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External Skull Base

Basis cranii externa

For medical students2 min readUpdated 2026-10-10

The external skull base (basis cranii externa) is the complex inferior surface of the neurocranium, visualized from the basal view. This region serves as a critical exit and entry zone for cranial nerves and major blood vessels, connecting the cranial cavity with surrounding topographic spaces.

PartsHard palate (anteriorly) and the proper external skull base (posteriorly)
ConnectionConnects the internal skull base with the facial skeleton
Lateral BoundaryTransitions freely into the infratemporal fossa
ForaminaContains apertures of all major temporal bone canals

General Characteristics and Boundaries

Anatomically, the external skull base is viewed from the inferior, or basal, aspect. For study purposes, it is conventionally divided into two unequal parts:

The spatial boundaries of the proper external skull base are delineated by surrounding bony structures. Anteriorly (excluding the hard palate), this region is bounded by the choanae (Choanae), also known as the posterior nasal apertures (Aperturae nasales posteriores). Laterally, the bony relief has no rigid boundary and continues freely into the infratemporal fossa (Fossa infratemporalis). Posteriorly and laterally, the base is bounded by complex anatomical formations of the temporal and occipital bones.

Connections with the Internal Skull Base

The external skull base is intimately connected with the internal skull base (its anterior, middle, and posterior cranial fossae) via numerous large foramina and canals. The basal surface displays the following key structures:

Special attention should be given to the foramina located in the greater wing of the sphenoid bone. These include the foramen ovale (Foramen ovale) and foramen spinosum (Foramen spinosum). Additionally, on a macerated (dried) skull, the petro-occipital fissure (Fissura petrooccipitalis) is clearly visible, though in living individuals this space is filled with cartilage and functions as a synchondrosis. Similarly, the foramen lacerum (Foramen lacerum) appears on a dry skull as a gaping fissure located at the posteromedial margin of the greater wing of the sphenoid bone and the apex of the temporal bone pyramid.

Temporal Bone Canals and Additional Communications

The external skull base acts as a topographic "receiver" for the exit and entry apertures of all major temporal bone canals. Opening externally are:

  1. Carotid canal (Canalis caroticus) — its external aperture.
  2. Facial canal (Canalis nervi facialis), whose exit aperture is the stylomastoid foramen (Foramen stylomastoideum).
  3. Musculotubal canal and its subdivisions.
  4. Numerous tympanic canaliculi and other minor pathways.

In addition to vertical connections with the cranial cavity, the basis cranii externa has crucial horizontal communications with the spaces of the facial skeleton:

Mnemonic

To avoid confusing the sphenoid bone foramina, remember: the greater wing contains the Oval foramen (Foramen ovale) and Spinous foramen (Foramen spinosum). Meanwhile, the stylomastoid foramen is always associated with the facial nerve—it marks the terminal point of its canal.

Frequently asked questions

What are the boundaries of the proper external skull base?
  • Anteriorly: bounded by the choanae / posterior nasal apertures (Choanae / Aperturae nasales posteriores).
  • Laterally: transitions freely into the infratemporal fossa (Fossa infratemporalis).
  • Posteriorly and laterally: bounded by anatomical structures of the temporal and occipital bones.
What passes through the jugular foramen?
  • Glossopharyngeal nerve (IX).
  • Vagus nerve (X).
  • Accessory nerve (XI).
  • Veins.
What passes through the foramen ovale?
  • Mandibular nerve (V3).
  • Venous plexus of the foramen ovale (plexus venosus foraminis ovalis).
What passes through the foramen spinosum?
  • Middle meningeal artery (a. meningea media).
  • Vein.
  • Meningeal branch of the mandibular nerve (r. meningeus spinosus).
What passes through the foramen magnum?
  • Medulla oblongata.
  • Arteries.
  • Nerves.
  • Spinal roots of the accessory nerve (radices spinales, pars spinalis).
What passes through the condylar canal?
  • Condylar emissary vein (v. emissaria condylaris).
What passes through the hypoglossal canal?
  • Hypoglossal nerve (CN XII).
  • Venous plexus of the hypoglossal canal (plexus venosus canalis nervi hypoglossi).
What are the conventional parts of the external skull base?

It is divided into an anterior smaller part formed by the hard palate and a posterior larger part representing the proper external skull base.

What bounds the external skull base anteriorly?

Anteriorly (except for the hard palate region), the boundary is formed by the choanae, or posterior nasal apertures (Choanae / Aperturae nasales posteriores).

How does the petro-occipital fissure appear in a living person?

Unlike a macerated (dry) skull where it appears as a bony fissure, in a living person this area is filled with cartilage, forming a synchondrosis.

Through which structure does the external skull base communicate with the pterygopalatine fossa?

Communication with the pterygopalatine fossa is established via the pterygoid canal (Canalis pterygoideus).

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