The fourth cranial nerve (BC4) is an exclu-sively somatomotor nerve and innervates the superior oblique muscle (E5) of the extra-ocular muscles. Its fibers originate from thelarge, multipolar neurons of the nucleus ofthe trochlear nerve (BC6), whichlies in the midbrain below the aqueduct at the level of the inferior colliculi. The fibers ascend dorsally in an arch, cross above the aqueduct, and leave the midbrain at the lower margin of the inferior colliculi. The nerve is the only cranial nerve leaving the brain stem at its dorsal aspect. It descends in the subarachnoid space to the base of the skull, where it enters the dura mater at the margin of the tentorium and continues through the lateral wall of the cavernous sinus. It enters the orbit through the superior orbital fissure.