Figure 4. Photoreceptor outer segments. The left part of the figure is reproduced from Figure 3. The detail on the right depicts some of the finer structures of the outer segments of the photoreceptors. The outer segment is the last part of the cell that light passes through as it travels through the living retina, although in some animals a reflective layer behind the retina causes a significant quantity of light to pass back through the retina in the opposite direction. In any case, the outer segments contain the molecules which absorb light and convert that absorption into a biochemical signal. The outer segments are comprised of a series of disks, probably to increase the amount of membrane and hence photopigment -- which is bound to these membranes -- in the cell. The drawings of the outer segments have been partially cut away to reveal the cross-sections of the disks. (Drawings after Walls 1942, and Bailey and Gouras 1985).