Posterior Views of the Roof of the Fourth Ventricle. A, The cerebellum has been sectioned in an oblique coronal plane to show the relationship of the rostral pole of the tonsil to the inferior medullary velum and the dentate nucleus. The left tonsil has been removed. The inferior medullary velum stretches across the rostral pole of the tonsil. The dentate nucleus is located above the superolateral portion of the apex of the roof of the fourth ventricle, wraps around the rostral pole of the tonsil, and is separated from the tonsil by the inferior medullary velum. B, Both tonsils have been removed, but the nodule and the inferior medullary velum have been preserved. The inferior medullary velum extends from the nodule to the flocculus. The cranial loop of the PICA passes through the supratonsillar cleft located between the tonsil on one side and the tela choroidea and inferior medullary velum on the opposite side; this is referred to as the “telovelotonsillar segment” or the “supratonsillar loop.” The rhomboid lip is a sheetlike layer of neural tissue that unites with the tela choroidea to form a pouch at the outer extremity of the lateral recess. C, The nodule and the inferior medullary velum have been removed to expose the full length of the floor of the fourth ventricle. The superior and inferior cerebellar peduncles face the ventricular surface of the fourth ventricle. The middle cerebellar peduncle, which passes from the pons to the cerebellum is separated from the ventricular surface by the superior and inferior peduncles. Each half of the floor is divided longitudinally by an irregular sulcus, the sulcus limitans, which deepens lateral to the facial colliculus and hypoglossal triangles to form the superior and inferior foveae. A darkened area of cells, the locus ceruleus, is located at the rostral end of the sulcus limitans. The striae medullares cross the floor at the level of the lateral recess. The hypoglossal and vagal nuclei and the area postrema are stacked one above the other in the lower portion of the floor, displaying the configuration of a pen nib and, thus, the area is referred to as the “calamus scriptorius.” (Images courtesy of AL Rhoton, Jr.)