Long after the guns fell silent at Antietam, the earth yielded up gruesome reminders of the bloodiest day of the American Civil War: bodies, bones, buttons and entire severed limbs – one of which is now the focus of intense study at the National Museum of Civil War Medicine. A Sharpsburg-area farmer is said to have found the human forearm while plowing a field two weeks after the 1862 battle. Officials at the museum in Frederick, Md., are trying to learn more about the limb in hopes of verifying...