The first triangle of the Tree of Life (the sephiroth Kether, Chokmah, and Binah) is ruled by the crown, Kether, related to our head, specifically, the two hemispheres of our brain and the pineal gland. The first triangle is controlled by the Father. Thereafter, comes the second triangle of the Tree of Life, formed by the sephiroth Chesed, Geburah and Tiphereth; this triangle is controlled by Chokmah, the second Sephirah of the crown.
This is why we say that the kingdom of the Father is in the head, but the kingdom of the Son is in the heart, because this second triangle is related with the lungs and heart. The lungs take the spirit, the breath, the air of the atmosphere; the lungs are related to the sephiroth Chesed and Geburah, the Spirit and the Spiritual Soul, while the heart relates to Tiphereth. Tiphereth is the human soul and is also called the son of man (Beni Adam), and Chokmah relates to the human soul. This is how you have to see it in your physicality.
Thereafter, below the second triangle, we find the third triangle formed by the sephiroth Netzach, Hod, and Yesod. This is called the triangle of priesthood or triangle of magic (the meaning is the same). The center of gravity of this triangle is in Yesod, which relates to the sexual organs, and it is ruled by the sephirah Binah, the Holy Spirit. So, the kingdom of the Holy Spirit, Binah, is in the sexual organs. In our organism, sexual potency relates to the kidneys; the sephiroth Netzach and Hod are related to the kidneys. This is how these sephiroth need to be seen alchemically in order to understand the meaning of the cross. Our physical body is in itself at the very bottom of the Tree of Life, it is Malkuth, the kingdom.
“And out of [Malkuth] the ground made Jehovah Elohim [Binah] to grow every tree that is pleasant to the sight, and good for food; the Tree of Life also in the midst of the garden, and the tree of knowledge of good and evil.” – Genesis 2:
Thus, if we alchemically observe our physical body, we will see that it is a tree pleasant to the sight, a kingdom, a fusion, of these Sephirothic forces. Thus, from these basics, we understand that we have two trees in our body, the Tree of Life within our central column which is the spinal medulla. Adjacent to the Tree of Life, we have two columns joint to each side of the central column; these are the columns of good and evil, sun and moon, positive and negative, Pingala and Ida, Adam and Eve, which are also related with the cross. Thus, Adam and Eve, positive and negative, Sun and Moon are related with the tree of good and evil, the Tree of Knowledge. The Tree of Life and the Tree of Knowledge share the roots in the sexual organs.