In 1664, the British scientist Thomas Willis (1621–1675) wrote about a circle of arteries at the base of the brain that act as a traffic circle for the blood flowing to the head. The two major arteries in the neck, the left and right carotids, meet at this circle and branch out into smaller blood vessels that nourish the face and brain. Because of Willis’s thorough illustrations and explanation of this structure, it is known today as the circle of Willis.

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Located on the underside of the brain and encircling the pituitary gland, the circle of Willis joins the carotid and basilar arteries with smaller arteries such as the anterior cerebral artery, the middle cerebral artery, and the posterior cerebral artery, which travel to all parts of the brain. The circle is anastomotic, meaning that its stems branch out and then reconnect, so all the different blood vessels are connected like roads that merge into a traffic circle.

Scientists had observed this circle as early as the 16th century, but it was Willis who first noted its importance in directing the flow of blood. He showed that people could go on living even if one carotid artery was completely blocked or didn’t work, and that when dye was injected into one carotid artery in animals, it stained all the vessels of the brain. This proved that the circle of vessels could redirect blood to both sides of the brain if one major artery became constricted by physical pressure, blocked by fat deposits known as plaque, or interrupted by disease or injury. This ensures that the brain will have the best possible blood supply.

ADDITIONAL FACTS

  1. Thomas Willis was an inaugural member of London’s Royal Society, largely considered the oldest scientific society in existence. His definitive work, Cerebri anatome (the title is Latin for “Anatomy of the Brain”), coined the term neurology.
  2. Willis attributed much of his knowledge and discovery to the English physician and writer Sir Thomas Millington (1628–1704) and to the scientist and architect Sir Christopher Wren (1632–1723).
  3. The circle of Willis varies greatly from person to person; the structure described in anatomical textbooks appears in only about 35 percent of patients.