The Níwán 泥丸 (Muddy Pellet) is one of the Nine Palaces in the head and it occupies the central position as the sum of the Nine Palaces.
“Since the center is located on the vertical axis that connects Heaven to Earth and Humanity, the níwán 泥丸 is also called “Kūnlún 崑崙,” a term that usually denotes the axis mundi. Hence the níwán is the Upper Cinnabar Field (Heaven), related to the Lower (Earth) and Middle (Humanity) Cinnabar Fields (see Dāntián 丹田 entry). It is “the One that connects the Three” (yīguàn sāntián 一貫三田).”–Monica Esposito, the author of this encyclopedia article.
Another fact I think it is useful to extract from this page: “Under the name băihuì 百會 (lit., “one hundred gatherings”), the níwán is the starting and arriving point of the circuit established by the Control Channel and the Function Channel (dūmài 督脈 and rènmài 任脈). Incorporating the Control Channel at its uppermost point at the crown of the head, it is the sanctuary in which the Yang Spirit (yángshén 陽神) is stored before its return to emptiness.”(M.E.)
ERRATUM in the description of nine palaces in the beginning: under number (9) should be “one inch above the Cavern Chamber” instead of “the Palace of the Jade Emperor.”
This comment would be much better with a better picture of the brain, yes, I agree.