Overview
This lecture covers the foramina of the skull base, focusing on their anatomical locations, how to identify them on CT scans, and the structures—especially cranial nerves and vessels—that pass through each.
Classification and Naming of Skull Openings
- Holes in the skull are classified as foramina (small holes), canals (longer, tube-like), or fissures (irregular spaces between bones).
- Foramina allow passage of nerves and blood vessels; canals are longer openings, fissures are irregular and often formed by adjacent bones.
Cranial Nerves and Associated Foramina
- Cranial Nerve I (Olfactory) passes through multiple small olfactory foramina in the cribriform plate of the ethmoid bone.
- Cranial Nerve II (Optic) passes through the optic canal in the sphenoid bone.
- Cranial Nerves III (Oculomotor), IV (Trochlear), VI (Abducent), and V1 (Ophthalmic division of Trigeminal) pass through the superior orbital fissure.
- Cranial Nerve V2 (Maxillary division of Trigeminal) passes through the foramen rotundum.
- Cranial Nerve V3 (Mandibular division of Trigeminal) passes through the foramen ovale.
- Cranial Nerve VII (Facial) and VIII (Vestibulocochlear) enter via the internal acoustic canal; facial nerve exits at the stylomastoid foramen.
- Cranial Nerves IX (Glossopharyngeal), X (Vagus), and XI (Spinal Accessory) exit via the jugular foramen; spinal accessory nerve also enters through the foramen magnum before exiting.
- Cranial Nerve XII (Hypoglossal) exits via the hypoglossal canal in the occipital condyle.
Foramina Without Cranial Nerves
- Foramen spinosum transmits the middle meningeal artery.
- Carotid canal transmits the internal carotid artery and runs horizontally through the petrous temporal bone.
- Foramen lacerum is filled with cartilage; carotid artery passes superior to it.
- Foramen magnum allows passage of the medulla oblongata, spinal cord, vertebral arteries, cerebellar tonsils, and the spinal accessory nerve.
Key Terms & Definitions
- Foramen — A small opening in bone for nerves or blood vessels.
- Canal — A longer, tube-like passage within bone.
- Fissure — An irregular gap between bones.
- Cribriform plate — Part of the ethmoid bone with olfactory foramina.
- Sphenoid bone — Complex bone containing several key foramina.
- Jugular foramen — Opening for cranial nerves IX, X, XI and the internal jugular vein.
Action Items / Next Steps
- Practice identifying each foramen and associated structures on CT scans.
- Review cranial nerve pathways and what passes through each skull base opening.