Although neither the vagus centre nor the vagus ends in the heart are completely paralysed by very high temperatures they are yet weakened so much that digitalis, and probably all drugs which act like it, such as adonidin (p. 331), no longer slow the pulse as they do at normal temperatures. This is shown in Fig. 216, where the pulse remained slow until the temperature rose to nearly 41° C. and then suddenly became very quick. Moderately high temperatures do not prevent digitalis from slowing the heart (Fig. 214).

Action of Different Preparations of Digitalis. - The two most marked effects of digitalis in disease are a reduction in the rate of the pulse, and an increase in the amount of urine. These effects are not coincident, and, according to Christison, the diuretic action is less when the heart is much affected. The preparation generally employed to act on the heart is the tincture, while the infusion is regarded as the best diuretic.1 The differences between the action of the infusion and tincture of digitalis are probably due, in part at least, to the different proportions in which the active principles of the plant are dissolved by alcohol and water.

1 The National Dispensatory.

In digitalis, as in physostigma (p. 904) and many other plants, there is a mixture of principles having antagonistic actions. Digitonin, which has an action very like saponin (pp. 307, 915), will to a greater or less extent antagonise the action of digitoxin, digitalin, and digitalein. Digitonin is readily soluble in water, forming, like saponin, a solution which froths easily. Digitalein is soluble in water, but digitalin is only sparingly so, and digitoxin is hardly soluble in water at all.

The solubility of these substances in alcohol is almost the converse of their solubility in water. Digitonin is sparingly soluble in alcohol, while digitalin and digitalein are readily soluble. Digitoxin is only sparingly soluble in cold alcohol.

From the ready solubility of digitonin in water, infusion of digitalis will contain it in much larger proportion than digitalin or digitalein. This, indeed, is readily seen by putting some infusion of digitalis into one bottle and a corresponding dose of the tincture diluted with water until both solutions have the same bulk. On shaking the bottles, the infusion will be found to froth much more strongly and to retain the froth much longer than the diluted tincture, although the latter also froths strongly.

Tincture of digitalis will, on the other hand, contain a larger proportion of digitalin and digitalein, with probably a small quantity of digitoxin.

It is quite possible, however, that in addition to differences in the preparations due to the menstruum, there may be differences in the same preparation due to the plants used. Thus in Edinburgh the usual dose of the infusion is half an ounce, and this is usually readily tolerated,.while in London I have frequently seen closes of one or two drachms produce considerable gastric disturbance. The infusion of the U.S.P. is nearly twice as strong as that of the B.P., and yet the recognised dose is considerably larger.

Whether these differences are or are not due to the amount and relative proportions of the active ingredients in digitalis plants grown in Scotland, England, and America, is a point which requires investigation, more especially when we have other examples, e.g. cannabis indica, where there is a notable difference between the action of plants of the same species growing in different climates.

Uses. - It is chiefly used as a tonic to the heart, when its action is irregular and feeble, and in dropsy, especially cardiac dropsy (pp. 332, 336).

It is used in functional palpitation, and in the irritable heart often seen in young soldiers, but its chief use is in mitral disease.

In pure aortic disease, with hypertrophy, it is not only injurious but dangerous, since by slowing the pulse-rate it lengthens the time during which blood can regurgitate (pp. 333, 334).

When the aortic disease is accompanied by mitral incompetence and the immediate danger is that from the mitral affection, it may be given with advantage (p. 334). In these cases, whilst taking the drug the patient must be kept perfectly quiet, as there is a great danger of sudden syncope (p. 335).

Digitalis is of great use as a soporific in sleeplessness at night, accompanied by drowsiness during the day, for both these symptoms depend on want of tone in the vessels, the blood gravitating to the feet when the patient is erect and to the head when in a lying posture (p. 194).

It is very useful in haemorrhages, especially when occurring in the lungs, and it has been added to cough mixtures to lessen congestion of the mucous membrane.

It was formerly used in fever and pneumonia, but is now discarded as being of very little use.

In delirium tremens it has been given in very large doses, but its use is dangerous.

In dropsy depending on mitral disease, also in renal dropsy and ascites, it has been used with good effect.

It is very serviceable in some cases of menorrhagia. Its action in this case is due not to contraction of the vessels of the uterus, but of the walls of the uterus itself, since digitalis did not affect haemorrhage from a fungoid growth in the cervix (Dickinson).

It is also useful in spermatorrhoea.

Precautions. - (1) Stop the administration of digitalis on the appearance of sickness or a tendency to faint, or change the preparation of digitalis and lower the dose.

(2) Do not give digitalis in large doses unless you see the patient frequently, and it is necessary to push the drug. Keep the patient in bed, and do not allow him even to sit up in bed, much less to rise, and above all not to rise up and make water, as otherwise fatal syncope may occur (p. 265).

Treatment on Poisoning. - Keep the patient recumbent and give stimulants, e.g. alcohol. Tannin has been recommended in order to precipitate digitalin in the stomach.