Structure: three zones from front to back
The external skull base is easiest to learn in three zones, from front to back.
- Anterior zone: the hard palate and the alveolar arch of the upper teeth, ending behind at the posterior nasal spine and the choanae.
- Middle zone: from the back of the hard palate to the front of the foramen magnum. It includes the pterygoid processes, the infratemporal surface of the greater wing of the sphenoid, the petrous temporal bone, the mandibular fossa for the jaw joint, and the styloid and mastoid processes.
- Posterior zone: the occipital bone, with the foramen magnum, the occipital condyles, the hypoglossal canals and the nuchal lines.
Most of the named foramina lie in the middle zone, arranged in a rough line running back and laterally from the pterygoid plates.
Hard palate and palatine foramina
The hard palate forms the roof of the mouth and the floor of the nasal cavity. The palatine processes of the maxillae form its front two-thirds, and the horizontal plates of the palatine bones its back third.
| Opening | Position | Transmits |
|---|---|---|
| Incisive fossa | Midline, behind the central incisors | Nasopalatine nerves, terminal branches of the greater palatine and sphenopalatine arteries |
| Greater palatine foramen | Posterolateral corner of the hard palate, medial to the third molar | Greater palatine nerve and vessels, which run forward to supply the hard palate and palatal gum |
| Lesser palatine foramina | Just behind the greater, in the pyramidal process of the palatine bone | Lesser palatine nerves and vessels to the soft palate and tonsil |
The palatine nerves are branches of the maxillary nerve (V2) that descend from the pterygopalatine fossa through the greater palatine canal.
Pterygoid process and hamulus
The pterygoid processes project down from the junction of the body and greater wing of the sphenoid, just behind the maxilla. Each has a medial and a lateral plate, with the pterygoid fossa between them.
- Lateral pterygoid plate: its outer surface gives origin to the lower head of lateral pterygoid, and its inner surface to the deep head of medial pterygoid.
- Medial pterygoid plate: forms the side of the choana and ends below in the pterygoid hamulus.
- Pterygoid hamulus: a small hook. The tendon of tensor veli palatini turns medially round it into the soft palate, and the pterygomandibular raphe attaches to it, linking buccinator and the superior constrictor. It can be felt in the mouth behind the last upper molar.
Foramen ovale and neighbouring openings
The foramen ovale is an oval opening in the greater wing of the sphenoid, just behind and lateral to the root of the lateral pterygoid plate. It connects the middle cranial fossa with the infratemporal fossa.
- Foramen ovale: the mandibular nerve (V3), the lesser petrosal nerve (usually), the accessory meningeal artery and an emissary vein from the cavernous sinus to the pterygoid plexus. The otic ganglion lies just below it.
- Foramen spinosum: small and round, posterolateral to the ovale and in front of the spine of the sphenoid. It transmits the middle meningeal artery and vein and the meningeal branch of V3.
- Foramen lacerum: the ragged gap at the apex of the petrous temporal bone, filled with cartilage in life. The internal carotid artery crosses its upper end.
- Carotid canal: in the petrous temporal bone, for the internal carotid artery and its sympathetic plexus.
- Jugular foramen: between the temporal and occipital bones, for the internal jugular vein, inferior petrosal sinus and CN IX, X and XI.
- Stylomastoid foramen: between the styloid and mastoid processes, for the facial nerve.
- Hypoglossal canal: above each occipital condyle, for CN XII.
Looking up into the infratemporal fossa, the inferior orbital fissure is the slit at its roof between the greater wing of the sphenoid and the maxilla. It connects the orbit with the infratemporal and pterygopalatine fossae.
Clinical relevance
The foramina of the skull base are the routes for nerve blocks, surgical access and the spread of disease.
- Trigeminal neuralgia: a needle passed through the cheek into the foramen ovale reaches the trigeminal ganglion for percutaneous treatment.
- Greater palatine nerve block: local anaesthetic at the greater palatine foramen numbs the back of the hard palate for dental work.
- Cleft palate: the incisive foramen marks the junction of the primary and secondary palate, and clefts are classified relative to it.
- Base of skull fractures: bruising behind the ear (Battle's sign), periorbital bruising and CSF leaking from the ear or nose.
- Emissary veins: the vein through the foramen ovale links the pterygoid plexus with the cavernous sinus, a route for infection.
On the specimen
The specimen is the skull viewed from below, with the teeth at the top of the view and the foramen magnum at the bottom.
- Greater or lesser palatine foramen: the greater is the single larger hole at the back corner of the hard palate; the lesser foramina are smaller holes just behind it.
- Foramen ovale or spinosum: the ovale is large and oval, just behind the lateral pterygoid plate; the spinosum is the small round hole posterolateral to it.
- Foramen ovale or lacerum: the lacerum is more medial, jagged and irregular, at the tip of the petrous bone beside the basilar occiput.
- Hamulus or styloid process: the hamulus is a tiny hook at the bottom of the medial pterygoid plate beside the choana; the styloid is a long, slender spike further back beside the mastoid.
- Inferior orbital fissure: seen high in the infratemporal fossa, above the back of the maxilla.