Peptic ulcer. Illustration of a peptic ulcer in the wall of the human duodenum. The black tube is an endoscope, a viewing device used by doctors to study the inside of the gut. The tip of the endo- scope has just passed through the pyloric sphinc- ter, the muscular aperture that separates the stomach (right) from the duodenum (left). A magni- fied view of the ulcer is shown below in a section of the duodenal wall. The finger-like projections around it are the absorptive villi that line the gut. Peptic ulcers are painful open sores that can occur in the stomach or duodenum. Their occurrence is linked to acidic conditions and presence of the bacteria Helicobacter pylori.