The sensation of pain cannot be produced from the normal gastric mucosa except by stimuli that evidently cause some destruction of cells and nerve-endings, such as strong acids, oil of mustard, absolute alcohol, excessive heat, etc. The normal mucosa can be pressed or squeezed with forceps until crushed, pins may be pushed into the mucosa anywhere, or the mucosa may be incised with a scalpel, without causing pain in abnormal person. The gastric pain caused by strong and destructive chemicals acting on the normal mucosa may be due to over-stimulation of nerve-fibers other than those of "protopathic" pain. In the case of a hyper-excitable stomach mucosa, as in certain types of gastritis, substances normal to the stomach cavity (water, gastric juice) may cause pain. Strong tonus and contraction of the stomach are painful, and it is not certain that the kinesthetic pain from gastric contraction is excluded in all cases of pain from destructive chemical stimulation of the normal mucosa, or normal stimulation of the hypersensitive mucosa, since the motor conditions of the stomach in these states have not been studied sufficiently. Hertz supports the view that all so-called gastric pains, even those induced by destructive chemical stimulation of the mucosa, are due to excessive contractions of the pylorus and the pyloric end of the stomach. There can be little doubt that strong acids on reaching the duodenum will cause temporary spasms or tetany of the upper part of the duodenum, and probably of the pylorus. The precise genesis of these pains must still be considered an open question. In our own experience these pains are too persistent to be due to peristalsis. It they are of purely muscular origin, the stomach and duodenal contractions giving rise to them must be prolonged tonus or tetany contractions. It is probable that the pain is essentially muscular, but the injured mucosa may also be a factor.
Excessive distension of the stomach by food, water, gas, or inflated balloons causes pain referred to the stomach, but there is no evidence that this pain is due to stimulation of the mucosa nerves.
We are thus forced to the conclusion that the only physiological pains from the stomach are the pangs of hunger, and these do not arise from the mucosa nerves. All pains possibly originating in the mucosa are indices of pathological processes, that is, either destructive stimuli acting on the normal nerve, or normal stimuli action on hyperexcitable nerves. These gastric pains will be considered in greater detail in the chapter on hunger and appetite in disease (chap. xvi).