When cold or hot water is introduced into the stomach through the tubes in quantities of 50 to 100 c.c. at a time, there is nearly always a shrinking of the arm, that is, vasoconstriction. Water at body temperature does not seem to affect the tonus of the vasomotor center. Water at 10° C. causes less vasoconstriction than water at 550 C.
In order to ascertain whether this reflex is due to the influence on the vasomotor center by conscious processes, similar experiments were performed while the subject was asleep. It is not an easy matter to go to sleep with a plethysmograph on one arm and two rubber tubes in one's throat and esophagus. But, after marked physical fatigue it is possible. The same rhythmic variation in the vasomotor tone is observed as before, but perfectly relaxed. There is a gradual increase in the size of the arm as the tonus of the stomach increases. This increase or vasodilation is greatest when the stomach contractions are highest and the size of the arm decreases as the stomach relaxes. From this it seems evident that this activity of the vasomotor center is not due to conscious processes.
Moderately hot or moderately cold water in the stomach does not seem to affect the activity of the vasomotor center during sleep. Very cold water either does not have any very marked effect, or else the stimulation is sufficient to awaken the subject. Warm water did not cause a marked vasoconstriction unless the subject woke up as a result of the injection. The marked vasoconstriction observed when the stomach mucosa is stimulated with very cold or very warm water, therefore, seems to be due mainly to the effect of conscious processes on the vasomotor center.