The Retina

The retina is the sensory part of the eye and consists of an outer avascular region of pigmented epithelium in close contact with the photoreceptors. Photoreceptor cells are rods and cones that transmit visual information to the bipolar, ganglion, Müller, horizontal, and amacrine cells of the inner retina ( Fig. 82-1 ). The photoreceptor layer has 6 to 7 million cones, which are necessary for central reading and color vision. Rod cells (120 million) permit night vision and motion detection. The inner two thirds of the retina contains the retinal vasculature and cell bodies, the bipolar cells that transmit impulses from the photoreceptors, and the ganglion cells whose axons transmit visual information first to the ON and then to the brain. The visual axis extends through the fovea centralis, an avascular zone with 650,000 cones concentrated for central vision.^[4]

Figure 82-1 A and B, Section of the human retina demonstrating its organization into layers. The outer retina contains the photoreceptors, whereas the inner retina contains neurons responsible for the transmission of visual information to the brain. (From Federman JL, Gouras P, Schubert H, et al [eds]: Retina and Vitreous, Textbook of Ophthalmology. St Louis, CV Mosby, 1994.)