Nerve · Head

Oculomotor nerve

Nervus oculomotorius

Four of the six muscles that aim the eye answer to the oculomotor nerve, and so does the muscle that holds the upper eyelid open. It leaves the front of the midbrain, passes forward between two of the arteries at the base of the brain, travels in the wall of the cavernous sinus and enters the orbit through the superior orbital fissure, where it splits into a superior division for the upper lid and the superior rectus and an inferior division for the medial and inferior recti and the inferior oblique. It also carries the parasympathetic fibres that constrict the pupil and thicken the lens for near vision — and those fibres run on the outer surface of the nerve, so they are the first ones reached by anything pressing on it from outside. That single fact of fibre topography is why the pupil is watched so closely whenever this nerve is in question.