Basis Cranii Externa
It would seem that there can be no ambiguity in the anatomy of the skull. However, the external cranial base is one such example. The issue, apparently, lies in the fact that in many anatomical atlases1,2, no distinction is made between the concepts of "basis externa cranii" and the "norma basalis crаnii". Yet, there is a difference between these concepts, and it is significant:
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‣ norma basalis (inferior) — inferior aspect of the cranium
‣ basis externa cranii — inferior aspect of the neurocranium
The fact that the base of the skull consists only of the bones of the neurocranium is clinically obvious3–5; otherwise, a nasal bone fracture could be interpreted as a skull base fracture. Therefore, the external base of the skull is anteriorly covered by the bones of the viscerocranium6, albeit not entirely. Only those parts of the viscerocranium that directly contact the cranial cavity contribute to this coverage (for example, the lamina cribrosa and crista galli are included, but not the labyrinthus ethmoidalis5).
On this page, we will adhere to a rational: we will depict what is typically asked of medical students within the topic of the external skull base, but we will label the illustrations appropriately as the basal norm (norma basalis) for accuracy.
Click on image to switch on comments focused on the borders and the bony components that build up the basis cranii. Note that the base of the skull includes some facial skeleton bones, like the maxilla and palatine.
The list of terms:
Basis cranii externa – External base of craniumFossa temporalis – Temporal fossa
Fossa infratemporalis – Infratemporal fossa
Cavitas nasi – Nasal cavity
Arcus alveolaris maxillae – Maxillary alveolar arch
Palatum osseum – Bony (hard) palate
Choana – Posterior aperture of the nose
Processus pterygoideus – Pterygoid process (of the sphenoid bone)
Pars petrosa – Petrous part (of temporal bone)
Pars tympanica – Tympanic part (of temporal bone)
Processus mastoideus – Mastoid process (of temporal bone)
Pars lateralis ossis occipitalis – Lateral part of the occipital bone
Squama occipitalis – Squamous part of occipital bone
Foramen magnum
The list of terms:
Arcus alveolaris – Alveolar archJuga alveolaria – Alveolar juga (eminences)
Foramen incisivum – Incisive foramen
Canalis incisivus – Incisive canal
Palatum osseum – Bony (hard) palate
Processus palatinus maxillae – Palatine process of maxilla
Lamina horizontalis ossis palatini – Horizontal plate of palatine bone
Sutura palatina mediana – Median palatine suture
Sutura palatina transversa – Transversal palatine suture
Canalis palatinus major – Greater palatine canal
Canalis palatinus minor – Lesser palatine canal
Processus pyramidalis ossis palatini – Pyramidal process of palatine bone
Cavitas nasi – Nasal cavity
Choana – Posterior aperture of the nose
Processus pterygoideus – Pterygoid process
Lamina lateralis processus pterygoidei – Lateral plate of the pterygoid process
Lamina medialis processus pterygoidei – Medial plate of the pterygoid process
Fossa pterygoidea – Pterygoid fossa
Vomer
Foramen ovale
Foramen spinosum
Tuberculum pharyngeum – Pharyngeal tubercle
Foramen lacerum
Tuberculum articulare – Articular tubercle
Fossa mandibularis – Mandibular fossa
Fissura petrotympanica – Petrotympanic (glasserian) fissure
Canalis caroticus – Carotid canal
Fossula petrosa – Petrosal fossula
Canaliculus tympanicus – Tympanic canaliculus
Foramen jugulare – Jugular foramen
Fossa jugularis – Jugular fossa
Foramen stylomastoideum – Stylomastoid foramen
Canalis n. facialis – Canal for the facial nerve
Processus styloideus – Styloid process
Processus mastoideus – Mastoid process
Incisura mastoidea – Mastoid notch
Sulcus a. occipitalis – Groove for occipital artery
Protuberantia occipitalis externa – External occipital protuberance
Linea nuchalis superior – Superior nuchal line
Linea nuchalis inferior – Inferior nuchal line
Crista occipitalis externa – External occipital crest
Foramen magnum
Canalis pterygoideus also opens into the basis cranii externa. However, this opening settles deep in the foramen lacerum, at the base of the fossa scaphoidea mentioned above.
The list of terms:
Fossa scaphoidea – Scaphoid fossaCanalis musculotubarius – Musculotubal canal
Canalis pterygoideus – Pterygoid canal
Canalis condylaris – Condylar canal
Canalis hypoglossalis – Hypoglossal canals
Last update: 08/Jan/2024