Abnormal dilation of part of an artery. Most aneurysms occur in the aorta, but they are also found in the femoral and cerebral arteries and the left ventricle of the heart. In most cases aortal aneurysm is caused by atherosclerosis: the calcified arterial wall loses elasticity and stretches through the pressure of the blood, increasing the diameter of the artery. In a cerebral aneurysm the cause is usually a congenitally weak spot in the wall of the blood vessel. Cardiac aneurysm develops after cardiac infarction has caused weakness in the wall of the left ventricle.