Part 13 · Chapter 107

And this is their enumeration: from the skull issued the thirteen white corrections, mentioned in Idra Rabba, page 128, side B. Therefore the aspect of the skull is always called the "white head" (Risha Hivra), because of these thirteen white paths within it, bare and without hair. Remember this, for the "white head" is said of the skull of Arich Anpin.