K, Enlarged view of the floor of the fourth ventricle. The median sulcus divides the floor longitudinally in the midline. 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 stria medullaris crosses 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 part of the floor to give the configuration of a pen nib and, thus, the area is referred to as the calamus scriptorius. L, Another fourth ventricular floor. The paired veins of the superior cerebellar peduncle course on the outer surface of the superior peduncles and join superiorly to form the vein of the cerebellomesencephalic fissure. The median posterior medullary vein ascends on the medulla and splits into the paired veins of the inferior cerebellar peduncle at the caudal margin of the floor. That left vein is hypoplastic. The left vein of the cerebellomedullary fissure passes along the lateral recess and ascends to join the petrosal group of veins in the cerebellopontine angle. (Images courtesy of AL Rhoton, Jr.)