The assignment of the head to Aries is the first term of melothesia, the scheme that maps the twelve signs onto the body from crown to sole in zodiacal order. The doctrine is ancient: Marcus Manilius sets it out in verse in the Astronomica in the first century, giving Aries the head, Taurus the neck, Gemini the arms and so on to Pisces at the feet, and Ptolemy treats the medical application of astrology at length in the Tetrabiblos. The head takes the first sign because the scheme begins where the body begins and because Aries begins the zodiac; the logic is one of correspondence between two ordered series rather than of any observed relation. Aries being ruled by Mars and hot in the classical scheme, the head was accordingly read for fevers, inflammations, injuries and complaints of the eyes and face.
Ptolemy's own treatment in the Tetrabiblos is less tidy than Manilius's simple head-to-foot list: his third book treats the body twice, once on form and temperament and once, separately, on the hurts and diseases specific afflicting planets can cause, a causal and diagnostic framework rather than a fixed chart of correspondences. Other writers filled out the doctrine independently. Teucer of Babylon, writing around the turn of the era, and Julius Firmicus Maternus, in the Mathesis of the 330s, both give their own versions of melothesia, Firmicus combining it with a separate scheme of bodily subdivisions called the dodekatemoria; the doctrine was therefore never a single fixed list but a family of related schemes sharing the same head-to-foot logic.