The dried rhizome and roots of Rhe'um officinale Baillon, Rheum palmatum Linné, and the var. Tanguticum Maximowicz, and probably other species of Rheum, deprived of most of the cortex and carefully dried.
BOTANICAL CHARACTERISTICS.—Botanical history somewhat obscure. It is known, however, from authentic specimens, that the plant is a herbaceous perennial with acidulous juice, resembling the garden rhubarb, but attaining a larger size than any other species. Leaves very large, roundish, cordate at base, and 5- to 7-lobed. The flower-stem, 6 to 8 feet high, bears flowers having a greenish perianth; ovary (and fruit) triangular, 1-celled.
SOURCE.—Rhubarb is obtained from many species of Rheum, mostly natives of Asia, especially of China, Chinese Tartary, and Thibet. Russian or Turkish rhubarb—so called because all of it imported into these countries from China had to be submitted to official inspection—is now never found in the market. The caravan commerce between Russia and China has been an important one for many generations, and the rhubarb in European commerce was almost entirely carried from China through Persia and Asia Minor; hence the old name of Turkey rhubarb. Later on it was brought through Northern China, Siberia, and European Russia (Kiachta) to St. Petersburg.
The "Russian rhubarb" of early times was evidently what is now known as Shensi variety. That brought into the trade by the port of Canton, known in Europe as Indian rhubarb, is now called Canton. The Chinese rhubarb is the variety recognized in commerce. The root, often attaining a weight of fifty pounds, is cut up into pieces of a suitable size for drying, holes being usually bored through the pieces and a string passed through for hanging them up.
DESCRIPTION OF DRUG.—In cylindrical, conical, or plano-convex pieces, or pieces with no regular shape, varying in size from 75 to 150 mm. (3 to 6 in.) long, and 50 to 75 mm. (2 to 3 in.) thick; they are usually sorted into "round" and "flat" rhubarb. Externally somewhat shriveled, often with portions of the cortical layer which have not been pared away; usually covered with a bright yellow dust, beneath which it is seen to have a rusty-brown hue; under the lens it is seen to be marked with the medullary rays (innumerable short, broken lines of a deep brown color) crossing a white ground, forming elongated whitish meshes. Well-formed pieces broken transversely display near the cambium zone dark lines arranged as an internal ring of star-like spots, with radiating, reddish medullary rays, marking the internal origin of the leaves. The tissue is made up of a white parenchyma, with reddish-brown or brownish-yellow medullary rays, so twisted, however, as to be scarcely recognizable as such, giving a cross-section a mottled appearance of red, white, and yellow. The white parenchyma cells are loaded with starch and crystals of calcium oxalate, which cause the grittiness between the teeth; the medullary rays contain the active constituents. Odor characteristics; taste bitter, aromatic, astringent, and gritty. When chewed, it tinges the saliva orange-yellow. It yields a yellowish powder with a reddish-brown tinge.
The common pie-plant, Crimean rhubarb, from Rheum rhaponticum Linné, is a European variety, having properties similar to that of rhubarb, but the astringent principles predominate. It is fusiform, about 100 mm. (4 in.) long and 20 mm. (4/5 in.) thick, with a thick orange-red cork, partially removed; a cross-section shows a comparatively regular, radiate structure of red medullary rays traversing a whitish parenchyma and extending into the cortical layer when present; its odor is less aromatic, is less gritty, and its taste more mucilaginous and astringent. Rumex hymenosepalus, Canaigre, has been used, in powder, to adulterate powdered rhubarb. For detection, follow general directions for examination of powders, see Part iv, Chap. I.
Choice of Rhubarb.—Select the moderately heavy and compact pieces, which should break with a brittle fracture, presenting a lively, mottled appearance of yellowish and reddish fibers intermingled with white parenchyma; odor decidedly aromatic; taste bitter, astringent, and gritty, not mucilaginous, tingeing the saliva orange-yellow when chewed. Very light, rotten, or worm-eaten pieces should be rejected. The yield of extractive using dilute alcohol should not be less than 30 per cent. The yield of ash should not exceed 13 per cent. It should be stored in air-tight containers with a few drops of chloroform to prevent the development of insects.
