The standard advice to bloodletters, especially in the eighteenth and nineteenth centuries, was "bleed to syncope." "Generally speaking," wrote the English physician and medical researcher, Marshall Hall, in 1836, "as long as bloodletting is required, it can be borne; and as long as it can be borne, it is required."[21] The American physician, Robley Dunglison, defined "syncope" in his 1848 medical dictionary as a "complete and, commonly, sudden loss of sensation and motion, with considerable diminution, or entire suspension of the pulsations of the heart and the respiratory movements."[22] Today little distinction is made between shock and collapse, or syncope, except to recognize that if collapse or syncope persists, shock will result. We know today that blood volume is about one-fifteenth to one-seventeenth the body weight of an adult. Thus an adult weighing 150 pounds has 9 or 10 pounds of blood in his body. Blood volume may increase at great heights, under tropical conditions, and in the rare disease polycythemia (excess red blood cells). After a pint of blood is withdrawn from a healthy individual, the organism replaces it to some degree within an hour or so. However, it takes weeks for the hemoglobin (the oxygen-bearing substance in the red blood cells) to be brought up to normal. If blood loss is great (more than 10 percent of the total blood volume) there occurs a sudden, systemic fall in blood pressure. This is a well-known protective mechanism to aid blood clotting. If the volume of blood lost does not exceed 30 to 40 percent, systolic, disastolic, and pulse pressures rise again after approximately 30 minutes as a result of various compensatory mechanisms.[23] [Illustration: FIGURE 2.--Venesection manikin, 16th century. Numbers indicate locations where in certain diseases venesection should be undertaken. (From Stoeffler, 1518, as illustrated in Heinrich Stern, _Theory and Practice of Bloodletting_, New York, 1915. Photo courtesy of NLM.)] If larger volumes than this are removed, the organism is usually unable to survive unless the loss is promptly replaced. Repeated smaller bleedings may produce a state of chronic anemia when the total amount of blood and hemoglobin removed is in excess of the natural recuperative powers. _When to Bleed_ Selecting a time for bleeding usually depended on the nature of the disease and the patient's ability to withstand the process. Galen's scheme, in contrast to the Hippocratic doctrine, recommended no specific days.[24] Hippocrates worked out an elaborate schedule, based on the onset and type of disease, to which the physician was instructed to adhere regardless of the patient's condition.