In reality only a few millimetres in diameter, it is made huge and menacing by the microscope – a great pink-red trunk of an artery which ominously pulses in time with the heart-beat. I need to follow it deep into the cleft – known as the Sylvian fissure – between the two lobes of the brain – to find the aneurysm in its lair, where it grows off the arterial trunk.
