Preparation. - Dissolving out digitalin from an alcoholic extract of the leaves by acetic acid and water, decolorising by animal charcoal. Neutralising by ammonia and precipitating the digitalin by tannic acid. Rubbing with oxide of lead and spirit, to remove the tannic acid. Dissolving out the digitalin with spirit, again decolorising by animal charcoal, evaporating, and purifying by washing with ether.

Characters. - In porous, mammillated masses or small scales, white, inodorous, and intensely bitter.

Solubility. - It is readily soluble in spirit, but almost insoluble in water and in pure ether; dissolves in acids, but does not form with them neutral compounds.

Reactions. - Its solution in hydrochloric acid is of a faint yellow colour, but rapidly becomes green. It leaves no residue when burned with free access of air. It powerfully irritates the nostrils, and is an active poison.

Dose. - 1/60-1/30 of a grain.

Chemistry of Digitalis. - Formerly the active principle of digitalis was said to be digitalin, but the substances prepared and sold by different manufacturers under this name varied much in their solubility and in the intensity of their physiological action. The most important varieties were Homolle's amorphous digitaline, Nativelle's crystallised digitaline, and soluble or German digitalin.

An examination of the chemistry of digitalis by Schmiedeberg has shown that there are at least five principles present in it, and possibly there are present also some products of their decomposition. They are all non-nitrogenous and, with the exception of one, digitoxin, are glucosides. They are : digitoxin, digitalin, digitalein, digitonin, and digitin. The first three of these are cardiac poisons. Digitonin has an action like that of saponin, and digitin appears to be inert.

Digitoxin is quite insoluble in water, and forms the chief constituent in Nativelle's digitaline. By boiling with dilute acids digitoxin yields toxiresin and digitalin yields digitaliresin.

Digitalin is also insoluble in water and is the active principle of Homolle's digitaline. Digitalein differs from the two former in being readily soluble in water, and forms a large proportion of he soluble digitalin. The digitalin of the B.P. 1867, being almost insoluble in water, probably consisted chiefly of digitoxin or digitalin.

General Action. - In large doses digitalis causes sickness, vomiting, muscular weakness, diuresis, subjective affections of vision, laboured respiration, and death; the heart usually failing before the respiration. The condition of the heart after death varies. Sometimes I have found it in diastole and sometimes in systole in dogs poisoned by digitalis.

Special Action. - On the muscles. In a number of unpublished experiments on this subject made in 1867-68 in the laboratories of Professors Brucke and J. Rosenthal, I found that soluble digitalin did not lessen the excitability of the unweighted muscle but diminished its power to lift a weight. According to Schmiedeberg and Koppe digitalis paralyses all voluntary muscles. Digitalin causes elongation of the muscle with increased elasticity in the frog. On the nervous system. It has no marked action on sensory or motor nerves. It has little action on the spinal cord. It has been stated to lessen reflex action in the frog by stimulation of Setchenow's centre, but this may be due to reflex irritation from the point of injection (p. 171). The brain is unaffected, and in cases of poisoning remains clear to the last. (Two of the products of the decomposition of digitalin, toxiresin and digitaliresin, however, produce convulsions like those of picrotoxin.) Large doses cause subjective affections of vision, consisting in dimness, occasional flashes of light, or in the constant appearance of a rainbow or bright light before the eyes. Locally applied to the eye it produces irritation at first, and afterwards causes a halo to surround bright objects.

The respiration is generally somewhat slowed, and occasionally before death may become excessively slow.

The effects produced on the circulation by the active principles of digitalis and by substances having a similar action, such as oleandrin, scillain, adonidin, neriin, convalamarin, antiarin, and helleborein, may be divided according to Schmiedeberg into four stages: 1. Rise of blood-pressure, usually though not invariably accompanied by slowing of the pulse.

2. Continued rise of blood-pressure, with a quick pulse.

3. Continued high pressure, with irregularity of the heart's action and pulse-rate.

4. Rapid fall of the blood-pressure, sudden stoppage of the heart, and death.

The rise in blood-pressure is regarded by Schmiedeberg, Boehm, and others as entirely due to increased action of the heart and not at all to contraction of the vessels. With this view I cannot agree, and I still hold to the opinion which I expressed many years ago that the rise in pressure is due in great measure to contraction of the arterioles. Not only is it more difficult to raise the pressure in the arterial system by alterations in the heart's action than by contraction of the arterioles, as we find from experiments on a schema (p. 266), but the form of the pulse-curve under the action of digitalis conclusively demonstrates that the arterioles are contracted (vide p. 276). This has also been demonstrated by Donaldson and Stevens,1 who found that the addition of digitalis to blood lessens the flow through vessels in which circulation was artificially maintained. A similar result has been obtained by Ringer.

The slow pulse in the first stage of digitalis-poisoning is partly due to stimulation of the vagus-roots of the medulla, and partly to increased sensibility or actual stimulation of the ends of the nerves in the heart. This increased sensibility has been shown to exist by Boehm, who found that after the administration of-digitalis, a faradaic current which previously had no action on the heart would not only slow the pulse but produce prolonged diastolic arrest.