The human/animal bond is never stronger or more tender than when your dog's muzzle turns grey and the spring in her step begins to diminish. After spending the better part of a decade with this beloved member of your family, making sure your canine friend ages comfortably, contentedly and well is a natural priority. And no one knows how to ensure healthy aging better than the renowned faculty of the Tufts Cummings Veterinary School, who treat more than eight thousand older dogs annually.