Many Ancient and Medieval ideas about how the human body worked seem laughable now, but before the science of molecular biology developed, a lot of conclusions just had to be conjecture (fueled also by cultural and confirmation bias). And that’s why – well into the early modern era – many scientists believed that menstrual blood in the uterus traveled up a vein and turned into breast milk.
Seriously. This vein — which some authors called the “kiveris vena” or “the female vein” turned out to be harder to find than the lost city of El Dorado. Of course, this is because it wasn’t actually a thing. But here is a quote from a 12th-century anatomist known as Master Nicolaus. “When the _os uteri_ closes after conception, the menstrual blood is retained and part of it is transported by these veins to the breasts, where it is modified and transsubstantiated into the essence of the milk which provident ministering nature provides and prepares as nutrient for the foetus when it sees the light. The remainder of the menstrual blood is taken to nourish the foetus while it is in the maternal uterus.
These ideas go back as far as the 1st century physician Galen. (P.S., many also thought sperm was transmorphed blood.) The first image shows the _kiveris vena_ of a pregnant woman very clearly. The second is from a sketch of Leonardo da Vinci. Even though he was famous for using cadavers to learn the correct anatomy of his drawings, he didn’t quite get everything right. You have to squint, but the “female vein” is there, too.
Source(s): First image Anatomische Abbildung der Frau Ausgabe Venedig 5 Feb 1493 from”Fasciculus Medicinae.” Second is Leonardo da Vinci, RL 19097v, copyright 2017 Windsor Castle, Royal Library. Quote from _Anatomia Magistri_ by Nicolai Physici and other information from “Leonardo da Vinci and Kethem- Kiveris Vena,” by Antonin Dolezal, Zita Skorepova, and Karel Jelen, in _Neuroendrocrinology Letters_, Volume 33, No 8, 2012.
