Why DOES the living stomach of a warm-blooded animal not digest itself? This question has intrigued physiologists for almost two centuries, yet it still constitutes one of the major mysteries of our science. To some people, the problem may seem to be purely academic—one that might have fascinated the medieval philosophers, had they had a suitable conception of the process of digestion to start with. In actuality, however, the answer to this question is the key to the etiology of peptic ulceration, because this disease process is essentially just such a process of autodigestion. In the normal person, digestion of the gastrointestinal mucosa by his own gastric juice is evidently prevented by the operation of some protective mechanism, but in the ulcer patient this agent appears to have suffered impairment, at least in isolated areas. Hence, the answer to our question rests entirely on the character and action of this