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Basis Cranii Externa

Ouroboros & Tree

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:

    ‣ 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.

  • 1 Feneis H, Dauber W. Pocket atlas of human anatomy. 4th ed. Thieme, Stuttgart, 2000.
  • 2 Schuenke M, Schulte E, et al. Thieme atlas of anatomy. Head and neuroanatomy. Thieme, Stuttgart, 2010.
  • 3 Lucifero A, Fernandez-Miranda J, et al. The modular concept in skull base surgery: anatomical basis of the median, paramedian and lateral corridors. Acta Biomed, 2021, 26;92(S4):e2021411
  • 4 Khalife S, Varshney R, Garg R. Anterior skull base anatomy. In: Kuan E, Tajudeen B, et al. (eds) Skull base reconstruction. Springer, Cham. 2023.
  • 5 Olivia F, Makalanda L, et al. Imaging review of the anterior skull base. Acta Radiol Open, 2018, 7(5):2058460118776487.
  • 6 Синельников Р, Синельников Я, и др. Атлас анатомии человека. Том 1. изд: 7-е. Новая волна, Москва, 2007.
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    Icon of crossfade image Comments for the borders and components of external surface of the base of skull
    Basis cranii externa
    Normal outline of skull base as viewed from below (norma basalis cranii)
    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 cranium
    Fossa 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
    Icon of crossfade image Basis cranii externa with the terminology markup
    Inferior and medal communications with pterygopalatine fossa
    Norma basalis cranii.
    The list of terms: Arcus alveolaris – Alveolar arch
    Juga 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
    Icon of crossfade image Cranial base to and the canalis musculotubarius
    Cranial base. Bottom and slightly dorsal view
    That illustration shows canals of the base of the skull, which usually is more challenging to localize. To find canalis musculotubarius and canalis pterygoideus it is recommended to first localize the scaphoid fossa on the pterygoid process's medial plate. After escaping from the canalis musculotubarius, the cartilage of the pharyngotympanic tube attaches to the fossa scaphoidea. Follow fossa in the direction toward temporal bone, and you will find the entrance to the musculotubal canal.
    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 fossa
    Canalis musculotubarius – Musculotubal canal
    Canalis pterygoideus – Pterygoid canal
    Canalis condylaris – Condylar canal
    Canalis hypoglossalis – Hypoglossal canals
    First published: Jan/2020
    Last update: 08/Jan/2024