The fourth ventricle. The fourth ventricle is a midline, CSF-filled cavity located posterior to the pons and rostral medulla, and anterior to the cerebellum.  In this dissection, the cerebellar tonsils have been retracted laterally and the inferior medullary velum (a thin membrane that forms the posterior roof of the IV ventricle) has been incised to expose the walls and floor of the ventricle.  The view is looking rostrally into the fourth ventricle, in the direction of the cerebral aqueduct.  The floor of the IV ventricle is diamond-shaped; it is widest in its central region, where it lies dorsal to the pons-medulla junction, and tapers gradually in both the rostral and caudal directions.  Rostrally, it communicates with the cerebral aqueduct whereas caudally it continues as the central canal.  Inferiorly, the caudal walls of the IV ventricle fuse at the midline, wedge-shaped obex.   A prominent tuft of choroid plexus is suspended from the undersurface of the inferior medullary velum and extends anteriorly and laterally on both sides, through the lateral recesses, to enter the foramina of Luschke.  The latter are paired, tunnel-like openings that curve anteriorly around the brainstem to connect the midline IV ventricle with the cerebellomedullary cistern (cisterna magna).  A third opening, the midline, funnel-shaped foramen of Magendie, has been removed by the dissection; it would be located in the inferior medullary velum immediately superior to the obex. (Image courtesy of AL Rhoton, Jr.)