General characteristics and boundaries
Anatomically, the external skull base is viewed from the inferior, or basal, norma. For ease of study, it is conventionally divided into two unequal parts:
- Anterior (smaller) part — represented by the structures of the hard palate.
- Posterior (larger) part — represents the external base proper.
The spatial boundaries of the external base proper are clearly outlined by surrounding bone structures. Anteriorly, this region (excluding the hard palate itself) is bounded by choanae (Choanae), also referred to 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). Posteriolaterally, the base is bounded by complex anatomical formations of the temporal and occipital bones.
Connections with the internal skull base
The external base is inextricably linked with the internal skull base (its anterior, middle, and posterior cranial fossae) via numerous large openings and canals. The following key structures are visualized on the basal surface:
- Foramen magnum (Foramen magnum) — the central and largest aperture of the base.
- Condylar canal (Canalis condylaris).
- Hypoglossal canal (Canalis nervi hypoglossi).
- Mastoid foramen (Foramen mastoideum).
- Jugular foramen (Foramen jugulare).
Particular attention should be paid to the openings located in the area of the greater wing of the sphenoid bone. These include the foramen ovale (Foramen ovale) and the foramen spinosum (Foramen spinosum). Also, on a macerated (dried) skull, the petro-occipital fissure (Fissura petrooccipitalis) is clearly visible, although it should be remembered that in a living person this space is filled with cartilaginous tissue and functions as a synchondrosis. Similarly, the foramen lacerum (Foramen lacerum) on a dry skull looks like a gaping fissure located at the posteromedial margin of the greater wing of the sphenoid bone and the apex of the pyramid of the temporal bone.
Canals of the temporal bone and additional communications
In fact, the external skull base serves as a topographic "receiver" for the exit and entrance apertures of all major canals of the temporal bone. Opening externally are:
- Carotid canal (Canalis caroticus) — its external aperture.
- Facial canal (Canalis nervi facialis), the exit opening of which is the stylomastoid foramen (Foramen stylomastoideum).
- Musculotubal canal and its secondary canals.
- Numerous tympanic canaliculi and other small 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:
- Free and wide connection with the infratemporal fossa (Fossa infratemporalis).
- Direct communication with the nasal cavity, which is achieved through the choanae.
- Communication with the pterygopalatine fossa (Fossa pterygopalatina), realized via the pterygoid canal (Canalis pterygoideus).
Briefly: answers to topical questions
What bounds the external skull base proper?
- Anteriorly, it is bounded by the choanae / posterior nasal apertures (Choanae / Aperturae nasales posteriores).
- Laterally, it continues freely into the infratemporal fossa (Fossa infratemporalis).
- Posteriolaterally, it is bounded by anatomical formations 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 (XII cranial nerve).
- Venous plexus of the hypoglossal canal (plexus venosus canalis nervi hypoglossi).
To avoid confusing the openings of the sphenoid bone, remember: the greater wing contains the Foramen ovale and Foramen spinosum. The stylomastoid foramen is always associated with the facial nerve — it is the endpoint of its canal.
Class and exam questions
Into what parts is the external cranial base conventionally divided?
It is divided into an anterior smaller part, formed by the hard palate, and a posterior larger part — the external base proper.
What is the anterior boundary of the external cranial base?
Anteriorly (except for the hard palate area), the boundary is formed by the choanae, or posterior nasal apertures (Choanae / Aperturae nasales posteriores).
What does the petro-occipital fissure look like in a living person?
Unlike the macerated (dry) skull, where it appears as a bony fissure, in a living person this area is filled with cartilage and functions as a synchondrosis.
Through which formation does the external cranial base communicate with the pterygopalatine fossa?
Connection with the pterygopalatine fossa is established via the pterygoid canal (Canalis pterygoideus).
What else to study on this topic
- Anatomy of the hard palate as the anterior part of the skull base
- Structure of the choanae and their connection with the nasal cavity
- Topography of the infratemporal fossa (Fossa infratemporalis)
- Canals of the temporal bone: musculotubal, facial, and tympanic
- Pterygopalatine fossa and pterygoid canal
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