CONSTITUENTS.—Seemingly a mixture of different coloring principles of a somewhat resinous quality, each having a peculiar solubility of its own: Chrysophan, C27H30O14 (and chrysophanic acid), emodin, aporetin, phaeoretin, erythroretin, rheumic acid, and rheo-tannic acid; also starch, calcium oxalate, pectin, and arabic acid. Chrysophan is a yellow glucoside yielding, with acidulated water, sugar and chrysophanic acid, C15H10O4, yellow crystals, one of the best solvents for which is hot benzol. According to Hagar, by proper extraction with chloroformic solvent, etc., rhubarb yields not less than 3 per cent. of chrysophanic acid. Chrysophanic acid, or dioxy-methyl-anthraquinone (C14H5CH3(OH)2O2) is closely related to emodin, which is a trioxy-methyl-anthraquinone (C14H4CH3(OH)3O2). Cathartic acid represents the cathartic principles of rhubarb in a crude but concentrated form. For its preparation, see Senna (240)
EMODIN TEST, in Rhubarb.—Boil 0.100 Gm. of powdered rhubarb with 10 mils of an aqueous solution of potassium hydroxide 1 in 100), allow it to cool, filter, acidulate the filtrate with hydrochloric acid and shake it with 10 mils of ether; on standing, the ethereal layer should be colored yellow. On shaking this ethereal solution with 5 mils of ammonia water, the latter should be colored cherry-red (presence of emodin) and the ethereal layer should remain yellow (presence of chrysophanic acid) U.S.P.
Preparation of Phaeoretin.—Wash alcoholic extract with water; dissolve residue in a little alcohol; add ether. This precipitates crude phaeoretin.
Preparation of Chrysophanic Acid.—Tincture of rhubarb, after standing for some time, deposits yellow sedimentary crystals. This sediment, dissolved in benzene, deposits the principle on evaporation.
Chrysarobin is a principle easily converted into chrysophanic acid by oxidation. The source of this is Goa powder (from Andira araroba). The powder is extracted with hot benzene (benzol), and the liquid allowed to cool. The orange-colored principle separates as the liquid cools.
ACTION AND USES.—Purgative and astringent. It has been highly esteemed as an antidysenteric remedy because of the fact that the cathartic principles are accompanied by the antiseptic action of chrysophan, and because catharsis is followed by an astringent and tonic effect upon the mucous lining. Roasting destroys the cathartic quality, when the root becomes simply a bitter astringent. Dose: 15 to 30 gr. (1 to 2 Gm.).
- OFFICIAL PREPARATIONS.
- Tinctura Rhei (20 per cent., with cardamom). Dose: 1 to 4 fl. dr. (4 to 15 mils).
- Tinctura Rhei Aromatica (20 per cent., with cassia cinnamon, cloves, and nutmeg), 1/2 to 3 fl. dr. (2 to 12 mils).
- Syrupus Rhei Aromaticus (15 per cent. of aromatic tincture), 2 to 6 fl. dr. (8 to 24 mils)
- Fluidextractum Rhei, 5 to 30 <minim> (0.3 to 2 mils)
- Mistura Rhei et Sodae (1.5 per cent. with sodium bicarbonate, fl'ext. of ipecac, and spirit peppermint),
- Syrupus Rhei (Fl'ext. 10 per cent), 2 to 6 fl. dr. (8 to 24 mils).
- Extractum Rhei Compositus (25 per cent., with magnesia and ginger), 1 to 3 dr. (4 to 12 Gm.).
- Pilulae Rhei Compositae (each pill containing about 2 gr. of rhubarb, with purified aloes 1 1/2 gr., myrrh, and oil of peppermint), 1 to 3 pills.
A Manual of Organic Materia Medica and Pharmacognosy, 1917, was written by Lucius E. Sayre, B.S. Ph. M.