Blood-filled protrusion in the wall of a blood vessel (usually an artery, and particularly the aorta). Disease or injury weakens the wall so that normal blood pressure makes it balloon out. Typically, the two inner layers rupture and the outer layer bulges. In a false aneurysm, all three layers rupture and surrounding tissues hold the blood in place. Symptoms vary with size and location. Aneurysms tend to enlarge over time, and blood-vessel walls weaken with age. Many aneurysms eventually burst, causing serious, even massive, internal bleeding; aortic aneurysm rupture causes severe pain and immediate collapse. Rupture of an aneurysm in the brain is a major cause of strokes. Treatment can consist of simply tying off a small vessel; more serious aneurysms require surgery to replace the diseased section of artery with a plastic graft.