Virchow regards embolism of a small vessel as the origin of ulcer of the stomach. Cohnheim disproved this beyond doubt by showing that there is an abundant circulation in the walls of the stomach by which the parts affected are again quickly supplied with blood. Klebs takes for granted a spasmodic contraction of single bloodvessels as the cause of the retardation of the circulation, while Rindfleich attributes it to the poor anastomotic connection of the gastric veins. He calls attention to the frequent coincidence of ulcer and hemorrhagic infarct in the walls of the stomach. Cohnheim injected chromate of lead into the gastric branch of the splenic artery in animals, and when he succeeded in cutting off the arterial supply of the mucous and submucous layers _only_, he found as a result large ulcers with sharp, well-defined margins and a circular base. If the animals were examined in the second week after the experiment, they showed several small ulcers in place of the larger one. In the third week the ulcers were found to have healed. From these experiments you can see that the gastric ulcer has a natural tendency to heal when not interfered with. By experiments such as these it has been proven beyond doubt that disturbances of circulation of a small part of the stomach may lead to ulcer. But the causes of these disturbances, and the reasons why some ulcers do not heal, are still disputed questions. Pavy claims that the alkalinity of the blood prevents the gastric juice from acting on the walls of the stomach. When he introduced acids into the stomach and allowed the circulation of the blood to continue, no ulcers resulted; if he impeded the circulation, the stomach was digested by its acid contents. Samelson instituted experiments to test the statement of Pavy. He introduced large quantities of various acids into the stomach of his animals without observing ulceration as a result; he also neutralized the blood by the injection of weakened acids into the bloodvessels, but no ulceration followed. But he did not impede the gastric circulation in his experiments, while Pavy did, hence the difference in their results. Clinical experience, however, favors Pavy's views. We can prevent the further progress of the gastric ulcer by the use of alkalies, while acids only favor its growth. These questions still need additional research before they are definitely solved. Gastric ulcer may occur in any part of the digestive tract which is exposed to the action of the gastric juice; hence it is found in the lower part of the œsophagus, any part of the stomach and the upper part of the duodenum. It is found most frequently in the pyloric end of the stomach, because this part is most frequently subjected to mechanical irritation and to the action of the gastric juice. The shape of the ulcer is usually conical or terraced, its diameter being largest in the mucous membrane and smallest at its base, in the deeper structures.