A glucoside extracted from galls.

Preparation. - By dissolving out the tannic acid from powdered galls with ether mixed with a very little water, gently evaporating the solution, and drying the acid. Although tannic acid is very sparingly soluble in pure ether, yet it appears to dissolve readily in ether containing a very little water.

Properties. - In pale yellow vesicular masses, or thin glistening scales, with a strongly astringent taste, and an acid reaction. On exposure to air or by the action of dilute acids, it splits up into glucose and gallic acid (q.v.). Tannin in its natural state appears to be a mixture of digallic acid (C14H10O9) with a glucoside of digallic acid. Schiff proposes to give the name tannic acid to the digallic acid, and that of tannin to the glucoside.

Solubility. - It is readily soluble in water and rectified spirit; very sparingly soluble in ether.

Reactions. - The aqueous solution precipitates solution of gelatine yellowish-white, and the persalts of iron of a bluish-black colour.

Impurities. - Mineral matter.

Tests. - It leaves no residue when burned with free access of air on platinum foil.

Dose. - 2 to 10 grains.

Preparations.

B.P.

Glycerinum Acidi Tannici.......................................................

1 part in 6 by weight.

Suppositoria ,, ,, ......................................................

3 grains in each.

,, ,, ,, cum Sapone................................

3 grains in each.

Trochisci ,, ,, ......................................................

1/2-grain in each.

Preparations - (continued).

U.S.P.

Collodium Stypticum (tannic acid 20, alcohol 5, stornger ether 20, collodion 55 parts)...................................

1 part in 5.

Trochisci Acidi Tannici..........................................................

1 grain in each.

Unguentum " " (with benzoated lard).......................

1 part in 10.

Action. - When applied externally to the unbroken skin tannic acid has little or no action; but applied to skin deprived of its epidermis, it coagulates the albumin and causes contraction of the cells of the skin. It coagulates blood and consequently acts as a local styptic.

It acts locally on mucous membranes, coagulating the mucus. On account of the dryness in the mouth produced by the drug, it was concluded that the vessels are contracted, and that the astringent action is due to this; but Rossbach found, from direct observation, that the vessels are dilated; in this particular tannin differs from other astringents, such as nitrate of silver. This dilatation is not due to paralysis of the coats of the arteries, since they contract on stimulation or subsequent application of silver nitrate.

Its astringent action on the skin and mucous membranes is probably clue to coagulation of albumin and a 'tanning ' of all the tissues to which it is applied.

When taken into the mouth it causes dryness, coagulation of mucus, and a partial paralysis of the ends of the sensory nerves (both the nerves of ordinary sensation and the special nerves of taste), so that it destroys to a great extent the sense of taste, and also lessens irritation in the throat.

When taken into the stomach in large doses it is irritant and causes vomiting. When given to animals it does not lessen either secretion or peristaltic action of the intestines, and yet in man, even from small doses, there is a dryness of the faeces and lessened peristalsis; probably these different results are due to some imperfection in the experiments or to a difference of dose.

Large doses cause diarrhoea, with subsequent constipation.

It is absorbed into the blood, and passes out as gallic acid or some product of the oxidation of gallic acid.

It restrains haemorrhage in distant organs, as the uterus, lungs, or kidneys, but the modus operandi is not known, and some authorities deny this action altogether.

Uses. - Externally applied to the skin tannic acid is used in intertrigo, impetigo, and eczema, especially when occurring behind the ears in children: in desquamating chronic eczema, a ten per cent. tannic acid ointment is useful, also in sycosis, applied after shaving. In hyperidrosis of the axillae, genitals, palms, and hands, and in sweating of the feet, frequent washing with a solution of tannin in diluted alcohol (1 in 250) is recommended.

It is also applied to mucous membranes, such as the external auditory meatus in otorrhoea (fill the meatus with glycerine of tannic acid and keep it there by a pledget of cotton wool). Also to the nasal mucous membrane, when there is ulceration and offensive discharge. In this case it is applied either in aqueous solution by means of the nasal douche or as glycerine of tannic acid with a brush. It is thus of use in ozaena after measles or scarlet fever, and in that form occurring in syphilitic children. In haemorrhages from the nose dry tannin may be snuffed up.

It is used in stomatitis and ulceration of gums; and as a gargle in relaxed sore-throat or applied locally as glycerine of tannic acid. It is very useful in the hacking cough often met with in children, and also in adults, which is due to an irritation at the back of the pharynx, often accompanied by inflamed throat, covered with mucus; in inflamed tonsils and deafness; also in whooping-cough and other throat affections, either in the form of the glycerine or as lozenges.

It has been used, dissolved in water, or mixed with olive oil, as an injection in leucorrhoea, gonorrhoea, and chronic discharges from the os uteri.

Internally tannin is used in haematemesis and intestinal haemorrhage; also as an antidote to poisoning by alkaloids, but when used for this purpose it must be followed by a purgative, as the tannates are all more or less soluble in the juices of the alimentary canal. It is also used in poisoning by tartar emetic, as tannic acid forms with antimony an insoluble tannate. It is used in diarrhoea, but usually the more sparingly soluble forms of tannin, such as kino, are preferred. Tannic acid lessens the amount of albumin in albuminuria.