Figure 82-4 The blood supply to the optic nerve is illustrated. The anterior portion of the optic nerve is located to the left, whereas the posterior portion (closer to the brain) is on the right. The anterior portion of the nerve derives its blood supply from the posterior ciliary arteries (PCA) and the choroid (C), whereas the posterior optic nerve derives its blood supply from penetrating pial arteries (Col br) and branches of the central retinal artery (CRA). A, arachnoid; C, choroid; CRV, central retinal vein; D, dura; LC, long ciliary artery; ON, optic nerve; PR, short posterior ciliary artery; R, retina; S, sclera; SAS, subarachnoid space. (From Hayreh SS: Ischemic optic neuropathy, University of lowa, Department of Opthalmology Internet Site. http://webeye.ophth.uiowa.edu/dept/aion/ion_fg2.jpg.)