Gallstones. Coloured X-ray of gallstones (green) in a patient's gall bladder. Part of the spine (top right to bottom centre) and ribs (upper left) are seen in this frontal view. An endoscope tube runs to lower left. Gallstones are hard deposits of cholesterol or salts that may cause acute pain. The gall bladder stores bile that arrives from the liver via the hepatic duct (white, above and left of endoscope). Bile passes into the digestive tract (white), along with secretions from the pancreatic duct (purple/white, below and right of endoscope). These ducts have been injected with a contrast agent from the endoscope in an ERCP (endoscopic retrograde cholangiopancreatography).