Their careful observations noted mucus discharges, menstrual bleeding, watery evacuations in dysentery, and they very likely observed blood left to stand, which over time separates into several layers, one nearly clear, one of somewhat yellowy serum, one of darker blood. Based on these observations, they hypothesized that there were four kinds of bodily fluids, or “humours”: blood, phlegm, bile, and black bile. (This terminology survives today in the phrase “humoral immunity,” which refers to elements of the immune system, such as antibodies, that circulate in the blood.)
WTF
