During the period of strong stomach contractions the rate of the heart beat is increased. At the cessation of the contraction period the heart rate becomes slower again. The return to the normal rate is gradual. If the motor activities of the empty stomach are very vigorous, so that the pauses between the contraction periods are relatively short, the pulse rate may not return quite to the normal during these pauses. The average increase in the rate of the heart beat during the hunger contractions of the stomach is eight to ten beats per minute. But strong individual contractions or tetanus periods may show an increase of thirty beats per minute.
When the details of the contraction periods are further analyzed, it is found that the greatest acceleration of the heart beat is on the whole synchronous with the individual strong contractions, and that the pauses between the contractions usually show less acceleration. It is not known whether this effect on the heart is a reflex from the stomach, or due to a direct lowering of the vagus tonus.
It is well known that the vasomotor center is acted upon not only by practically all afferent impulses, but also by the conscious centrifugal impulses from the cerebrum. None of these disturbing factors can be controlled completely. The most we can do is to endeavor to make the external conditions and the cerebral processes as uniform as possible. This was attempted in two ways during these experiments. The subject was permitted to read stories; he was required to add figures, or his eyes were covered and he was instructed to think of nothing in particular. Aside from the varying cerebral states, auditory stimuli are the greatest disturbing factors.
The periods of strong contractions of the empty stomach are synchronous with great variations in the vasomotor tone, and in most cases the vasomotor variations exhibit a rhythm similar to that of the stomach. There is an increase in volume of the arm (vasodilation) pari passu with an increasing tonus of the stomach and with the beginning of the individual contractions, but the arm begins to shrink (vasoconstriction) before the stomach contraction has reached its maximum. Or the volume of the arm shows a definite increase parallel with the strong contractions, and a corresponding diminution in the arm volume during the stomach pauses. These two types of vasomotor rhythms were the ones usually obtained during periods of moderate hunger contractions. If the experiments were continued for long periods (4 to 6 hours), so that the hunger contractions and hunger tetanus became very strong, the vasomotor variations were in evidence, but the rhythms were rarely synchronous with the stomach rhythms. In such instances the subject gave unmistakable signs of restlessness. Under these conditions the vasomotor rhythm may be slightly faster or slightly slower than the stomach rhythm, or the two rhythms may be practically identical, but the contraction phase of the stomach activity may be synchronous with a decrease in the volume of the arm (vasoconstriction).
Before the appearance of the first period of hunger contractions after a meal, and during the pause or relative quiescence of the stomach between the periods of moderate hunger contractions, another type of vasomotor rhythm appears in the plethysmographic records. Considerable attention was given to this rhythm, because the rate of it suggested some correlation with the "20-seconds rhythm" of the empty stomach. These slight variations in the arm volume are frequently irregular, so that its rate cannot be made out with certainty, but when the fluctuations are fairly regular the rate corresponds closely to the "20-seconds rhythm" of the empty stomach. The contraction and relaxation phases of the two rhythms do not seem to correspond, but that is of little significance in view of probable difference in the latent time of the respective neuro-muscular apparatus as well as of the recording devices. This type of vasomotor rhythm is most marked when the "20-seconds rhythm" of the stomach is the strongest, that is, at the beginning of a period of strong hunger contractions.
In view of the fact that the vasomotor center is acted on by so many factors, central and peripheral, the parallelism between the vasomotor tone and the motor activity of the empty stomach during the hunger period is too regular to be accidental. When the synchrony fails, this is probably due to central or peripheral factors that cannot be controlled or recorded. The question, then, remains how this synchrony is brought about, and what is the biological significance of it. (i) The co-ordination may be due to associations between the vasomotor center and the center for gastric tonus in the medulla. (2) It may be due to a direct action on the vasomotor center by afferent impulses from the stomach initiated by the stomach contractions. (3) Or, it may be due to the influence on the vasomotor center of the conscious processes of hunger, which are caused by the stomach contractions. Brodie and Russel, and Miller have studied the effect on the blood pressure of stimulation of the central end of the gastric branches of the vagi in animals under general anesthesia. The changes in the blood pressure are variable and complicated by respiratory changes and vomiting movements. In dogs and cats the primary effects may be either an increase or a decrease in the arterial pressure, while in rabbits the stimulation seems to cause a rise in the blood pressure only. It will probably be difficult to secure experimentally the selective stimulation of the afferent gastric nerve fibers that are stimulated by the contraction of the empty stomach. There may be some connection between the vasomotor rhythms described above and the well-known Traube-Hering blood-pressure variations, as the latter are frequently induced by experimental interferences with the vagi.