120 MANUAL OF PHARMACOLOGY
PLASTERS
These serve for protections against irritation, for mechanical support; and for the slow conveyance of drugs to or through the skin. They are somewhat irritant, partly by preventing evaporation from the skin, and partly by the small quantities of volatile oils contained in the resins from which they are often prepared. The official plaster-masses are unimportant, since the commercial plasters are usually made by private formulas from some rubber mixture. "Court plaster" is made of isinglass (Ichthyocolla), a variety of gelatin prepared from the swimming bladder of the sturgeon.
PREPARATIONS—PLASTER MASSES
Emplastrum Elasticum (Emp. Blast.), U.S.P.; Rubber (Adhesive), Plaster.—A mixture of rubber, pitch, wax and an absorbent powder (starch, etc.), spread on cloth.
Emplastrum Resina (Emp. Res.), U.S.P., B.P. (Rosin Adhesive Plaster).—Lead plaster with rosin and yellow wax, U.S.P.; hard soap instead of wax, B.P.
Emplastrum Plumbi (Emp. Plumb.), U.S.P.. B.P.; Lead Plaster (Diachylon Plaster).—A soap made by boiling lead oxid with olive oil and lard, U.S.P.; with olive oil, B.P.
Emp. Saponis (Emp. Sap.), B.P.; Soap Plaster.—Similar to preceding, with greater proportion of soap.
MECHANCAL PROTECTIVES
Certain substances which are used for this purpose may be mentioned in this place.
PREPARATIONS—MECHANICAL PROTECTIVES
*Collodium (Collod.), U.S.P., B.P.; Collodion.—A 4 per cent. solution, U.S.P., 2.1 per cent. B.P., of Pyroxylin (gun-cotton) in a mixture of 3 vol. of ether and 1 vol. of alc. Should be kept in a cool place remote from fire. Clear, or slightly opalescent, syrupy liquid; colorless, or slightly yellowish and having the odor of ether. Highly inflammable. Collodion dries rapidly into a firm film, which is used to protect small wounds. The film cracks easily and contracts considerably, exerting pressure on the tissues. Similar solutions may be prepared with acetone or other solvents.
*Collodium Flexile (Collod. Flex.), U.S.P., B.P.; Flexible Collodion.—Collodion rendered flexible by the addition of a little Castor Oil and Camphor, U.S.P.; Castor Oil and Canada Turpentine, B.P. It does not contract or crack, but does not adhere quite so well.
Pyroxylinum (Pyroxylin), U.S.P., B.P.; Pyroxylin (Soluble Gun-cotton).—Obtained by acting on cotton with a mixture of nitric and sulphuric acids. Consists chiefly of cellulose tetranitrate, C12H16(ONO2)4O6. Slowly sol. in 25 parts of a mixture of 3 vol. of ether and 1 vol. of alc. (equal vol. of 90 per cent. alc. and eth., B.P.); Sol. in acetone. Celloidin, N.N.R., is a purified form. Ordinary gun-cotton is a hexanitrate.
*Gossypium Purificatum (Gossyp. Purif.), U.S.P.; (Gossyp.), B.P.; Purified (Absorbent) Cotton.—The hairs of the seed of Gossypium herbaceum freed from adhering impurities and deprived of fatty matter (so as to make it absorbent) and often sterilized. Consists of almost pure cellulose (C6H10O5) Used in bandaging, etc., either as such, or as gauze (Tela, Carbasus), lint (Lintum), etc., or impregnated with antiseptics, astringents, etc.
Calcii Sulphas Exsiccatus (Plaster of Paris, Burnt Gypsum), CaSO4 + ½H2O.—A fine white powder, which sets into a stone-like mass when mixed with half its weight of water. Used in bandaging. The "setting" takes place in fifteen to twenty minutes. It may be delayed to an hour by the addition of 5 per cent, of glycerin, or hastened by the addition of sodium silicate. Plaster of Paris must be kept dry.
Liquor Sodii Silicatis (Waterglass).—A colorless syrupy liquid, of alkaline properties, but not corrosive. Contains about 20 per cent. of sodium silicate. Forms a solid glassy mass on exposure. Used in bandaging.
Actions of Sodium Silicate.—When given by the mouth or skin, this acts like a typical mild alkali without showing any specific features. It is readily absorbed from the alimentary canal, and excreted by the urine. Injected intravenously it causes agglutination of blood corpuscles, and consequently intravascular clotting.
Silicates are normally present in all tissues, but in very small amount (Siegfried, 1901). There is no evidence that they play any physiologic rôle. Silica is also constantly present in hairs and feathers (H. Schulz, 1901). It constitutes up to 77 per cent. of the total ash of the feathers of the dove (Gonnermann, 19x8).
