Figure 3.

Schematic representation of the embryological mechanisms underlying anatomic variation involving anomalous origins of the subclavian artery, the vertebral artery, and persistent trigeminal artery. Left, Normal embryological development. Right, Present case in which the right VA arises at the 4^th CIA level, with involution of the right dorsal aorta distal to the 4^th CIA. The PTA is associated with hypoplastic VAs. BA, basilar artery; CIA, cervical segmental artery; HA, hypoglossal artery; IC, internal carotid artery; PA, proatlantal artery; PTA, persistent trigeminal artery; SA, subclavian artery; VA, vertebral artery.