This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
Many of these drugs are sufficiently powerful irritants to cause inflammation. If this goes no further than the exudation of plasma from the vessels, and this plasma collects under the epidermis to form vesicles, the drug causing the production of vesicles is said to be a vesicant; e.g., cantharides.
Pustulants are such of the above drugs as are sufficiently powerful irritants to cause the inflammatory process to proceed to the passage of leucocytes through the-walls of the capillaries. They collect in the vesicles, which consequently become pustules; e.g., croton oil.
Escharotics or caustics are the most powerful of all the above drugs. Their local application destroys the vitality of the part to which they are immediately applied, and sets up vascular dilatation of the surrounding area; e.g., strong nitric acid, zinc chloride, silver nitrate, and arsenous acid.
It has been shown by experiments on animals that when the vessels of the skin are dilated by the application of an irritant, those of the subjacent viscera are often reflexly altered in size. The same is probably true of man. An irritant is called a counter-irritant when it is applied to the skin with the object of altering the size of the vessels of the subjacent viscera. It is particularly to be remembered that the action is a reflex nervous one, and is in no way due to the withdrawal of blood into the dilated vessels of the skin.
The following, when inhaled, dilate peripheral vessels by acting locally on them:
(1) Amyl nitrite.
(3) Sodium nitrite.
(4) Ethyl nitrite.
(5) Spiritus aetheris nitrosi.
(6) Erythrol tetranitrate.
Drugs which, taken by the mouth, dilate arterioles by acting locally on them:
(2) Amyl nitrite.
(4) Sodium nitrite.
(5) Ethyl nitrite.
(6) Spiritus aetheris nitrosi.
(7) Erythrol tetranitrate.
Drugs which, taken by the mouth, contract arterioles by acting locally on them:
(2) Caffeine (early in its action).
The following have been shown by experiments in the laboratory to cause contraction of small arteries through which they circulate: copper, zinc, tin, platinum salts all cause powerful contraction. Lithium, calcium, strontium, magnesium, cadmium, nickel, cobalt and iron salts cause slight contraction.
Drugs which, when locally applied to vessels, contract them:
These may act in two ways, either by contracting the muscular coat of the vessels, or by coagulating the albuminous fluids around them, the coagulum by its contraction constricting the vessels.
Those which act on the muscular coat of the vessels:
(1) Cold, however produced; hence rapidly volatilizing substances as ether. This effect is but temporary. If cold is long continued it dilates blood-vessels.
(3) Lead salts.
(4) Dilute solutions of silver salts.
Those which coagulate the albuminous fluids around the vessels:
(2) Lead salts.
(3) Silver salts.
(4) Zinc salts.
(5) Copper salts.
(7) Ferric salts.
(8) Bismuth salts to a slight extent.
B. Drugs which act on the Vaso-motor Centres.
Drugs which, by their action on the vaso-motor centres, dilate the vessels:
(7) Chloral hydrate.
(14) Hydrocyanic acid.
Some of the substances, which in small doses contract the vessels by central action, in large doses dilate them; viz., digitalis and squill.
Drugs which, by their action on vaso-motor centres, cause contraction of vessels:
(9) Hamamelis. (ID) Strychnine.
(11) Lead salts
Also, for a very short early period of their action, some substances whose main action is to dilate the vessels by their central action; viz., belladonna, stramonium, hyoscyamus, alcohol, ether, chloroform, hydrocyanic acid and veratrine.
Astringents are drugs which diminish the size of the vessels, and thus decrease the amount of exudation from them.
Styptics, or Haemostatics, are drugs which stop bleeding. They comprehend all astringents, especially cold, lead and copper salts, hamamelis, ergot, hydrastis, tannic acid, and, above all, ferric salts, for they coagulate the blood which is flowing from the vessel, and the clot prevents further bleeding. Matico leaves, because of the numerous hairs on their under surface, favor coagulation of blood when locally applied to a bleeding surface. Cobwebs act in the same way.
Drugs which locally dilate vessels are frequently applied to stimulate sores to heal, and to promote absorption of inflammatory products, as seen in the application of iodine over joints in certain forms of joint disease; and as counter-irritants in many forms of disease of deep-seated organs, as in the application of a blister in pleurisy. Drugs which by their central action cause dilatation of all the vessels of the body are used in certain forms of heart disease, as in the use of amyl nitrite in angina pectoris; and some suppose that the good they do is brought about by dilating the vessels, and so rendering the work of the heart easier. Amyl nitrite and nitroglycerin are much used for this purpose. Drugs causing general vascular dilatation are also employed to cause dilatation of the vessels of the skin with the object of thereby leading to an increase of perspiration and an increased radiation of heat. Alcohol,
Spiritus AEtheris Nitrosi, and Ipecacuanha, amongst others, are used in this way.
The most important use of astringents is as styptics; they are also used to check excessive discharges of all sorts, as in diarrhoea, leucorrhoea, etc., and in relaxed conditions of vessels, such as are Often seen in pharyngitis.
There is perhaps no better opportunity than this of mentioning emollients and demulcents.
Emollients are substances which soften and protect parts. The word is usually employed for substances applied to the skin.
Common emollients are substances soaked in warm water, as hot fomenta-Sons and poultices, fats of various sorts, as lard and lanolin (hydrous wool fat), non-irritating oils, as olive oil, spermaceti, petrolatum, vaseline, etc.
Demulcents are substances which protect and soothe parts. They are generally applied to mucous membranes, especially when unduly dry, and thus they are often used for the mouth.