Th e retinal architecture is disrupted at two locations: the optic disc and the fovea (Figure 1). Th e optic disc, which creates each eye’s “blind spot,” is a region devoid of neurons, in which the axons of retinal ganglion cells exit the eye and enter the optic nerve. Th ese axons are the only means by which visual information is transmitted from the eye to the rest of the brain. Th eir loss or dysfunction in diseases such as glaucoma or optic neuritis causes blindness. Th e fovea is a retinal specialization dedicated to achieving high-resolution vision along the visual axis of the eye. In this small region, the many neural and vascular layers of the retina are displaced sideways to allow light rays to reach photoreceptors directly. Unlike photoreceptors in imaging systems, the photosensitive elements of the eye are not distributed randomly. Th e fovea contains the highest concentration of photoreceptors in the retina (about 200,000 cones/mm2). Its view of the world is relatively unimpeded by distortions due to overlying neural tissue and blood supply. Th e concentration of cones decreases rapidly away from the fovea, to a density of 5000/mm2, at the outer edges of the retina. On the other hand, there are no rods in the fovea, but their concentration increases rapidly

to a maximum about 20 degrees off the visual axis and then decreases gradually toward the outer edges of the retina.