The concept of microcosm and macrocosm is both common and fundamental within the Western Occult Tradition. It is represented in the hermetic statement "As above, so below," meaning that actions in one sphere reflects changes in the other.
The MacrocosmThe three arching divisions here represent the three realms of existence: physical, celestial, and spiritual. The physical realm is the realm in which we exist and the realm of the elements. The Celestial realm houses the stars and planets, and the spiritual realm is the realm of the angels. Beyond all of these is God, represented by a triquetra (symbolizing the trinity) glowing with divine light.
The Microcosm - The Intellect
The three arching divisions separate the human form into three corresponding sections. The head and the associated intellect corresponds with the spiritual realm. This was particularly emphasized with occult philosophers, as they considered their pursuits to be ruled by logic and rationality.
The Torso and HeartThe torso is associated with the celestial realm. In particular, the Sun's orbit cuts through the center of the celestial realm and lines up with the heart. The Sun was often considered the physical representation of God in the visible universe. With its central location in the hierarchy, it worked as a sort of regulator and reflector of God. So to does the heart, considered the seat of the soul.
The GenitalsFinally, the genitals and intestines are associated with the physical realm. These are the more coarse parts of the human anatomy, dedicated solely to physical needs and wants: consumption, excretion, reproduction and lust.
Fludd seems to have particular disdain for the genitals, which he depicts as descending below even the physical realm. Sex was commonly seen as more of a pollutant than food or other physical wants and more of a threat to spirituality.
The genitals and intestines have no higher purpose, no connection to spirituality or rationality. They are part of our animal nature, and those who cannot see beyond physical desires are little more than animals in the eyes of occultists and theologists alike in the Renaissance.
Rising and Descending TrianglesA common symbol used by Fludd are two intersecting triangles. One is wide at the top and descends downward. This is the descent of divine light into matter. The upper realms contain more divine light, while the lower realms contain very little, overwhelmed by matter.
The second triangle is wide at the bottom and points upward. This is the presence of matter. All realms contain at least a trace of matter (except God himself, which is why he is outside of the system of realms). Most of it exists in the lower realm and becomes steadily more rarefied as you travel upward.
Together, the two triangles depict an equilibrium between spirit and matter. Things that are more material inherently are less spiritual and vice-versa. The Sun, at the very center of the system, is composed equally of both.