i.e. the fevers caused by blood called synocha and synochus. Synochus comes from corrupt blood within the vessels, and synocha from an excess of blood without putrefaction. Galen1 says, since the blood is the friend of nature it may be corrupted and2 brought under her ruling; if it be brought on account of sharpness of choler and become corrupt, the thin part of it turns to choler and the thick part to melancholy; and the contrary of this i.e. that sanguine humour is capable of putrefaction and corruption and generates fever. The doctors agree with this statement, and it is not right to oppose their sayings, but to treat them with honour: nevertheless we have said the blood is naturally in the veins and arteries and is ruled by nature and her instruments, that is by the natural heat. When choler penetrates thither it disturbs their natural moderation, and sometimes it grows hot of itself naturally and becomes corrupted [or] the thin part is burnt and nature heats it. And when nature overcomes it, it is evacuated by crisis, or by nose-bleeding, or the haemorrhoidal or menstrual flow. At another time it cannot be evacuated from the veins, but settles in some member forming the imposthume called phlegmon.
p.21Sometimes nature can do neither of these things, but [the matter] remains in the veins externally, and when brought under [the ruling of] nature it is corrupted and causes acute fever.