Posterior View of Dissected Brainstem and Fourth Ventricle. The fourth ventricle surface anatomy is visible on the right, whereas the dissected brainstem is visible on the left.  The facial nerve makes a characteristic loop (internal genu) around the abducens nucleus before exiting the brainstem. The facial colliculus is the surface elevation in the fourth ventricular floor formed by the abducens nucleus and arching fibers of the facial nerve. The vestibulocochlear nerve fibers are seen existing the brainstem just posterior to the facial nerve fibers. (Image courtesy of M Nunez)