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A historical explanatory system in which health and disease depend on the mixture and balance of bodily fluids. The four-fluid scheme includes blood, humoral phlegm, yellow bile and black bile. These categories are not a modern analysis of blood composition. ^[ref]

By 1867 the central change was from confidence in bloodletting as a general remedy to a disputed, increasingly restricted practice tested against clinical outcomes. Bloodletting includes opening a vein, local wet cupping, and the application of medicinal leeches. The history must explain both the theories that supported it and why clinical comparisons weakened that support.

Ancient explanations and qualifications. In the Hippocratic Corpus, disease could be explained through disturbances of bodily fluids. The four-fluid account underlying humorism named blood, humoral phlegm, yellow bile and black bile; restoring their proper mixture offered a rationale for evacuation. This was a historical explanatory system, not a modern description of blood composition. Nor was ancient practice uniformly indiscriminate. Aulus Cornelius Celsus describes bloodletting as a powerful intervention whose suitability depends on bodily strength and whether the condition represents excess rather than exhaustion. Galen likewise distinguishes whether evacuation is needed, whether it should be by venesection, and whether the patient can tolerate it; he discusses the site, quantity and repetition. Thus the ancient debate already included selection and restraint. ^{[ref] [ref] [ref]}

Medieval continuity and the instruments. The Canon of Medicine by Ibn Sina preserves a carefully qualified doctrine of venesection, including cases where removing good blood would leave harmful fluids behind. Its reasoning belongs to humorism; its distinctions should not be mistaken for verified modern indications. In late medieval Europe, bloodletting was also organized by calendars: a surviving Augsburg calendar for 1472 specifies supposedly favourable times according to the patient and the Moon. This documents an additional astrological framework, not evidence of therapeutic efficacy. ^{[ref] [ref]}

The material distinction between general and local removal mattered. Venesection opened a vein with a sharp instrument. Wet cupping combined superficial cuts in the skin with suction under a cup; Celsus describes bronze cups in which burning material reduced the enclosed air's pressure, and horn cups from which air was sucked through a small opening. Dry cupping did not include the cuts and therefore did not itself remove blood. Medicinal leeches provided another means of local removal. These instruments made depletion visibly demonstrable, but the visible removal of blood did not establish that a disease had been removed. ^[ref]

New physiology did not immediately settle treatment. William Harvey's 1628 account of blood circulation undermined the older picture of blood simply being supplied and consumed in the parts. His observations of venous valves and experiments with arm ligatures showed directional movement: filling below a ligature and the behaviour of compressed veins helped demonstrate return towards the heart. Nevertheless, a circulating blood supply could still be invoked to explain relief through depletion or redistribution. Establishing blood circulation and establishing the benefit of bloodletting were different questions. ^[ref]

In the early nineteenth century, François-Joseph-Victor Broussais's emphasis on local irritation and inflammation encouraged extensive use of medicinal leeches. This illustrates why changing theory could increase bloodletting: a local inflammatory explanation supplied a fresh reason to remove blood near the supposed seat of disease. The accompanying claims of efficacy still needed comparison with what happened under other treatments. ^[ref]

Comparative evidence weakened general claims. In 1835 Pierre Charles Alexandre Louis asked whether comparable patients with pneumonia did better when first subjected to bloodletting during days 1–4 than when first bled later. He explicitly considered age and other differences. This numerical method in medicine shifted attention from selected recoveries to outcomes in defined groups. His work restricted confidence in a large, general benefit; it should not be presented as a randomized controlled trial proving that every use was ineffective. In modern terms, these were observational studies: confounding by indication could make the earlier-treated group differ in severity, and surviving long enough to enter the later-treated group could introduce selection bias. Counting cases was an advance, but did not automatically remove these problems. ^[ref]

The dispute still alive in 1867. William Pulteney Alison defended earlier experience of benefit, while John Hughes Bennett opposed routine depletion and developed a restorative approach. One defence of the altered practice was the change-of-type theory of disease: formerly vigorous or “sthenic” inflammatory illness was said to have become weak or “asthenic”, so that measures once borne well were now inappropriate. The rival explanation was that understanding of pathology and the evidence about treatment had improved. Alison's 1856 argument and the contemporary 1858 review show that this was an active disagreement, not a completed unanimous rejection. ^{[ref] [ref]}

Bennett's 1864 discussion made the theoretical contrast explicit: an account centred on altered blood and vessels favoured depletion, whereas his account of tissue irritation and inflammatory exudation led him towards the opposite practice. That historical account of inflammation should itself be evaluated historically, rather than adopted as current pathology. The durable methodological lesson is that recovery after treatment is insufficient evidence of recovery because of it: acute disease may resolve naturally, as even the Latin extract acknowledges. Changes in theory, recognition of the patient's limited reserve, and comparative clinical observation together explain the retreat from routine bloodletting. ^[ref]

Present-day distinction. Contemporary venesection has specific indications: in haemochromatosis, removal of red blood cells helps reduce excess iron; in polycythaemia vera, it can reduce an excessive red blood cell burden. Those uses concern defined abnormalities, rather than correction of historical humorism or a general remedy for fever and inflammation. They are a modern explanatory coda, not knowledge attributed to the 1867 candidate. ^{[ref] [ref]}