Posterior Perspective of Caudal Fourth Ventricle and Craniocervical Junction. At the top of the image is seen the inferior margin of the fourth ventricle ending at the obex and central canal. On the left, hypoglossal nerve rootlets eminate from the preolivary sulcus on the ventrolateral surface of the medulla.  Further laterally, the spinal accessory nerves are visible.  The biventral lobule and cerebellar tonsils are the visible near the top of the image and are separated by the secondary fissure (not highlighted).  The posterior inferior cerebellar artery (PICA) branches off the vertebral artery and supplies the dorsolateral medulla and posterior inferior surface of the cerebellum. (Image courtesy of M Nunez)