Certain precautions are necessary in its administration. The patient should be generally confined to the recumbent posture; for if, while under the influence of Digitalis, he should suddenly rise, and the heart, already weakened, have further to contend with the force of gravity in the propulsion of the blood upwards, it may actually stop. The dose also should be cautiously regulated, and, if it do not operate at once, it must not be increased, or too soon repeated; for this medicine is apt sometimes to accumulate in the blood, and though several doses may perhaps be given without effect, they are afterwards liable at any time to be suddenly discharged upon the nervous system, and may then produce by their united action a dangerous result. A considerable dose of Digitalis, as of Tartar Emetic, deranges the control of the Vagus nerve over the function of the stomach, and may in this way produce nausea and vomiting. There is no reason for supposing that it irritates the coat of the stomach. (_Vide_ p. 93.) The active principle of Digitalis is secreted from the system in the urine, and probably acts as a true Eliminative Diuretic. But it tends more powerfully to promote diuresis in another indirect way. Any thing which diminishes the pressure on the vascular system, as Digitalis, by weakening the force of the heart, tends thereby to promote absorption, and to increase the secretion of urine. So that the employment of Digitalis in Dropsy is attended with a twofold advantage; by its cardiac action, it prevents a renewal of the dropsical effusion; by its diuretic power, it causes a quantity of fluid to be drained out of the system through the kidneys. That it may do this the more effectually, it is generally considered advisable to combine it with other diuretics. (_Vide_ pp. 251, 304, 308.) TANNIC ACID. Class III. Ord. II. ASTRINGENTIA VEGETABILIA. This is the chief astringent principle of vegetables. It is very soluble in water, crystallizable, and has a strong astringent taste. It is not possessed of those marked properties that are vulgarly attributed to acids; but its solution reddens litmus-paper, and it combines with bases to form salts.