Cerebellomesencephalic Fissure and Trochlear Nerve. The pineal gland has been dissected to allow unimpeded visualization of the tectum, which is composed of the paired superior and inferior colliculi.  The trochlear nerve exits the brainstem immediately inferior to the inferior colliculus and then passes within the ambient cistern along the lateral margin of the midbrain. The anterior lobe of the cerebellum obscures the superior and middle cerebellar peduncles. (Image courtesy of AL Rhoton, Jr.)