The substances used for stimulation were sugar (solid and in solution), quinine in weak solution, sodium chloride (solid and in solution), weak solutions of acetic and hydrochloric acids. Tests were made at all stages of activity of the empty stomach. The results were uniform and practically identical for the four kinds of stimuli employed. If the substances were used in sufficient concentrations to affect the stomach activity, the effects were inhibition of the tonus and contractions. These inhibitory effects follow promptly on placing the substances in the mouth, and disappear, on the whole, very soon after removing the substances from the mouth and rinsing the mouth with warm water. Quinine and the acid produced the longest inhibitory after-effects, probably because of the difficulty in completely removing these substances by rinsing the mouth with water.
This gustatory inhibition is, on the whole, proportional to the strength of the stimuli (i.e., the concentration of the substance) and varies inversely with the degree of the stomach activity. Thus a weak solution of acetic acid that produced distinct inhibition during the first stage of a period of hunger contraction when the individual contractions are relatively weak may have little or no effect when placed in the mouth during the tetanus stage of the contractions.
If the gustatory stimuli are weak and allowed to act in the mouth for 5 to 15 minutes, the stomach "escapes" from the inhibition gradually. This is practically true of sweet (sugar). Moderate strength of acids and quinine may hold the stomach in nearly complete inhibition up to 15 minutes. The stimulating substances are, of course, gradually diluted by the secretion of saliva.
Are these gustatory inhibitions primary and relatively simple reflexes independent of the states of consciousness, or are they of the type of conditional reflexes, and therefore due to cerebral states of unpleasant affective tone ? This question must be answered by experiments on lower animals with less development of the cerebrum and especially on decerebrated mammals and on so-called "acephalic" infants.