The Proatlantal artery is a persistent carotid-vertebrobasilar connection. In the Proatlantal artery type 1 configuration, the artery arises from the cervical internal carotid artery and joins the vertebrobasilar system via the first cervical space. The Proatlantal artery type 2 variant arises from the external carotid artery and joins the vertebrobasilar system via the second cervical space.

Above (Via Lasjaunias et al. 2001): “The occipital artery viewed from the side. The asterisks represent site of vascular regression(s). A The type I proatlantal artery. The basilar system originates from the cervical internal carotid artery; it joins the posterior fossa via the first cervical space (broken arrow). B The type II proatlantal artery. The basilar artery originates from the external carotid artery; it joins the posterior fossa via the second cervical space (curved arrow).”

References:

1. Lasjaunias P, Berenstein A, Brugge KT. Clinical Vascular Anatomy and Variations. Springer Berlin Heidelberg; 2001.