Iulius Casserius and the first anatomically correct depiction of the circulus arteriosus cerebri (of Willis).

The circulus arteriosus cerebri is the arterial anastomotic circle at the base of the brain, now better known as the circle or polygon of Willis. The British physician and anatomist Thomas Willis (1621-1675) was the first to demonstrate the physiologic function and observe the clinical significance of the circulus. It has been overlooked, however, that the first accurate depiction of the circulus was provided by the Paduan anatomist Giulio Cesare Casseri (Iulius Casserius) (1552-1616) in two engravings published posthumously in multiple formats, including the Tabulae anatomicae LXXIIX (1627). Casserius was the fifth of the six "Vesalian anatomists" at the University of Padua, Italy, which was the site of the most important discoveries in anatomy in the 16th and 17th centuries. Here we review the life of Casserius, his rise from servant to Girolamo Fabrizio D'Acquapendente (Fabricius) (1533-1619) to Professor of Surgery at the University of Padua, his research in comparative anatomy, and his depiction of the circulus arteriosus cerebri. Although previous authors have commented on Casserius's portrayal of the circulus arteriosus in Table 10 of Tabulae anatomicae LXXIIX, none have discussed Figure 2 of Table 9. This is important because whereas the anterior communicating artery complex is depicted clearly in one table, the accurate course of the posterior communicating arteries is shown in the other. Together, Tables 9 and 10 represent a sophisticated, sequential dissection, which deserves recognition as the first accurate portrayal of the arterial anastomosis at the base of the brain.