Abstract
For centuries, Galen's humoral theory held sway. It explained diseases as resulting from an imbalance of the body's four humours: blood, phlegm, black bile, and yellow bile. Since the second century AD many body fluids and the channels they use have been identified: for example, tiny systems are found in the eye, where aqueous humour, constantly being formed by capillaries of the ciliary processes, drain away into Schlemm's canal; and the much larger lymphatic system that conveys many substances and ducts into the main cardiovascular system. Cerebro-spinal fluid has its own circulatory system and also ducts into the blood. These linked circulatory systems also convey invaders, either unbidden or self-inflicted, that may wreak havoc in the body. But these systems can also be deliberately punctured to withdraw fluids for study, analysis, or for other uses, such as the introduction of drugs that are part of an increasing number of combatweapons. Today, we say that humoral immunity requires circulating antibodies. Galen would surely be astonished.
Published Version
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