The causal (deepest) spine. (1)
Raising one’s energy through the brahmanadi is the last test a yogi must go through to attain full union with the Divine. The opening of the brahmanadi is at the top of the head, and when one draws one’s energy to this point, one becomes reunited with God. (1)
For more explanation, see The Art and Science of Raja Yoga by Swami Kriyananda.
