An anastomosis is a union between the branches of smaller arteries in the form of a tubular loop. Obvious anastomoses occur around the joints of a limb, and they may be of importance in maintaining the circulation if a main vessel has to be ligatured. In other places the anastomosis of arteries provides for an even distribution of blood to the parts supplied; notable exceptions are the "arterial circle" on the base of the brain, and the series of arcades formed by the arteries of the intestines.