Willis, Thomas (1621–1675), British physician. One of the major figures of English medicine in the 17th century, Willis was a founder of the Royal Society. With all his notable achievements, he is known especially for his extensive study of the nervous system. In 1664 he produced Cerebri Anatome (“Anatomy of the Brain”), the most complete and accurate description of the nervous system up to that time. This work contained his description of the circular anastomosis of arteries at the base of the brain that is now known as the circle of Willis. His description was not the first but it was the first complete one and was accompanied by an equally complete illustration.