Waist

Libra takes the lower back, the loins and the kidneys, and this is one of the places where the zodiacal and planetary schemes agree partially, since Libra is ruled by Venus and the planetary scheme also gives the kidneys to Venus. The reasoning offered in the tradition is the paired and balanced character of the organs, which filter and regulate the fluids of the body and hold it in equilibrium, matching the scales of the sign and the temperate nature of its ruler. Saturn's exaltation in Libra was read as adding the sign's association with the bones of the lower back. The correspondence is a good example of how the two schemes were harmonised where they could be and simply used in parallel where they could not.

Libra is the one sign of the twelve with no animal or human form of its own, its symbol a pair of scales rather than a living thing, and some astrological writers took this as fitting the region it governs: the kidneys perform a mechanical, regulating function, filtering and balancing the body's fluids, which was read as more akin to the impersonal operation of a scale than to the animate qualities the animal signs were thought to lend their regions. The reading is an interpretive gloss rather than a claim found stated outright in Manilius or Ptolemy, and it is recorded here as part of how later astrologers rationalised the scheme rather than as part of its classical foundation.

The lower back and loins go to Libra by the positional rule, and the kidneys come with them. But the kidneys are also given to Venus in the parallel scheme of planetary rulership of the organs, which this atlas records on the Organ's own page, and the two claims are made by the same authors in the same books without any sense of conflict.

That is not carelessness. Medieval and early modern astrological medicine ran two mappings at once and used them for different purposes. The zodiacal mapping is positional and answers the surgeon's question: which member is at risk today, given where the Moon is. The planetary mapping is temperamental and answers the physician's question: what is this organ's nature, and what humour does it govern. A part could be Libra's for the first purpose and Venus's for the second with no contradiction, and in the case of the kidneys the two even agree, since Libra is itself ruled by Venus.

Where they do not agree the atlas records both, because collapsing them would misrepresent how the material was used.

Neither mapping is a statement about renal function. This entry records a historical doctrine of correspondence, and nothing in it is medical guidance.