The sensation complex of nausea and vomiting can be initiated by stimulation of nerves in the gastric mucosa; but many of the sensory elements in nausea involve mechanism outside the stomach. When nausea is induced by the odor, sight, or taste of disgusting substances, or by the memory of such substances we may call it strictly physiological processes. But most cases of nausea are due to pathological processes or disease. Some of these pathological conditions are pressure, partial anemia, or action of specific toxic substance on the medulla; intense and prolonged pains of peripheral origin, toxemias, hyperexcitability, or excessive stimulation of the visceral nerves.
The sensation of nausea is very complex. A certain feeling of bodily weakness and a characteristic pain and distress ("sinking feeling") in the stomach are always present. With these one may also experience headache, dizziness, chills (from cutaneous vasoconstriction and perspiration), formication, etc. The violent contractions of the pyloric end of the stomach preceding and accompanying the act of vomiting may even be felt as painful.
When nausea is produced by excessive or destructive stimulation of the normal gastric mucosa, or by what may be called normal stimulation (water, food, gastric juice, etc.) of a hyperexcitable mucosa, what sensory elements in nausea are due to this stimulation directly? Since strong contractions of the stomach cause a pressure-pain sensation, and excessive stimulation of the mucosa may also cause pain, it is evident that the tension, pressure, and pain referred to the epigastric region in nausea are of peripheral or gastric origin. The peculiar "sinking feeling" or the feeling of "helplessness" is probably in the main also a direct effect of the gastric states. At any rate that feeling is not experienced except in nausea sufficiently intense to approach vomiting. The weakness, dizziness, and headache are probably in the main indirect effects, that is, due to disturbances in the circulation.
Michael Foster considered "nausea allied to hunger." Boring states that some of his subjects (normal men) confused mild nausea with £he sensation of hunger. Thus 5 per cent of hydrochloric acid put directly into the stomach by a tube produced gastric pain identical with hunger, except that it was more continuous. One of Boring's men who was subject to spells of indigestion reports "when I feel nauseated [from indigestion] I generally stop eating. I begin with my meals again as soon as I feel hungry, but I cannot always tell whether I am hungry or still nauseated."
We believe this fusion or confusion of nausea with hunger is either pathological or else due to superficial analysis. It is true that the gastric factors in both hunger and nausea are uncomfortable tension and pain; that hunger as well as nausea causes salivation ; that in some people hunger as well as nausea includes bodily weakness, headache, etc., but the distinct "sickness" character of the gastric distress in nausea is not even present in any stage of hunger in normal persons. Hunger involves the kinesthetic nerves of the stomach, while nausea is caused by stimulation of nerves in the gastric mucosa. The central effects of hunger and nausea in normal persons are also different. Hunger is compatible with and intensifies appetite and desire for food, while nausea and appetite are mutually exclusive. But since hunger is, and nausea may be, caused by stimulation of gastric nerves, it is likely that in persons with hypersensitive vagi, or with unstable central nervous organization, strong hunger may contain elements of nausea, or actually pass into nausea.
Normal persons may experience hunger pangs and something like mild nausea or "sick stomach,, at the same time, without confusing the two sensations, provided there is a certain degree of hyperexcitability of the nerves in the gastric mucosa. This was noted by the author on the fifth day of starvation. On that day the sensation of hunger was tinged with a "burning" sensation also referred to the stomach. This sensation was probably caused by pressure or acid stimulation of hyperexcitable nerve-endings in the gastric mucosa. It is known that prolonged starvation leads to such hyperexcitability, and in this case there was additional indications of it in the nausea caused by smoking.