Peter Niebyl, who has traced the rationale for bloodletting from the time of Hippocrates to the seventeenth century, concluded that bloodletting was practiced more to remove excess good blood rather than to eliminate inherently bad blood or foreign matter. Generally, venesection was regarded as an equivalent to a reduction of food, since according to ancient physiological theory, food was converted to blood.[9] [Illustration: FIGURE 1.--Chart of elements, seasons, and humors.] Galen defined the criteria for bloodletting in terms of extent, intensity, and severity of the disease, whether the disease was "incipient," "present," or "prospective," and on the maturity and strength of the patient.[10] Only a skilled physician would thus know when it was proper to bleed a patient. Venesection could be extremely dangerous if not correctly administered, but in the hands of a good physician, venesection was regarded by Galen as a more accurate treatment than drugs. While one could measure with great accuracy the dosages of such drugs as emetics, diuretics, and purgatives, Galen argued that their action on the body was directed by chance and could not easily be observed by the physician.[11] However, the effects of bloodletting were readily observed. One could note the change in the color of the blood removed, the complexion of the patient, and the point at which the patient was about to become unconscious, and know precisely when to stop the bleeding. Galen discussed in great detail the selection of veins to open and the number of times blood might be withdrawn.[12] In choosing the vein to open, its location in respect to the disease was important. Galen recommended that bleeding be done from a blood vessel on the same side of the body as the disease. For example, he explained that blood from the right elbow be removed to stop a nosebleed from the right nostril.[13] Celsus had argued for withdrawing blood near the site of the disease for "bloodletting draws blood out of the nearest place first, and thereupon blood from more distant parts follows so long as the letting out of blood is continued."[14]