Pisces takes the feet and closes the sequence that Aries opened at the head. The tidiness of that closure is often offered as evidence for the scheme, and it is worth being clear about what it actually shows: twelve signs distributed in order over a body must end somewhere, and ending at the feet is the only available outcome once the first sign is placed at the head. The fit is a consequence of the construction.
What happened to the doctrine afterwards is the more useful history. Astrological medicine was standard university teaching in Europe into the seventeenth century. It came apart not through a single refutation but through the same period that produced Harvey's circulation, the chemical medicine of the Paracelsians, and the mechanical philosophy, and by the eighteenth century a physician who set his practice by the Moon's sign was outside the profession rather than in its mainstream. The zodiac man survived that separation intact, because the almanac trade had no reason to drop a woodcut its readers expected, and the figure is still printed in popular almanacs today.
That long afterlife in print, detached from the medicine it belonged to, is why the doctrine is so much better known than it is understood. The atlas records it as a historical system of correspondence, and asserts nothing about the body.
Sources AstronomicaMarcus Manilius with A History of Horoscopic AstrologyJames Herschel Holden