The pineal gland is lined on the inside with tissue called pinealocytes. These pinealocytes are similar to the rods and cones in the retina of your eye. That’s right; it’s the third eye! Your brain has a third eye in the center and it has retinal tissue. It also has the same wiring that your physical eyes do to the visual cortex in your brain! How did they know that all the way back when?
Source: wiktionary