If you suggest somebody has Gone Round The Bend you're unkindly inferring that they've gone mad. In the 18th century, the Victorians built hospitals in which to confine the mentally unsound and, at the time, stately homes were built with long, straight driveways so that the building could be seen from the main road in all its splendour. Mental homes, on the other hand, were placed at the end of long, curved driveways in order that they remained unseen, and so therefore if a person had "gone round the bend" it meant they'd been confined.