Midsagittal Perspective of Brainstem and Fourth Ventricle.   The floor of the tent-shaped fourth ventricle is formed by posterior surfaces of the pons and medulla, while its roof is formed by the superior and inferior medullary vela.  The midbrain has been partially dissected to reveal, from ventral-to-dorsal, the substantia nigra, red nucleus, central tegmental tract, and tectum.  The fibers of the corticospinal tract has been dissected as they course inferiorly through the basis pons. (Image courtesy of M Nunez)