Thomas Willis was a 17th-century English doctor who, among other things, coined the term "neurology" and collaborated with Christopher Wren (who did the drawings) to produce the first accurate description of the structures of the human brain. The story of Willis's research and medical practice is told in Carl Zimmer's book Soul Made Flesh: The Discovery of the Brain ... (there's more to its title, but it's just marketing ...).
Zimmer talks a great deal about the historical context of seventeenth-century England, not just to add color but because Willis's biography (like that of anyone living through it) was strongly influenced by the English Civil War. After the Restoration and the Great Fire of London, Willis moved his practice from Oxford to London and became the most famous physician in England (in part because of his work with Wren describing the brain). Zimmer writes of his medical practice as follows:
His theories about how people became sick and how the body worked were hugely popular. Yet his remedies remained, as ever, generally useless. He continued prescribing his eclectic blend of medicines—a blistering plaster for one patient, a ground-up millipede for another—and claimed success when his patients recovered and escaped blame when they didn't. If they recuperated, they usually did so in spite of his attentions.
The popularity of a medical treatment bears no necessary relationship to its effectiveness. Its popularity might well be based on other things—in this case, the fame of Willis's research—or on logical fallacies, such as the clear post hoc, ergo proper hoc moment in the above passage. Perhaps ground-up millipedes are good for something, but no matter how much he had discovered about the structure of the brain, Willis was probably just making it up when he decided that millipedes would do the job.