All who have studied the origin of the sensations of fulness and satiety seem to agree that they do not originate in the mucosa of the stomach, and our own results agree with this view. The literature is extensively reviewed by Hertz.
Hertz and his co-workers inflated the stomach of two healthy men with air through a tube connected with a manometer. They found that a "sensation of fulness or tightness in the upper part of the abdomen, associated with a desire to eructate, was felt as soon as the intragastric pressure reached respectively 12 and 14 mm. of mercury in the two cases. The pressure fell after 20 seconds by approximately 2 mm., owing apparently to relaxation of the tone of the stomach, and simultaneously the sensation of fulness disappeared. Now, on slowly injecting more air, the pressure gradually rose to its original height and the sensation reappeared; it again disappeared after 20 seconds, the pressure simultaneously falling 2 mm., after which it remained constant. Exactly the same rise in pressure and the same sensation of fulness, followed by a fall of pressure and a disappearance of the sensation, were produced four times in succession by injecting air, none being allowed to escape in the interval. These observations proved that the tension exerted from within on the circular muscle fibers of the stomach is the cause of the sensation of fulness."
That this is the main source of its origin seems quite clear. There is a possibility that the stretching of the abdominal muscles as well as the pressure on other structures in the abdominal cavity contribute to the sensation of fulness. As a proof that this element has little if any significance in giving rise to the sensation, Hertz related experiments on persons with atonic stomachs. When 6 gm. each of sodium bicarbonate and tartaric acid are swallowed separately, 1,700 c.c. of carbon dioxide are given off under atmospheric pressure at body temperature. This invariably causes an unpleasant and sometimes painful sensation of fulness in normal individuals, while in persons with atonic stomachs it does not cause any sensation of fulness. Hertz, therefore, concludes that "the sensation of fulness in the stomach is due to tension on its muscular coat, and depends very little and only in extreme cases on the stretching of the abdominal wall."
To these observations of Hertz we can add our negative findings in regard to the gastric mucosa as a contributing factor. Chemical or mechanical stimulation of the mucosa (whether the stomach is in strong or in feeble tonus) never produces a sensation similar to that of fulness. At the same time, it must be noted that mere tension on the muscular coats, that is, intragastric pressure, will not under all conditions give rise to the sensation. The degree of intragastric pressure required to cause a feeling of fulness, according to Hertz, is frequently exceeded at the height of a period of hunger contractions of the empty stomach, when a distended balloon is in the stomach, yet the sensation referred to the epigastrium in these conditions is that of emptiness, not fulness. It is therefore clear that a certain amount of tonus reaction of the stomach must be present before tension or pressure on the walls of the stomach produce the sensation of fulness.
The sensation of satiety felt after an abundant and palatable meal involves several factors, none of which appear to depend primarily on the nerves in the gastric mucosa. One must have some degree of hunger and appetite before eating, the food must be palatable, and enough food must be consumed to produce moderate distension of the stomach. If either of these factors is lacking, complete satiety is not felt after the meal. One may feel great hunger and consume unpalatable food till the sensation of fulness develops, but satiety does not follow. If the hunger factor is present and the food is to the king's taste, but of insufficient bulk, although more than ample for the needs of nutrition, satiety is also lacking. Hence the sensation complex of satiety involves the element of contrast between the uncomfortable tension of j hunger and the sensation of fulness, together with the lingering' memories of the taste and smell of the food. Normal motor, secretory, and sensory functions in the stomach are thus a prerequisite rather than a direct factor in the genesis oi satiety.