Heart surgery. Image 4 of 9. Artificial valve in place in the heart during an operation to repair an aortic aneurysm. The valve (black) was inserted during a previous operation. A second operation is necessary because the wall of the aorta (the largest artery that leaves the heart) has become weakened, forming a dilation known as an aneurysm. The dilated wall of the aorta can be seen (creamy white surrounding the valve). Without surgery, an aneurysm can rupture with fatal consequences. During this operation, part of the aorta will be replaced with a prosthetic vessel. For a sequence showing the operation, see images M560/461 to M560/469.