The circle of Willis was dissected in 75 Macaca mulatta monkeys, and the anatomy is described and compared with several series of human specimens. The principal difference between monkey and man is the presence of a single distal anterior cerebral artery in the former, and the most common anomaly in the moneky is an anterior communicating artery proximal to the junction of the anterior cerebral arteries.
A large vessel joining the internal carotid arteries and an accessory branch of the anterior cerebral artery, probably communicating with the external carotid circulation, were found in a small number of our specimens. In man the posterior communicating is significantly reduced in diameter far more frequently than the anterior communicating artery, whereas the reverse situation obtains in the monkey.
In general, the intracranial distribution of blood in the moneky is the same as in man, because the similarities in the normal anatomy and variations i the circle of Willis outweight their differences. However, physiological data on collateral cerebral circulation will be more meaningful when it is based on the arterial anatomy in each preparation.