CORTEX ULMI FULVÆ.
Slippery Elm Bark.
Botanical Origin—Ulmus fulva Michaux, the Red or Slippery Elm, a small or middle-sized tree,[2067] seldom more than 30 to 40 feet high, growing on the banks of streams in the central and northern United States from Western New England to Wisconsin and Kentucky, and found also in Canada.
History—The Indians of North America attributed medicinal virtues to the bark of the Slippery Elm, which they used as a healing application to wounds, and in decoction as a wash for skin diseases. It is the “Salve Bark” or “Cortex unguentarius” of Schöpf.[2068] Bigelow, writing in 1824, remarks that the mucilaginous qualities of the inner bark are well known.
Description—The Slippery Elm Bark used in medicine consists of the liber only. It forms large flat pieces, often 2 to 3 feet long by several inches broad, and usually ¹/₂₀ to ²/₂₀ of an inch thick, of an extremely tough and fibrous texture. It has a light reddish-brown colour, an odour resembling that of fenugreek (which is common to the leaves also), and a simply mucilaginous taste.
In collecting the bark the tree is destroyed, and no effort is made to replace it, the wood being nearly valueless. Thus the supply is diminishing year by year, and the collectors who formerly obtained large quantities of the bark in New York and other eastern states have now to go westward for supplies.[2069]
Microscopic Structure—The transverse section shows a series of undulating layers of large yellowish bundles of soft liber-fibres, alternating with small brown parenchymatous bands. The whole tissue is traversed by numerous narrow medullary rays, and interrupted by large intercellular mucilage-ducts. In order to examine the latter, longitudinal sections ought to be moistened with benzol, aqueous liquids causing great alteration. In a longitudinal section, the mucilage-ducts are seen to be 70 to 100 mkm. long, and to contain colourless masses of mucilage, distinctly showing a series of layers. Crystals of calcium oxalate, as well as small starch grains, are very plentiful throughout the surrounding parenchyme.
Chemical Composition—The most interesting constituent of the bark is mucilage, which is imparted to either cold or hot water, but does not form a true solution. The bark moistened with 20 parts of water swells considerably, and becomes enveloped by a thick neutral mucilage, which is not altered either by iodine or perchloride of iron. This mucilage when diluted, even with a triple volume of water, will yield only a few drops when thrown on a paper filter. The liquid which drains out is precipitable by neutral acetate of lead. By addition of absolute alcohol, the concentrated mucilage is not rendered turbid, but forms a colourless transparent fluid deposit.
Adulteration—Farinaceous substances admixed to the powdered drug may be detected by means of the microscope.
Uses—Slippery Elm Bark is a demulcent like althæa or linseed. The powder is much used in America for making poultices; it is said to preserve lard from rancidity, if the latter is melted with it and kept in contact for a short time.