Two saccular aneurysms: In ten to twenty percent of cases in which one aneurysm is found, a careful search will reveal one or more additional aneurysms. In this slide there are two saccular aneurysms. One (the one on your right) is at the junction of anterior cerebral artery and anterior communicating artery. The one on your left lies at the first branch of the middle cerebral artery. Note that the segment of anterior cerebral artery between the anterior communicating and internal carotid arteries on the left side of the slide is considerably attenuated in comparison with the right. This is an occasional developmental anomaly, and means, of course, that most of the blood supply for both anterior cerebral arteries distal to the anterior communicating artery would have been derived from the carotid system on the right side of the slide. Such anomalies can be of great significance in influencing the outcome of vascular surgery.