A, The PICA courses around the medulla, enters the cerebellomedullary fissure, and exits the fissure to supply the suboccipital surface. The fissure extends upward between the cerebellar tonsils on one side and the medulla and inferior half of the ventricle roof on the other side. The PICAs frequently form a caudal loop at the lower pole of the cerebellar tonsils. B, Enlarged view. The left tonsil has been removed to expose the course of the PICA within the cerebellomedullary fissure. The PICAs often loop upward around the rostral pole of the tonsil, where they course between the rostral pole of the tonsil on the lower side and the tela choroidea and inferior medullary velum on the upper side. C, Both tonsils and the adjacent part of the biventral lobule have been removed to expose the PICA trunks. The PICAs divide into a medial trunk, which supplies the vermis and adjacent part of the hemisphere, and a caudal trunk, which loops around the tonsil to supply the largest part of the hemispheric surface. Choroidal branches pass to the tela choroidea and choroid plexus in the roof. The vein of the cerebellomedullary fissure crosses the tela and velum and passes above the flocculus to join the veins in the cerebellopontine angle that empty into the superior petrosal sinus. D, Another dissection showing the relationship of the cranial loop of the PICA to the tonsils and inferior medullary velum. Both tonsils and the nodule and uvula have been preserved. The inferior medullary velum has been preserved on the right side. The left half of the inferior medullary velum has been removed to expose the supratonsillar loop of the PICA, which courses between the velum and the tonsil. The velum stretches laterally from the nodule across the rostral pole of the tonsil to blend into the flocculus. (Images courtesy of AL Rhoton, Jr.)