Posterior dissection of the posterior cranial fossa and foramen magnum. A suboccipital craniectomy and an upper cervical laminectomy have been performed in order to expose the contents of the posterior cranial fossa and the foramen magnum.  Several lobules of the posterior lobe of the cerebellum are visible, as are the cerebellar tonsils.  The latter structures, parts of the posterior lobe, are ovoid structures located on either side of the midline cerebellar vermis.  The tonsils are of clinical significance because with increased intracranial pressure (ICP) the tonsils can herniate into the foramen magnum (tonsillar herniation) and compress the caudal brainstem.  Immediately caudal to the inferior pole of the tonsils  the foramen magnum, a large, ovoid shaped opening in the occipital bone through which passes the C1 spinal cord, the vertebral arteries, and the spinal accessory nerves.  The dorsal rootlets of spinal nerves C2 and C3, and a cervical denticulate ligament, are visible near the bottom of the image attached to the upper cervical spinal cord. (Image courtesy of AL Rhoton, Jr.)