Cerebral artery aneurysm, illustration. Aneurysm of the basilar artery with effusion of blood forming a subarachnoid haemorrhage at the base of the brain. This is a fatal condition, and commonly is a congenital defect in the arterial wall but may occur due to traumatic head injury, or arising from atherosclerosis. If the latter, the media of the arterial wall is replaced by fibrous tissues and destruction of the internal elastic lamina causing a weakness with aneurysmal dilation. Basilar aneurysms may compress cranial nerves and result in neurological deficits. From Bollinger, O. 1901 Atlas und Gundriss der Pathologischen Anatomie, vol 1. Lehmann, Munich.