By means of compositional devices and a magisterial use of the Greek language, Galen achieves an accurate and didactic exposition of the vivacity of oral speech. Dealing with a humoral unbalance that is the basis of many local and general affections, this small treatise also presents key notions of Anatomy and Physiology. On those grounds it became a bestseller in the Middle Ages and the Renaissance with translations into Syrian, Arabic, Latin (8 versions) and Hebrew, as well as 14 commentaries from 1290 to 1567. This first critical edition presents this classic as a continuous text without chapter divisions and points to the internal structure created by the author in order to make his text readily accessible -- and unforgettable. A thorough introduction, explaining the textual transmission and the treaty's fundamental topics, is joined by a comprehensive commentary that deals with alteration and pain, qualities and humours, inflammation and fevers.