So far we have been unable to initiate or augment the gastric hunger contractions in man or experimental animals by any sensory stimulation or central nervous processes. We have seen that so far as these nervous processes affect hunger it is in the direction of inhibition. It is singular, indeed, that the inhibitory mechanisms are so readily called into play, while motor reflexes are either inaccessible or lacking, especially since the utility of some of the inhibitory reflexes are open to question at present.
From the point of view of biological adaptation we might expect the vagogastric tonus to be directly affected by voluntary muscular activity and by exposure to cold, since both conditions involve increased oxidation and consequently increased need of food.
Muscular activity may augment the gastric hunger activity by increasing the vagus tonus as well as by chemical changes in the blood. The same applies to stimulation of the cold nerve-endings - of the skin. However, it is probable that if these conditions cause increase in the vagus tonus reflexly, this response is more prompt than that induced by the changes in the blood following the increase or decrease in body metabolism due to stimulation. It is generally recognized that exercise, cold climate, and cold baths increase appetite and hunger. It does not follow that these conditions actually augment the gastric hunger contractions. The increase in hunger and appetite may be only apparent, that is, may reflect a condition of increased excitability of parts of the central nervous system, so that the afferent impulses that give rise to the sensation of hunger and appetite produce a greater central effect. If the gastric hunger contractions are actually increased, this may be due to changes in the blood rather than to increased vagus tonus.
It is well known that exposure of the skin to cold (as by bathing in ice water) may induce contracture or "cramps" of the digestive tract. This is especially the case during the height of gastric and intestinal digestion. These cramps and contractures may be the result of circulatory disturbances or of changes in the blood rather than a direct reflex effect. Central processes are also able to induce contraction of the large intestine and the rectum, as shown by involuntary defecation in cases of great anxiety or fear.