In the dog the cerebral processes of joy, fear, anger, eagerness (for food), attention, etc., cause temporary inhibition of the gastric hunger contractions. This inhibition takes place by way of the splanchnic nerves, not by a .depression of the vagus tonus. This, again, points to an unusual independence of the vagogastric tonus apparatus. The sight or smell of food on the part of the starving dog does not initiate or augment the gastric hunger contractions. Dr. Luckhardt has recently shown that when the sleeping dog dreams the gastric hunger contractions are inhibited in the same way that cerebral and emotional processes tend to inhibition of the contraction when the animal is awake.
In man intellectual processes (attention, reading, figuring, arguing) have no distinct influence on the course of the hunger period. Actual anxiety causes temporary inhibition (probably through the splanchnics). We have not been in a position to make observations on the effect of actual 4anger, fear, and joy, but there is no reason to believe that these processes act differently in man from that in the dog. In man we have paid particular attention to the effects of seeing and smelhng palatable food, as it seemed a priori reasonable that the impulses generated by these stimuli might make more intimate connection with the vagogastric tonus apparatus. Cannon assumes a "pyschic gastric tonus" analogous to the "psychic secretion" of gastric juice. Glucksmann states that the borborygmi are increased in rate and intensity on seeing and smelling palatable food. He ascribes this to increased gastric contraction. Extensive experiments on Mr. V. and on the author seem to show that this is not the case. These stimuli neither initiate nor augment the gastric tonus and hunger contractions; so far as they influence them at all, it is in the direction of inhibition. One of the tests pn the author might be given. Before beginning the five days' starvation period, our colleague, Dr. Luckhardt, was asked to bring in, unknown to the author, a tray of choice food in the midst of a hunger period. The arrangements being made the matter was dismissed from the author's thoughts. At one o'clock on the morning of the fourth starvation day the subject was asleep and the record showed him to be in the midst of a period of vigorous . and regular hunger contractions. He was awakened to behold Dr. Luckhardt and the assistant enjoying a feast of porterhouse steak with onions, fried potatoes, and a tomato salad. The tray of edibles was placed not more than 4 inches from the subject's face, and the delicious odor of the food filled his nostrils. He felt the hunger pangs as unusually intense, and there was considerable salivation. However, the gastric hunger contractions were not increased either in rate or intensity. In a few minutes, on the contrary, the hunger contractions became weaker and the intervals between them greater, and the period terminated by this gradual depression much sooner than it probably would have done in the absence of the dinner scene. This was undoubtedly due to local acid inhibition from copious secretion of appetite gastric juice.
When the hungry individual sees or smells good food the gastric hunger pangs are more intense, although there is no change or even when there is some decrease in the strength of the gastric hunger contractions. This is therefore a phenomenon of central reinforcement.
Our data on normal men and dogs seem incapable of any other interpretation than that the vagogastric tonus apparatus, so far as it concerns the empty stomach, occupies a unique and physiologically isolated position in the way of nervous control, while the inhibitory apparatus via the splanchnic nerves is readily influenced by central and reflex processes. We feel, however, that these observations must be extended to other groups of vertebrates as well as to such pathological cases in man in which there are indications of abnormalities of the vagogastric tonus before final explanations are attempted or speculation indulged in as to the usefulness of this physiological isolation.
This evidence for the physiological isolation of the hunger mechanism in the way of positive cerebral or central control is of interest in connection with the view that the cravings of hunger and appetite are subjective and largely a matter of habit, and that the periodicity or intensity of these cravings may be altered almost at the will of the individual. Chittenden states this view as follows:
The so-called cravings of appetite are largely artificial and mainly the result of habit. Anyone with a little persistence can change his or her habits of life, change the whole order of cravings, thereby indicating that the latter are essentially artificial and have no necessary connection with the welfare or needs of the body. The man who for some reason deems it advisable to adopt two meals a day in place of three or four at first experiences a certain amount of discomfort, but eventually the new habit becomes a part of the daily routine, and the man's life moves forward as before, with perfect comfort and without a suggestion of craving or a pang of hunger.
Our studies of the hunger mechanism seem to show that the foregoing view is essentially wrong. In the normal individual the gastric hunger periods begin as soon as the stomach is empty and continue (in the absence of inhibitory processes) as long as the stomach is empty, irrespective of the time of day or night, and without reference to the time the individual is accustomed to eat. In individuals accustomed to the usual three meals in daytime and to sleep during the night the gastric hunger periods are more frequent and usually more vigorous during the night (in sleep) than during the day, provided, of course, the stomach is empty. In the normal individual the empty stomach exhibits periodic hunger activity, and there is no evidence to show that this primary automatism of the empty stomach is in the least influenced by eating one or by eating five meals a day. The basis for the view that the time of appearance of the "cravings of hunger" can be changed at will is probably to be sought in the fact that the milder hunger contractions do not enter consciousness as pangs of hunger if the individual's attention is directed into other channels, j They are felt as hunger pangs if the individual's attention is directed toward food and eating. The attention is thus directed, consciously or subconsciously, about the time the individual is accustomed to eat. The periodicity of this subjective attention to the milder hunger cravings can probably be altered by training. But this applies only to relatively mild pangs of hunger. The more severe " cravings of hunger" caused by the gastric hunger tetanus rise above the limen of consciousness, except in deep sleep or under conditions of cerebral process involving intense interest. When an individual who is accustomed to eat three times a day turns to a regimen of one meal a day, the quantity of food ingested in that one meal is much greater than that at any one of the three meals a day regimen. The emptying of the stomach and the appearance of the pangs of hunger are correspondingly delayed. The view that prompt appearance and the persistence of the gastric hunger activity in the empty stomach have no relation to the actual need of the individual for food cannot be seriously maintained for the normal animal.