[The developmental mechanism of occipital-vertebral anastomosis (author's transl)].

Four cases of the occipital-vertebral anastomosis, demonstrated on vertebral arteriogram, were reported; three on the left side and one on the right side. The occlusion in the vertebral artery at the posterior end of the horizontal segment or the more distal segment of the vertebral artery was found in two cases and no occlusion involving the carotid and vertebral arteries was present in the remaining two cases. The direct type of the occipital-vertebral anastomosis was found in all of the four cases in the atlanto-occipital space. The developmental mechanism of the anastomosis was explained based upon the author's hypothesis on the embryonic development of the occipital artery.