Fundus camera image of the retina of a normal eye, showing the distribution of the retinal veins & arteries: the central retinal artery (a branch of the opthalmic artery) enters the optic nerve 2 cm before it reaches the eyeball, emerging through the retina in the centre of the optic disc (the blind spot of the eye, the pale central area). An extension of the optic nerve, the retina consists of photosensitive cells (rods & cones) which translate light energy into nervous impulses. The orange-red appearance is caused by the pigment & abundance of blood vessels in the choroid layer, visible through the normally transparent retina.