FIG 2. Schematic representation of the carotid bifurcation development.

A, Normal development of the carotid bifurcation. The third arch (III) constitutes the proximal part of the carotid axis, which is divided into the CCA and proximal ICA either by in situ sprouting of the ventral pharyngeal artery or by its migration from the aortic sac. The distal ICA is formed by the dorsal aorta distal to the ductus caroticus. The ductus caroticus is the segment of dorsal aorta located between the third (III) and fourth (IV) aortic arches. Its regression at the 12–14-mm stage allows the carotid axis to take its definitive appearance.

B, Separate origins for the ECA and ICA from the aorta occur when the third (III) aortic arch involutes instead of the ductus caroticus (illustrated on the left side). The same process may occur on the right side, the separate origins for the ECA and ICA arising in that case from the innominate artery.

C, Low carotid bifurcations are formed when, in addition to the situation described in B, there is migration of the ECA origin toward the ductus caroticus, and reconstitution of a short CCA.