Third nerve anatomy

The third cranial nerve begins as a nucleus in the midbrain. The fascicle of the third nerve leaves the nucleus and passes ventrally near important structures in the midbrain (eg, red nucleus, corticospinal tract). It then enters the subarachnoid space, passes into the lateral wall of the cavernous sinus, and finally divides into superior and inferior branches as it enters the superior orbital fissure in the orbit. The pupil fibers are shown; the innervation to individual muscles (superior rectus, inferior rectus, medial rectus, and inferior oblique) are not shown.