M01.02.017 Cranial Foramina

The cranial foramina are vital openings in the skull base, permitting the passage of nerves, blood vessels, and other structures between various regions. Understanding their anatomy is essential for mastering neuroanatomy and preparing for USMLE Step 1.

Classification of Cranial Foramina

The cranial foramina are classified based on the cranial fossa and the structures they transmit:

• Anterior Cranial Fossa
• Middle Cranial Fossa
• Posterior Cranial Fossa

Major Cranial Foramina and Their Contents

1. Anterior Cranial Fossa

Cribriform Foramina

• Location: Cribriform plate of the ethmoid bone
• Structures Transmitted:
  • Olfactory nerve (CN I)
  • Anterior ethmoidal nerves

2. Middle Cranial Fossa

Optic Canal

• Location: Sphenoid bone (body and lesser wing)
• Structures Transmitted:
  • Optic nerve (CN II)
  • Ophthalmic artery

Superior Orbital Fissure

• Location: Between the lesser and greater wings of the sphenoid
• Structures Transmitted:
  • Nerves:
    • Lacrimal nerve (branch of CN V1)
    • Frontal nerve (branch of CN V1)
    • Trochlear nerve (CN IV)
    • Superior and inferior divisions of the oculomotor nerve (CN III)
    • Nasociliary nerve (branch of CN V1)
    • Abducens nerve (CN VI)
  • Vessels:
    • Superior ophthalmic vein
    • Branch of the inferior ophthalmic vein

Foramen Rotundum

• Location: Greater wing of the sphenoid
• Structures Transmitted:
  • Maxillary branch of the trigeminal nerve (CN V2)

Foramen Ovale

• Location: Greater wing of the sphenoid
• Structures Transmitted:
  • Mandibular branch of the trigeminal nerve (CN V3)
  • Accessory meningeal artery

Foramen Spinosum

• Location: Lateral to the foramen ovale
• Structures Transmitted:
  • Middle meningeal artery and vein
  • Meningeal branch of CN V3

Foramen Lacerum

• Location: Junction of sphenoid, temporal, and occipital bones
• Structures Transmitted:
  • Filled with cartilage in life, permitting passage of minor vessels.

Internal Acoustic Meatus

• Location: Petrous part of the temporal bone
• Structures Transmitted:
  • Facial nerve (CN VII)
  • Vestibulocochlear nerve (CN VIII)
  • Vestibular ganglion
  • Labyrinthine artery

3. Posterior Cranial Fossa

Jugular Foramen

• Location: Anteriorly by the petrous temporal bone, posteriorly by the occipital bone
• Structures Transmitted:
  • Anterior compartment: Inferior petrosal sinus
  • Middle compartment:
    • Glossopharyngeal nerve (CN IX)
    • Vagus nerve (CN X)
    • Accessory nerve (CN XI)
  • Posterior compartment: Sigmoid sinus, meningeal branches of occipital and ascending pharyngeal arteries

Hypoglossal Canal

• Location: Occipital bone
• Structures Transmitted:
  • The hypoglossal nerve (CN XII)

Foramen Magnum

• Location: Occipital bone
• Structures Transmitted:
  • Medulla and meninges
  • Vertebral arteries
  • Spinal division of the accessory nerve (CN XI)
  • Anterior and posterior spinal arteries
  • Dural veins

Summary Table of Cranial Foramina

Key Points to Remember

1. Mnemonic for Superior Orbital Fissure: Lazy French Tarts Sit Naked In Anticipation (Lacrimal, Frontal, Trochlear, Superior division of oculomotor, Nasociliary, Inferior division of oculomotor, Abducens).
2. The Foramen Magnum is the largest and serves as the gateway for brainstem continuation and vertebral arteries.
3. Understanding the relationships of cranial foramina to their contents aids in diagnosing cranial nerve pathologies.