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