Inferior surface: posterior nasal vault, inferior sphenoidal crest, sphenoidal rostrum, vaginal processes
  • Posterior surface: fuses with occipital clivus via spheno-basilar synchondrosis
  • Lateral surfaces: lesser wings (superior-anterior), greater wings (inferior-posterior) with superior orbital fissure between
• Lesser wings: Two triangular plates from body, form optic canal boundaries, anterior clinoid processes at posterior corners
• Greater wings: Form anterolateral cranial floor and orbit lateral wall, features foramen rotundum, ovale, spinosum (neurovascular foramina) and spines for ligament attachment
• Pterygoid processes: descend from body, composed of medial and lateral plates separated by pterygoid notch, form pterygoid fossa for muscle attachments, end medially with pterygoid hamulus (ligament attachment)
• Articulations: with palatine bone (perpendicular and pyramidal processes), lateral wall of nasal cavities

Temporal Bone

• Localization: Lateral skull base and sides, lateral to temporal lobes
• Functions: Houses hearing (cochlea) and balance organs (semicircular canals), protects neurovascular structures (internal carotid artery, facial nerve, sigmoid sinus)
• Parts:
  • Squamous part: large, thin, convex, upper/anterior temporal bone; attachment for temporalis muscle
  • Mastoid portion: posterior to external acoustic meatus, contains mastoid cells, mastoid process (muscle attachments: sternocleidomastoid, splenius capitis, digastric, longissimus capitis)
  • Petrous portion: pyramid-shaped between sphenoid and occipital, encloses auditory and vestibular apparatus; four surfaces (superior-anterior, superior-posterior, inferior-anterior, inferior-posterior) with multiple foramina and landmarks
• Squamous part details:
  • Zygomatic process with anterior and posterior roots, mandibular fossa (glenoid fossa), articular tubercle, temporal line, grooves for deep temporal artery
  • Inferior surface landmarks: articular eminence, mandibular fossa, petrotympanic fissure (chorda tympani), tympanomastoid fissure (auricular branch of vagus)
• Mastoid portion details:
  • Mastoid foramen for emissary veins, sigmoid sulcus for sigmoid sinus, digastric fossa
• Petrous portion details:
  • Tegmen tympani (roof of tympanic cavity), arcuate eminence (semicircular canals), hiatus for greater/lesser petrosal nerves
  • Internal acoustic meatus (cranial posterior fossa), subarcuate fossa
  • Styloid process attachments (stylohyoid, stylopharynx, styloglossus muscles, stylomandibular ligament)
  • Stylomastoid foramen (motor fibers of facial nerve exit)
  • Carotid canal (entering point of internal carotid artery)
  • Jugular fossa and surface (articulation with occipital bone, origin of internal jugular vein)

3. High-Yield Facts

• Ethmoid bone: cribriform plate allows olfactory nerve passage (CN I); crista galli anchors falx cerebri
• Sphenoid bone:
  • Sella turcica houses pituitary gland
  • Optic canal length approx. 5 mm
  • Foramina: rotundum (maxillary nerve, V2), ovale (mandibular nerve, V3), spinosum (middle meningeal vessels)
  • Sphenomandibular ligament attaches to sphenoid spine
  • Pterygoid hamulus is a ligament attachment point
• Temporal bone:
  • Mastoid cells vary in size, important in ear infections risk
  • Internal acoustic meatus transmits facial nerve and vestibulocochlear nerve
  • Stylomastoid foramen is facial nerve exit
  • Carotid canal transmits internal carotid artery into brain
  • Sigmoid sinus drains to internal jugular vein through jugular foramen
• Clinical relevance: ethmoid bone fractures can damage olfactory nerves; sphenoid foramina are key for trigeminal nerve branches; temporal bone critical in hearing, balance, and facial nerve pathologies.

4. Summary Table

5. Mini-Schema (ASCII)

Cranial Bones I
 ├─ Ethmoid Bone
 │    ├─ Cribriform Plate (olfactory foramina, crista galli)
 │    ├─ Perpendicular Plate (nasal septum)
 │    └─ Lateral Masses (ethmoidal sinuses, conchae, lamina papyracea)
 ├─ Sphenoid Bone
 │    ├─ Body (sphenoid sinuses, sella turcica)
 │    ├─ Lesser Wings (optic canal, anterior clinoid processes)
 │    ├─ Greater Wings (foramina rotundum, ovale, spinosum, spines)
 │    └─ Pterygoid Processes (medial/lateral plates, pterygoid hamulus)
 └─ Temporal Bone
      ├─ Squama (temporalis muscle, mandibular fossa)
      ├─ Mastoid Portion (mastoid process, cells, muscle attachments)
      └─ Petrous Portion (auditory apparatus, internal acoustic meatus, carotid canal, stylomastoid foramen)

6. Rapid-Review Bullets

• Ethmoid bone is unpaired, forms anterior skull base, orbit medial wall, nasal septum
• Cribriform plate allows CN I passage; crista galli attaches falx cerebri
• Perpendicular plate forms superior nasal septum
• Ethmoidal labyrinth contains air cells (sinuses) and conchae
• Sphenoid bone articulates with all neurocranium bones
• Body contains sphenoidal sinuses; sella turcica lodges pituitary
• Optic canal is approx. 5 mm, between lesser wing roots
• Foramina rotundum, ovale, and spinosum transmit CN V branches and vessels
• Pterygoid processes have medial/lateral plates and pterygoid hamulus
• Temporal bone houses auditory/vestibular organs
• Squama is attachment for temporalis muscle, forms mandibular fossa
• Mastoid process attaches neck muscles, contains mastoid air cells
• Petrous portion encloses internal acoustic meatus and carotid canal
• Stylomastoid foramen transmits facial nerve motor fibers
• Carotid canal transmits internal carotid artery into skull
• Sigmoid sinus drains to internal jugular vein via jugular foramen
• Petrotympanic fissure allows chorda tympani nerve passage
• Tegmen tympani separates tympanic cavity from cranial cavity
• Temporal bone fractures can affect hearing and facial nerve function