Posterior View of Dissected Fourth Ventricle.  On the right side, the floor of the fourth ventricle remains intact, while on the left side it has been removed to reveal deeper lying structures in the low pons and upper medulla.   The abducens nucleus can be seen in the caudal pons, deep to the fourth ventricular floor. The facial nerve makes a characteristic loop around the abducens nucleus before exiting the brainstem. The facial colliculus is the term that refers to the surface elevation in the floor of the fourth ventricle corresponding to the location of the abducens nucleus and the looping fibers of the facial nerve. The vestibulocochlear nerve fibers can be seen exiting the brainstem just posterior to the facial nerve fibers. (Image courtesy of M Nunez)