Floor of the fourth ventricle. The cerebellar peduncles have been sectioned and the cerebellum removed in order to expose the floor of the fourth ventricle.  The floor of the IV ventricle is formed by the dorsal surfaces of the pons and the rostral medulla.  As seen from above, the IV ventricle is diamond-shaped.  It is widest at the point where the pons and medulla merge, i.e., at the level of the lateral recess.  The lateral recess is a wedge-shaped, tapered region of the IV ventricle that merges laterally with the Foramen of Luschke.  The paired Foramina of Luschke are two of three openings (the third opening is the unpaired, midline Foramen of Magendie) through which CSF in the IV ventricle drains into the cisterna magnum.  Caudally, the ventrolateral walls of the ventricle fuse at midline as the obex.  At the obex, the IV ventricle continues caudally as the central canal (not visible in this dissection).  Rostrally, the IV ventricle tapers and continues into the midbrain as the tunnel-like cerebral aqueduct, also not visible in this image.  The floor of the IV ventricle is divided into symmetrical halves by the midline median sulcus.  Located immediately lateral to the median sulcus in the caudal pons are paired prominent bulges, the facial colliculi.  These subtle surface elevations are formed by the underlying abducens nucleus and the arching fibers (interal genu) of the facial nerve.  Further caudally, the columnar-shaped hypoglossal nucleus and dorsal motor nucleus of the vagus form subtle surface elevations on the dorsal surface of the medulla, the hypoglossal and vagal trigones, respectively. (Image courtesy of AL Rhoton, Jr.)