From 53 cases of thrombosis of the internal carotid artery in the neck, collateral circulation through external carotid branches was angiographically demonstrated in 34 subjects. Anastomosis between external and intracranial circulations was seen to take place through three ways formed by internal maxillary, superficial temporal and facial arteries. The role of internal maxillary artery is emphasised as the most important pathway in the connections between the two circles.