The fascicles formed by the axons of the third cranial nerve nucleus complex travel across the midbrain crossing through the red nuclei and the pyramidal tracts (Figure 189.1). They leave the brainstem anteriorly and advance in the subarachnoid space (Figure 189.1 ), go through the cavernous sinus (Figure 189.1 ), and enter into the orbits through the superior orbital fissure (Figure 189.1 ). In the orbits it splits into two groups to innervate the muscles of the eyes. Lesions to the oculomotor system usually occur in the subarachnoid space prior to the cavernous sinus (Figure 199.1 4) or at the levator palpebrae muscle (Figure 189.1 LPM).
Figure 189.1.— Schematic representation of the left oculomotor nerve nuclei, oculomotor fascicle, oculomotor nerve and the muscles inervated by this nerve 1: aqueduct of Sylvius; 2: red nucleus; 3: pyramidal tract; 4: subarachnoid space; 5: cavernous sinus; 6: superior orbital fissure; 7: cilliary ganglion. See the figure in page 188.