Since Parkinson's original description, the C5 collaterals are always presented as originating from a common trunk (meningo-hypophyseal). This is the least frequent anatomical variation. In fact, the posterior (C5) group of collaterals corresponds to the remnant of two transient embryonic vessels (primitive maxillary and trigeminal arteries), branches of which will give rise to the two main groups of collaterals originating from this area: medially, the inferior hypophyseal and the medial clival arteries; laterally, the basal tentorial and lateral clival arteries, which are constantly anastomosed with the ascending pharyngeal artery. Only in some cases (10%) do these two groups arise in common from the C5 carotid siphon. Apart from these branches the so-called Bernasconi's artery may originate from eight different pedicles, the C5 siphon representing only one of them.