"Nature is not always the best designer, at least when it comes to things that humans must build and maintain. So the newest artificial heart doesn’t imitate the cardiac muscle at all. Instead, it whirs like a little propeller, pushing blood through the body at a steady rate. After 500 million years of evolution accustoming the human body to blood moving through us in spurts, a pulse may not be necessary. That, in any case, is the point of view of the 50-odd calves, and no fewer than three human beings, who have gotten along just fine with their blood coursing through them as evenly as Freon through an air conditioner."