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FIG 6. A, Persistent proatlantal artery type I (PPA 1) arises from the caudal part of the internal carotid artery and courses along the anterior aspect of the vertebral bodies to the level of the occipitoatlantal space before coursing dorsally.
B, Persistent proatlantal artery type II (PPA 2) arises from the external carotid artery; it crosses the C1 or C2 vertebra obliquely. Both PPA-1 and PPA-2 enter the skull via the foramen magnum.
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