To examine the inner wall of a patient's stomach, a light tube is inserted into the stomach through the mouth. One set of these light tubes illuminates the corresponding part of the stomach wall by sending light through the optical fiber , the other set shows this illuminated part from the outside. This procedure is known as endoscopy. In this way, the action of the arteries or veins of the blood vessels or the valves of the heart can be seen by inserting the light tube.