Your retina is brain tissue, and it is wired back to front
The retina grows out of the embryonic brain, making it the one piece of the central nervous system anyone can look at without surgery. It is also built inside out: light must pass through nerve cells and blood vessels before reaching the sensors at the back. Squid eyes, built the other way round, have no blind spot.
Although it is no more than half a millimetre thick, the retina does a camera sensor's job and more. In an adult it covers about 72 percent of a sphere some 22 mm across and holds roughly 7 million cones and between 75 and 150 million rods. Rods handle dim light in shades of grey; cones need brighter light and deliver colour and fine detail. A third light-sensitive type, the photosensitive ganglion cell, helps set circadian rhythms and drives the pupil's response to light. The retina also has the greatest continuous energy demand of any tissue in the body.
Ten layers are stacked between the vitreous gel and the choroid. Because ganglion cells sit at the front, their axons must gather and exit through the retina to form the optic nerve, leaving a patch of about 3 square millimetres with no photoreceptors: the blind spot. The fovea compensates. This small pit, free of blood vessels and with little tissue over it, packs long, slender cones in a hexagonal mosaic, giving the sharpest vision the eye can manage, though it is less sensitive in dim light because it lacks rods.
Why build it inverted? One view calls it an evolutionary leftover; another says the arrangement keeps photoreceptors well maintained by the pigment layer behind them. Cephalopod retinas, which did not grow from the brain and evolved separately, have no such pigment layer, and their photoreceptors probably wear out sooner. A study reported by the American Physical Society found that glial cells act like optical fibres, funnelling green and red light five to ten times more effectively than blue into cones, which sharpens daytime colour vision at little cost to night vision.
The wiring even leaves a visible trace. Stare at a bright blue sky and tiny moving dots may appear: white blood cells passing through capillaries in front of the photoreceptors, known as Scheerer's phenomenon.
Source: Retina
