The exact anatomy of the central retinal vein as it exits the eye is unknown. In this study serial sections of the central retinal vein and artery in the anterior optic nerve from six globes (five from cornea donors and one exenteration specimen) were examined by image analysis and three-dimensional reconstruction to determine their luminal characteristics. The retinal artery was found to have a uniform perimetric length and cross-sectional area. The vein, however, had a reduction in these measurements at the level of the lamina cribrosa, signifying a constriction of the vessel at this site. In addition, the cross section of the vein usually formed a "D" or crescentic shape adjacent to the circular artery. We propose that the constriction of the vein acts as a "throttle" mechanism on venous blood flow. Thus, a relatively high venous blood pressure is maintained within the high-pressure intraocular environment, thereby maintaining the patency of the retinal venules and capillaries.