Astringents are agents which cause constriction or contraction of the tissues to which they are applied, and diminish the amount of secretion from mucous membranes.
The action of astringents may be local or remote. In the former case they are brought directly into contact with the part, as in the case of gargles, injections, eye-washes, etc. In the latter it is necessary that they be first absorbed into the blood, and by it be conveyed to the part to be acted upon. Cold, in the form of ice, evaporating lotions, or freezing-mixtures, is spoken of as an astringent. The mineral acids, and their salts also, if soluble, have a similar effect, particularly the salts of lead, zinc, iron, copper, aluminium, silver, and mercury. Among the chief vegetable astringents used in veterinary medicines may be mentioned gallic and tannic acids, and the barks of trees which have astringing or binding properties due to these or similar constituents. Oak bark and oak-galls are among the most powerful; white elm bark, catechu, witch-hazel, and others are more or less employed on account of their astringent properties.
The immediate effect of such agents upon mucous surfaces, and soft parts generally, is to contract them, whilst some also coagulate the albumen with which they are brought into contact.
The action upon living tissues is somewhat similar to that of tanning skins, by the formation of a tanno-gelatin.
Some astringents administered as internal medicines counteract the relaxed condition of mucous surfaces, and some possess the property of arresting haemorrhage.
Externally applied, remedies of this class arrest excessive secretions and discharges of pus, serum, and mucous or muco-purulent discharges, as, for example, in abscesses, ulcers, and catarrhal diseases of a more or less chronic character.
Astringents in the more popular acceptation of the term are those remedies usually employed to arrest diarrhoea only, but it will be seen from the foregoing remarks that their uses are very diversified.
Certain astringents have the property of causing contraction of the blood-vessels, decreasing the amount of blood circulating in the parts, and so lessening the quantity of secretion given off by it. The mineral acids, sulphuric especially, have this astringent effect.
Although tannic acid is extracted from nut-galls (fig. 428), yet in practice we find powdered galls have certain advantages over the active principle for some purposes, while tannic acid is best suited to others. Powdered galls are less astringent than tannic acid, and rather more so than oak bark, elm bark, or catechu. They are not absorbed with the facility of alum, or iron, but have the effect of coagulating albumen. In horse practice they are used in electuaries for relaxed and sore throat, and abrasions of the mouth and fauces generally, where a somewhat con-stringing effect is desired from an agent that is not likely to be taken up into the circulation. The powder is also prescribed with success, as a ball, in cases of polyuria or profuse staling.
The action and uses of tannic acid, or tannin as it is sometimes called, are similar to those of nut-galls above described, but it has the additional advantage of solubility.
Where the bowel discharges are of such a character as to suggest abraded surfaces, tannin is administered with great advantage not only on account of its power to check secretion, but also because of its action on the bleeding vessels. It is soluble in water and spirit, and has a special affinity for glycerine.
This astringent is nearly allied to tannic acid, both in chemical composition and therapeutic value and uses.
The. bark from young trees and the smaller branches is preferred for medicinal purposes, as containing a relatively larger amount of the astringent principle than is to be found in other portions. It has a special value in the treatment of chronic diarrhoea and bowel discharges of a dysenteric type. It is usually prescribed in the form of decoction, both as an internal remedy and as an application to languid sores and indolent ulcers.
Catechu is a vegetable extract of special value as a bowel astringent, and is frequently combined with opium and chalk. It is prescribed as a powder, tincture, and infusion.
Kino is similar in its action, and chiefly employed as an intestinal astringent or gargle.
There are a number of compounds of alumina, but that commonly known as "alum" is a sulphate of ammonia and alumina. Veterinary surgeons value the drug highly, and employ it for a variety of purposes. It is used in several forms - as a crystal of irregular shape, as a powder, and as burnt, exsiccated, or dried alum, that is after it has been deprived of its water of crystallization.
Fig. 428. - Nut-Galls.
1, 2, Nut-Galls (Aleppo) from Quer-cus infectoria. 3, Transverse section.
Externally it is used for all the purposes to which other astringents are applied, except as a styptic, as it has no very marked influence in arresting haemorrhage of a serious character. In the dried form, alum is very much more active than in the other conditions, and is a valuable dressing for wounds, ulcers, and foetid discharges of various kinds. In. combination with flour and oxide of zinc it is used for cracked and chapped heels, and for grease and skin eruptions upon the legs and belly.
For injuries to the tongue, or any part of the oral cavity, alum is invaluable. It has been found, in cases where the cheeks and gums have been denuded of membrane, or the tongue has suffered serious laceration, that a solution of alum speedily brings about the formation of a protective covering. In some forms of diarrhoea, and in prolapsus of the womb or bowel, it is an excellent astringent. It is also employed as an eye lotion, in powder as an insufflation, and in solution as an injection up the nostrils in cases of ozena. It has long been esteemed for drying up the milk of mares when, through loss of the foal or other causes, it is desirable to arrest the mammary secretion.
In addition to its great value as a tonic, iron is one of our most valued astringents. There are a great many preparations of it, of which the following are in most common use in veterinary practice: sulphate, carbonate, and solution of perchloride.
For arresting haemorrhage outside the body, the solution of perchloride is a most efficient preparation. It enters into the astringent lotions employed in the treatment of thrush, grease, and other diseases attended with foul discharges and fungoid growths.
The salts of copper are used in much the same way as those of iron, but the sulphate, which is the preparation most employed, is more astringent than that of iron. In grease, thrush, and canker it is employed as a lotion or in powder, either alone or in combination with other astringents. It has the most drying effect of any of the astringent metallic salts when applied to raw surfaces. In special cases it is prescribed as an intestinal astringent.
The preparations of zinc used in the medical treatment of horses are the sulphate, acetate, chloride, oxide, carbonate, and oleate. The salts vary very much in their astringent and caustic properties.
The oxide and carbonate are but very slightly astringent, and in this connection are used only as cooling and drying agents. The acetate, sulphate, and chloride in weak solutions are astringent.
Chestnut Hunter Gelding. Artist.By Highthorn.
The Property of John Hadland, Esq., Beverley. Winner of Numerous Prizes.
Only one salt of silver is used in horse practice, and that is the nitrate, or lunar caustic as it is also called. As a weak solution it may be prescribed as a mouth-wash, when a spongy state of the gums indicates the need of a sharp and quickly-acting astringent that will not require frequent repetition, as might be the case if tannin or some of the simpler washes were employed. For the suppression of soft granulations and the formation of a level scab, nitrate of silver is probably the best of all the metallic salts, and is much used in the treatment of broken knees and other skin wounds, when the object in view is to secure the least possible permanent blemish.
As an eye lotion it is and has been long in repute for a variety of affections of the visual organs. By its effects upon the vascular structures of the eye it causes absorption of specks or cloudiness of the cornea if they are not of long standing. In small doses internally administered it is astringent, and occasionally prescribed when ulceration of the stomach or bowels is suspected.
As a bowel astringent this drug is valuable either in solution or powder. In the latter form and with chalk it appears to act mechanically by forming a smooth coating over the mucous membrane.
Chalk, or carbonate of lime, is a safe and often effective astringent in cases of diarrhoea, particularly in those instances in which a general acidity of the intestines gives rise to it.