Fig. 17. The optic nerve (ON). A parasagittal MR image demonstrates the 7-mm excess of intraorbital optic nerve, seen as an S shape. Also note the perpendicular relation of the inferior oblique muscle (IOM) to the inferior rectus muscle (IRM). Other findings of anatomic interest in this image include the levator (Lev), the superior rectus muscle (SRM), the orbital septum (OS) arising from the arcus marginalis, and the preaponeurotic fat pad (PF) between the septum and the levator. Note that the orbital floor (FL) is angled upward by 15° to 20° from anterior to posterior.