By 1664, the year he published his most famous book of neuroanatomy, Cerebri Anatome, Dr. Thomas Willis was already renowned in Britain for saving lives.

Fourteen years earlier, the corpse of executed murderer Anne Green had been delivered to Willis and some of his colleagues for autopsy. Upon opening the coffin–the story goes–the doctors heard a gasp. Ms. Green, they discovered, had been hanged but not executed. Thanks to the resuscitation efforts of Willis and his colleagues, Green survived, and was given a stay of execution. She died fifteen years later.

The episode supposedly drew jealousy from Willis’s contemporaries, who could have had no idea just how many lives Willis’s work would one day save. Among the important discoveries included in Cerebri Anatome, considered the founding text of neurology, is the Circle of Willis, a map of the interconnecting arteries at the base of the brain.

Such circular connections among arteries are called anastomoses. They enable blood to reach vital tissue along multiple routes so that when one is blocked, the blood has an alternative outlet.

The Circle of Willis is perhaps most important because of its implications for stroke. Stroke, which is the third leading cause of death in this country, occurs when blood flow to the brain is disrupted. This can occur when an artery gets blocked with plaque or a clot (called an ischemic stroke) or when at artery bursts (called hemorrhagic stroke). Many of these problems, particularly the latter kind of stroke, occur in the Circle of Willis.

In Willis’ day, the only way to see these arteries was in autopsies, when–with the exception, perhaps, of Ms. Green–it was too late to help. By 1793, however, doctors were already intervening surgically, on the basis of discoveries by Willis and others, in stroke cases to save lives.

Nowadays, patients presenting to the emergency room with symptoms of stroke will undergo brain imaging. Radiologists will use the Circle of Willis, among other clues, to help figure out where the problem is. The appearance of the circle can also help them to figure out whether a blood vessel is clogged (ischemic stroke) or bleeding (hemorrhagic), an important piece of deciding whether to use clot-busting medications like TPA. (This type of medication must be used within about four and a half hours of the start of stroke symptoms. Call 911 if you or anyone else is experiencing the symptoms of a stroke.)

Of course, there is no substitute for prevention. Lifestyle changes like diet and exercise can help prevent up to 80 percent of strokes. Willis studied the effects of conditions like diabetes on blood vessels in the brain, too (in fact, an outmoded name for diabetes is Willis Disease 1).

May is stroke awareness month. Let’s celebrate the lives saved thanks to a 350-year-old neuroanatomical discovery!