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.