We have now discussed in some detail the tonus and the contractions in the empty or partly empty stomach, esophagus, and intestine of man and of a number of other species of vertebrates. Do these contractions initiate the sensation of hunger, does the central state or sensation of hunger initiate the contractions, or is there no genetic relation between the two series ? We have seen that these questions have been answered in various ways by physiologists, psychologists, and clinicians, but without conclusive proofs.
We owe the actual demonstration of the synchrony of the hunger sensations with the strong contractions of the empty stomach to Cannon and Washburn. These observers recorded by means of an electric signal the subjective sense of hunger simultaneously with the intragastric pressure, and found that the stomach contractions and hunger sensations ran parallel. The fact that the beginning of the stomach contractions is in evidence before the hunger sensation is felt and that the sensation lasts longer than the active phase of the contraction is adduced by them in support of the view that contractions in some way stimulate afferent nerves in the stomach, and that these impulses give rise to the hunger pangs.
The beautiful demonstration of Cannon and Washburn leaves undecided the question where the contraction stimuli act in the stomach, and the cause of the peculiar periodicity of these contractions. But there can be no further question of the parallel between the stomach contractions and the hunger sensation, even though the evidence that the former are the cause of the latter is incomplete. We do not appreciate the force of Cannon's argument that no other condition than the contractions as the cause can account for the periodicity or intermittency of the hunger sensations. Assuming that the stomach contractions constitute the primary stimuli in the genesis of hunger, does that really solve the problem of periodicity ? It would seem that the problem is only-shifted a little; for these stomach contractions must depend on corresponding periodic rhythmical activities in the gastric muscles (idio-muscular contractions, "nodal tissue" rhythm) or in central or peripheral motor nervous mechanisms. That such rhythm should give rise to the hunger sensations indirectly through contractions in the digestive tract is just as difficult to explain as a similar central nervous rhythm giving rise to or constituting the hunger sensation directly.
In our experiments the subject was either standing, sitting, or lying down. His position was such that he could not see the kymograph or any of the recording apparatus. The signal key or keys for recording the hunger sensation were placed in his hand, and he was instructed to press the key as soon as he felt hunger and to keep on pressing it till the hunger was no longer felt. There was no difficulty in keeping a person's attention fixed on this for shorter periods of 1 or 2 hours and under conditions of hunger of moderate intensity. But in the case of Mr. V., when the observations were continued without interruption for 5 to 6 hours and, therefore, during several periods of strong hunger, he would usually become restless, and unable to give undivided attention to the introspection.
Most of the observations were made within a period of from 4 to 10 hours after meals, and only a few as long as 24 to 120 hours after a meal, for the reason that the hunger pains in many persons become gradually severe to the point of discomfort, and the man becomes restless and tired.
As a check on the intragastric respiratory pressure, records of the respiratory movements (chest pneumograph) were usually taken simultaneously with that of the stomach movements.
The general results of our work on more than fifty men are in complete accord with those of Cannon and Washburn. When the empty stomach shows strong contraction, the subject invariably signals that he feels hunger, and, on being questioned, he invariably replies that he feels the hunger in his stomach. There is, on the whole, a fairly close correspondence between the duration of stomach contractions and duration of the subjective sensations of hunger. On days when the stomach does not exhibit these strong contractions, the person does not feel hungry. These "hunger contractions" of the empty stomach are primarily those of the strong periodic rhythm already described.
Relation between the strength of the stomach contractions and the intensity of the hunger sensation.-Data on this point were obtained in the following manner: Three signal magnets were arranged to record on the drum perpendicularly to the recording point of the bromoform manometer, and the corresponding keys were placed in the subject's hand. He was then instructed to press key No. i when he felt, without question, even the faintest hunger; No. 2 when he felt hunger of moderate strength; and No. 3 when he felt the strongest hunger.
The subject presses key No. 1 (weak hunger) at the beginning of a contraction period when the individual contractions are relatively feeble. Then, as the contraction increases in strength, there comes a period of vacillation between key No. 1 and key No. 2 (moderate hunger). As the contractions grow still stronger, key No. 2 is used for a while without any change. Then follows a period of alternation between key No. 2 and key No. 3, and in the final stage of maximum activity of the contraction period the signal is made with key No. 3 exclusively. In other words, there is a fairly close correspondence between the strength of the stomach contractions and the degree of hunger sensations experienced simultaneously.
This account applies particularly to the first hunger period appearing after a meal and for the milder hunger periods in general. On more prolonged fast, that is, after having experienced several hunger periods in succession, the subject may not signal with key
No. 1 at all, and sometimes not even with key No. 2, but starts in with key No. 3 (strongest hunger) at the very beginning of a period, despite the fact that the strength of the stomach contractions is not greater (or might be even less) than those designated as very mild or moderate hunger some hours or days earlier. This seems to indicate an increased excitability of the afferent nerves in the stomach, or an increased excitability of some parts of the brain.