Posterior Perspective of Caudal Fourth Ventricle and Foramen Magnum. At the top of the image can be seen the floor of the fourth ventricle, formed by the dorsal surface of the medulla.  Within the floor of the ventricle lie the hypoglossal and vagal trigones and, further laterally, the vestibular area.  The inferior margin of the ventricle tapers to a point, the obex, and continues as the central canal.  Cranial rootlets of the spinal accessory nerve eminate from the caudal brainstem to join the spinal rootlets that come from the upper cervical spinal cord and enter the posterior cranial fossa via the foramen magnum. The inferior medullary velum forms the posterior roof of the fourth ventricle and has a midline hole, the foramen of Magendie, through which CSF drains from the IV ventricle to the cisterna magna.    (Image courtesy of M Nunez)