A, Inferior view. The left tonsil has been removed at the level of the tonsillar peduncle, its site of attachment to the remainder of the hemisphere. The anterior medullary segment (green) extends from the origin at the vertebral artery to the level of the inferior olive. This segment courses rostral or caudal to or between the rootlets of the hypoglossal nerve. The lateral medullary segment (orange) extends from the level of the most prominent part of the olive to the level of the rootlets of the glossopharyngeal, vagus, and accessory nerves. The tonsillomedullary segment (blue) extends from the level of the latter nerves around the caudal half of the tonsil and often forms a caudally convex loop. The telovelotonsillar segment (yellow) extends from the midlevel of the tonsil to the exit from the cleft located between the tela choroidea and the inferior medullary velum superiorly and the superior pole of the tonsil inferiorly. The cortical segment (red) extends from where the artery and its branches exit the fissures between the tonsil, vermis, and hemisphere to reach the cortical surface. The bifurcation of the main trunk into medial and lateral trunks is often located at the level of the tonsillomedullary or the telovelotonsillar segments. The medial trunk gives rise to median and paramedian vermian arteries. The lateral trunk gives rise to lateral, intermediate, and medial hemispheric and tonsillar arteries. B, Enlarged posterior view. The left and part of the right halves of the cerebellum was removed to show the relationship of the PICA to the roof of the fourth ventricle. The dentate nucleus wraps around the superior pole of the tonsil. The telovelotonsillar fissure is below the inferior half of the roof of the fourth ventricle between the tonsil, tela choroidea, and inferior medullary velum. The caudal loop of the PICA is near the caudal pole of the tonsil, and the cranial loop is above the rostral pole of the tonsil. (Images courtesy of AL Rhoton, Jr.)