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In [[1938]] Fischer, described five [[internal carotid artery segments]] in the opposite direction to the [[blood flow]]
((Fischer E. Die Lageabweichungen der vorderen hirnarterie im gefässbild. Zentralbl Neurochir. 1938;3:300–313.)).

These segments were based on the angiographic course of the intracranial ICA rather than its arterial branches or anatomic compartments. Subsequent attempts to apply modern nomenclature to these numerical segments failed to recognize Fischer's original intent of describing patterns of arterial displacement by tumors and, therefore, resulted in a nomenclature that was anatomically inaccurate. Fischer's system was further limited, because segments were numbered opposite the direction of blood flow and the extracranial ICA was excluded
((Bouthillier A, van Loveren HR, Keller JT. Segments of the internal carotid
artery: a new classification. Neurosurgery. 1996 Mar;38(3):425-32; discussion
432-3. PubMed PMID: 8837792.
)).