Fig. 4. Blood supply of the optic nervehead. 1, central retinal artery; 2, arterial circle of Zinn-Haller; 3, pial arterial network. Contribution to Zinn-Haller circle from posterior ciliary arteries, pial plexus, and peripapillary choroid. The latter also sends branches directly to prelaminar disc substance. (Modified from Kolker AE, Hetherington J Jr: Becker-Shaffer's Diagnosis and Therapy of the Glaucomas, 3rd ed. St. Louis, C.V. Mosby, 1970)