I think it's the rolled up small snake seen by the bottom of the spine, mentioned in yoga texts. Inb4 "that's kundalini": no, kundalini is the spine. The main book of Hatha Yoga, the Pradipika, uses "spine" and "kundalini" interchangeably, they are the same thing.
I had a dream some while ago, told of it on here at the time, it was about seeing a large yellow snake bite a small green snake. Some week after I went to the graveyard by the church and saw a large yellow watering hose connected to a thin green hose. I felt this meant something, but wasn't sure what, although I did think it was about the kundalini.
I now think it's this way: the kundalini is the large yellow snake - the spine, which has all nerves and creates stability. The small green snake is the one seen rolled up, but because people don't know what "kundalini" means in the first place, they confuse them. The small green snake is the "kundabuffer" which is not named in yoga, and it connects the first 3 chakras, where the spine/kundalini connects 1-6.
Because this small snake is not a visible united organ, but a "system" in the physical body, it's not easy to understand what it is from conventional anatomy. Yogis can see it, but they can't easily say "it's this organ" as they can do with the kundalini and the spine (or the "demon" and the brain).
