Cerebral aneurysm. Coloured X-ray angiogram of a berry aneurysm (black, lower centre) at the end of the internal carotid artery, which supplies the brain, eyes and forehead with blood. The big black vessel at the bottom is the internal carotid artery in the base of the brain. This branches in the middle of the picture into a network of smaller vessels (top) that feed the brain. Aneurysms are abnormal swellings in arteries caused by the pressure of blood flowing through a weakened area. Cerebral aneurysms are especially dangerous as they can rupture, causing a life- threatening brain haemorrhage with symptoms similar to stroke.