William Harvey's monumental achievement in discovering the circulatory system inspired two of his friends to dabble in the study of blood — Christopher Wren, the architect who designed St. Paul's Cathedral and other remarkable London buildings (Wren was an astronomer before he turned to architecture), and Robert Boyle, a pioneer in modern chemistry.
The men were all members of the Experimental Philosophy Club in Oxford, England, and admirers of the work of Francis Bacon, who advocated first-hand investigations into the natural world, rather than accepting long-held orthodoxies.
At the time, it was thought that the blood was impervious to anything that came from the outside world. Using a prototypical syringe made of a quill and a bladder, Wren and Boyle injected dogs with opium and other drugs, and showed that the dogs were affected — that they reacted to the opium, for example, by falling asleep.
These experiments inflamed the scientific community, and no end of creatures were injected with every kind of fluid, from urine to milk, sometimes with fatal results.
Richard Lower, an Oxford-trained doctor and protege of Wren and Boyle's, in 1665 decided to see what happened when he injected a dog with blood from another dog, connecting the two vein-to-vein. The experiment failed. The blood just pooled up in the connecting tube, Douglas Starr relates in his book, Blood: An Epic History of Medicine and Commerce.
Then, Lower tried tapping an artery in the donor dog, and this time the experiment worked. The stronger pressure from the arterial blood made for a successful transfusion, leading Lower to reason that "one Animal may live with the blood of another," Starr writes. Lower's experiments set off a frenzy for transfusions in England and, soon, in France.
Jean-Baptiste Denis, one of the French King Louis XIV's doctors, thought he might cure violent people of their rages by transfusing them with the blood of gentle animals like calves and sheep. At the time, people believed that blood contained a sterotypical set of characteristics of the creature that possessed it. For a while, it looked like Denis had had a stroke of genius, as one violent character in particular seemed for awhile utterly transformed.
Lower was furious, accusing Denis of stealing his work. Meanwhile, some human transfusion subjects began to die (blood being much more complicated than these men understood), including some high-profile patients of Denis. The French Parliament banned transfusions in 1670, followed by the British Parliament and eventually the pope.
That was the end of transfusions in Europe until the early 19th century.
Still, Starr writes, these early researchers "cracked the wall of humoral medicine, showing that the body was ruled not by vague humors but by chemicals, vessels and pumps."