Arms

Gemini takes the arms, hands and shoulders, and in most versions the lungs and the breath as well. The doubling of the figure and the pairing of the limbs made the correspondence a natural one, and the assignment of the lungs to a Mercurial air sign connects the region to speech, which in the classical scheme belongs to Mercury. Medieval and Renaissance texts read the region for complaints of the chest and the breathing as well as for injuries to the arms. The overlap with Virgo, the other Mercurial sign, which takes the bowels, and with the separate scheme of planetary rulership of organs, which gives the lungs to Mercury directly, shows that the two body schemes, the zodiacal one recorded here and the planetary one recorded under Organs, are distinct systems that were used side by side and do not always agree.

The term for the whole doctrine, melothesia, is itself built from melos, limb or bodily part, and thesis, a setting or position, so the word describes exactly what the diagram does, setting each limb or region in its place on the zodiacal sequence; it is a genuine technical term from the ancient sources, not a modern coinage for older material, and is distinct from iatromathematica, the broader field of astrological medicine of which melothesia is one part among others, including electional timing for treatment and the planetary rulership of organs recorded separately under Organs.

Sources AstronomicaMarcus Manilius with Astrological Melothesia in Ancient Rome and India: Intercultural Conversations in Religion, Medicine, and Technical ScienceTejas Subramanya Aralere

The assignment of the arms and hands to Gemini is one of the places where melothesia's reasoning is easiest to see, because the reasoning is the sign's own image. Gemini is the Twins, and the parts of the body that come in pairs and work together are given to it. The same logic recurs elsewhere in the scheme and is worth naming, since it explains attributions that otherwise look arbitrary.

The scheme has three kinds of argument in it, layered rather than consistent. There is the positional argument, which simply runs the twelve signs down the body from head to feet and is the backbone of the whole thing. There is the iconographic argument, which reads the sign's image, so Gemini takes the paired limbs and Virgo the process of sorting. And there is the planetary argument, which imports the ruling planet's associations, so Gemini's rulership by Mercury pulls the lungs and the breath into its territory in many versions.

Where the three agree, the assignment is stable across sources. Where they conflict, the sources disagree with each other, which is why the stomach appears under both Cancer and Virgo and the kidneys under both Libra and Venus in this atlas.

Recognising the three arguments is the most useful thing a reader can take from the doctrine, because it shows the scheme being constructed rather than received.