Retina. Coloured scanning electron micrograph (SEM) of a resin cast of the retina, the light- sensitive membrane that lines the back of the eye. Blood vessels (red, purple) radiate out from the optic disc (yellow). The optic disc is the area where the optic nerve and blood vessels enter the eye. Due to this, it has no light-sensitive cells and is the cause of the blind spot. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissues.