Jean Baptiste Denys: The blood of the calf.

I am the first to recognise that a great deal of scientific advancement has developed from accidents, mistakes or radical misconceptions. I also believe that in science even a negative result is a good thing. Advancement in any direction is still advancement. However rarely in history when a scientist starts with a profound misunderstanding about how the human body works, develops a theory that I am confident today would be considered “conspiracy to commit murder” and executes a plan based on that theory that we hail it as a epoch shattering success purely because nobody actually died.

Jean Baptiste Denys was a doctor. By modern standards he was a doctor in very much the same way I am the Duchess of Cornwall but this is seventeenth century France we are talking about so I will allow the term doctor. He, through a process of highly advanced science noted that the stuff in our bodies is red, and the stuff in the bodies of animals is also red, so it must be the same. Further more it must be interchangeable.

Side Note: I apologise if this is getting a bit advanced, I appreciate not all of my readers will have an MSc in Biochemistry.

On the fifteenth of June 1667 he injected (I refuse to use the word transfused in this case) twelve ounces of sheep blood into a fifteen year old boy who had been bleed with leaches twenty times in order to treat a fever.

I think its worth exploring this a little bit. Our boy Jean, using the rigorous and time honoured ethical standard of “its just a kid, who the fuck cares” applied his theory of all red things must be the same. But first he exsanguinated the boy.

Now I have found about a dozen different types of ounce none of which convert into anything less than 340 ml. I am a nurse and whilst I haven’t been clinical for a while I know one unit of tranfusable blood is usually about 250ml. With only 250ml its not uncommon for transfusion reactions to occur and for patients to become critically unwell and sometimes die. There are strict measures in place to monitor the patient and reverse the reaction if it starts. This can happen with human to human transfusions of the same blood type. There is no way 340ml was pumped into this kids veins and he walks away from it. My best guess here is that he injected it all subcutaneously. I would not be amazed if the child died of infection some days or weeks later. Unfortunately Denys did not record the boys name for me to check.

Side Note: I am not joking, check out my maths here

Following the resounding success of this first human trial Denys moved on to perform a number of other red-liquid exchanges. The next appears to have been on a labourer who under went two of these procedures and survived. However the third and final was on a man called Antoine Mauroy. Not much is known about Mauroy except that he was a “madman” (thankfully his doctor was completely sane) and he was to undergo a number of these procedures until his insanity was fixed. He underwent two successfully however as they were exchanging the third round of calfs blood Mauroy became unwell and died.

Mauroy’s wife, quite reasonably felt that Denys was responsible for her husbands death and took him to court. However Denys successfully convinced the judge that all red things are the same and that Mauroy probably died of arsenic poisoning almost certainly administered by the wife. In the face of such bulletproof logic the judge had no choice but to let Denys go and charge the wife.

Thankfully after this Denys never attempted “transfusions again” and retired from medicine entirely. I would argue this probably indicates a certain amount of guilt or remorse.


I have been gloriously irreverent of Jean Baptiste Denys efforts to understand the human body here. I have compared his lack of ethics to the rigid ethics applied to the world in which I work. Whilst his original notes are freely available, I lack the ability to read early-modern French so I have relied on a translated letter he sent in which he outlines his logic and motives. I have also read a number of articles written by medical historians, all of whom write about this chap glowingly. Most notably “Blood Transfusion and the Body in Early Modern France” written by B. Chin-Yee and I. Chin-Yee and published in Bulletin canadien d’hisoire de la medecine in March 2016. Its actually a really interesting read for anyone interested in medical history.

I genuinely don’t know how so many procedures were performed before anyone died. He seemed to favour drawing blood from an artery in the leg and inserting it into a vein the neck however given I have no idea how bovine cardio-vascular systems work I don’t know if that is relevant.

This work was never really expanded on as other doctors were working on similar processes at the same time. Mostly more successfully, and certainly more ethically. Denys lacked a number of discoveries, including knowledge of the four human blood groups which wouldn’t arrive until 1900.

I recognise that he was trying to find his way out of the dark ages of medicine where interventions like blood letting were common place however I am struck by how much religious zeal his notes contain. Including his choice of the doner animal. According to the bible the blood of the calf is pure of sin.