Altho the further development of Occidental as well as Oriental astrology drew its resources from the primeval Assyrian, Babylonian, and Egyptian doctrines, yet from the second century, A.D., the astronomic work of Ptolemy and the exhaustive description of antique medicine by Galen derive their inspiration from _Medicina Astrologica_. Whatever these two great masters were able to report of the dependence of the functions of the body upon celestial bodies was from then on, without further inspection and examination, acknowledged to be true by the great majority of physicians. Only occasionally this or that practitioner is bold enough to oppose the intrusion of astrologic vagaries into the art of healing; among these radicals was the philosophically trained physician, Sextus Empiricus, who lived about the year 193, A.D. However, this protest of brave Sextus, as well as all subsequent ones, scarcely had any influence upon the astrological development of medicine. Astrology could not be arrested on its road to the domination of the world, and until the seventeenth century it controlled the thought of physicians with the same invincible sway that it exercised over the mental life of all other professions and classes. Medico-astrological superstition had become legalized, and this in spite of the fact that Galen himself at last expressed his distrust of the _Medicina Astrologica_, and at least endeavored to extenuate his part in its dissemination. Let us now scrutinize more minutely the condition of _Medicina Astrologica_ in the second century, A.D. The works of Ptolemy, the “Iatromathematica” of the mysterious Hermes Trismegistus, and the third book of Galen’s writing on the “critical days” furnish sufficient material for outlining the medico-astrological system of that period.