pineal gland

- sits outside BBB to manage blood 

- but needs blue light notification from retina

- retina routes to CNS's SCN, which routes to PNS's SCG, then to pineal

- pineal was first evolved eye, which converted to an internal organ in mammals

- our paired eyes evolved later, from neural tissue

- light has to pass through the optic nerves to reach the photoreceptors before being chemically signalled back to the optic nerves