Virgo takes the abdomen, the intestines and the process of digestion and assimilation, which is the same idea of sorting and separating that the sign carries generally, applied to the body. Medieval and early modern medicine, working within humoral theory inherited from Hippocrates and Galen, understood digestion as a series of concoctions in which the useful was separated from the superfluous, so the sign's general character and the region's function were held to correspond closely. Virgo being mutable, earthy and Mercurial, the texts read the region for complaints held to arise from anxiety and from irregularity as much as from diet, a connection modern medicine also makes, though for entirely different reasons and with an actual mechanism.
Galen's physiology, which medieval medicine inherited alongside its astrology, described digestion as proceeding through three successive concoctions, the first in the stomach, turning food into chyle, the second in the liver, turning chyle into blood, and the third in the tissues themselves, turning blood into flesh; Virgo's rulership of the first of these gave the sign a foundational role in the larger digestive process even though later concoctions were assigned, in the planetary scheme recorded separately under Organs, to different rulers entirely. The overlap and disagreement between the zodiacal and planetary body schemes recorded across this family of articles is therefore not confined to a single case, but recurs wherever Galenic physiology's own multi-stage processes met a single-organ zodiacal assignment.
Sources AstronomicaMarcus Manilius with Ancient MedicineVivian Nutton