Until William Harvey's discovery of circulation in 1628, blood was thought to be produced continually in the liver and sent around the body by a sort of boiling, fuelled by heat generated from the digestion of food. A normal body nourished itself from this blood, excreting waste material in the form of urine, faeces, sweat, and mucus. Haemorrhoidal and menstrual blood were also considered normal forms of excrement, eliminating from the body the ‘dregs’ of blood — clotted, dark, and contaminated by black bile or melancholy, one of the four humours. Certain kinds of ‘blood lettings’ were thus considered quite normal, and were the body's way of ridding itself of waste products.
Blood letting as a form of medical therapy arose from a belief that fever and inflammation, characterized by redness, swelling, heat, and a quick pulse, were the result of too much blood, which had to be eliminated. The most usual way of accomplishing this was to open a vein, usually the basilic vein at the front of the elbow. Alternatively, blood was drawn to the skin's surface, sometimes by applying suction to an animal's horn with a small hole cut in the tip, or by placing a heated glass cup on the skin. When the air inside the cup cooled, it contracted and drew blood to the skin's surface. This was called dry cupping. Commonly, the bleeder would make small cuts in the skin's surface with a lancet or by using a small box with trigger-loaded knives inside, before applying the cup, which would then fill with blood. Another popular method was to apply leeches, ideally selected with great care for their cleanliness and fitness to the task, which were allowed to drop off after drawing their fill. Three or four were usually recommended, but some nineteenth-century physicians advised 50 at once, a blood letting of nearly vampiric proportions.
One of the purposes of blood letting was to reduce fever, which was indicated by a drop in the pulse after phlebotomy was accomplished. Another was to aid nature, that is to help the body rid itself of ‘peccant’ or harmful matter that was causing inflammation at a particular site. Medieval Islamic physicians advocated what is generally called ‘revulsive’ bleeding — blood letting at a place remote from the site of inflammation, in order to draw the peccant matter back into the body where it could be ‘digested’ normally. Followers of the ancient Greek texts attributed to Hippocrates, like the Frenchman Pierre Brissot (1478–1522) and the Englishman Thomas Sydenham (1624–89), advocated the opposite — a ‘derivative’ method in which the blood and peccant matter were evacuated close to the site of the inflammation and on the same side of the body. The latter method was in common use after the end of the medieval period.
Medieval medical thinkers often commented on the attraction of the moon both on the tides and on the flow of blood in the body. Many also seem to have associated increased production of blood with the rise of sap in trees during early springtime. Both these beliefs affected the way blood was let. Barbers, surgeons, and the keepers of bath houses, all of whom bled as part of the practice of their trade, were warned not to open veins when the moon caused a high tide, lest too much blood be released. They also typically let blood in the springtime from people who were not ill as a kind of ‘spring tonic’.
The discovery of the circulation of the blood only increased the popularity of the practice. By the end of the eighteenth century, blood letting was depicted in novels being performed by lay people as a kind of life-saving ‘first aid’ to relieve ‘congestion’ caused by fainting and as the result of accidents. Medical texts before World War II still advocated phlebotomy, either by leeches or by venesection, to ‘lower arterial tension’ and to ‘relieve right side stagnation’ of blood in the heart. Nowadays blood letting seems to us a relic of a former age. Its only common use is to reduce the damaging effects of an excess of red blood cells in cases of polycythaemia vera. Recently, however, the use of leeches has returned, to remove blood from capillaries after delicate surgery.
See also humours; Islamic medicine; medicine.
"blood letting." The Oxford Companion to the Body. . Encyclopedia.com. (May 24, 2019). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/blood-letting
"blood letting." The Oxford Companion to the Body. . Retrieved May 24, 2019 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/blood-letting
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