This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Rhubarb is the prepared root of different species of Rheum, herbaceous perennial plants, natives of the interior of Asia, several of which have been introduced into Europe, where they are cultivated to a limited extent, especially in France and England. But the species which yields the only valuable rhubarb, that which is brought exclusively from Central Asia, is wholly unknown. The drug has been conjecturally ascribed to Rheum palmatum, R. undulatum, R. compactum, and R. australe, all of which are said to grow wild in Tartary, and have been cultivated in Europe from seeds brought from Asia; and it is not impossible that one or more of these species may contribute to furnish the rhubarb of commerce; but we have no proof of the fact; and the failure to obtain from them a product, having the precise properties of the root as imported from the East, renders it extremely doubtful. The root of R. palmatum approaches nearest in character to the Asiatic rhubarb.
The leafstalks of the different species have an agreeable acidulous taste, owing to the presence of oxalic acid, and are used in making pies, tarts, etc., for which purpose the plants are cultivated in our gardens.
Rhubarb is collected and prepared in Chinese Tartary. After being dug up, the root is deprived of its smaller branches and cortical portion, cut into pieces of convenient size, and dried either in the sun, or with the aid of artificial heat. It is sent into commerce in two directions; one portion being taken eastward to Canton, and thence exported under the name of Chinese rhubarb; the other northwestward to St. Petersburg, whence it is distributed into commerce under the title of Russia or Turkey rhubarb; the latter name having originated in the circumstance, that it formerly reached the markets of Europe through Turkey. As brought by these two routes, the drug differs considerably in quality, being prepared in a different manner, and possibly derived from different species, though of this latter fact we have no certain knowledge. Hence it is classed, both in commerce and pharmacy, in two varieties, which must be separately considered.
Rheum Sinense. This is usually in cylindrical pieces, often somewhat flattened and irregular, of a brownish-yellow colour, and of an aspect as though the cortical part had been removed by grating or scraping. in each piece there is almost always a small hole, passing quite through it, and obviously intended for the insertion of a cord, by which it was probably hung up when dried, and portions of which may often be seen remaining in the opening. Though somewhat light and spongy, compared with many other roots, this variety is in general rather more heavy and compact than the Russian. When broken, it exhibits a rough uneven fracture, and a grayish colour consisting of intermingled shades of brownish-red and white. By pulverization, it yields a yellow powder, tinged with reddish-brown, and portions of this powder, produced by attrition, are often seen on the outside of the pieces, giving them a yellow colour. Mixed with the better pieces, others often come, worm-eaten, more or less extensively decayed, or otherwise injured, so that the general character of a package is usually much inferior to that of selected portions; and, as the whole contents of a package are often powdered together, it happens that the pulverized Chinese rhubarb of the shops, even though genuine, is not unfrequently of low quality. it is, moreover, liable to adulteration with the rhapon-tic root, as it is called, an inferior variety of rhubarb cultivated in France.
A variety of rhubarb is sometimes imported from Canton, consisting of pieces, the surface of which has been trimmed so as to imitate the Russian; but close examination will generally detect in them the small perforating passage, or some remains of it, which has been mentioned as one of the characteristics of the proper Chinese rhubarb. it is, however, superior to the common unassorted variety, because selected from the soundest pieces.
Turkey Rhubarb. - Rheum Russicum. - Rheum Turcicum. As before stated, this comes from St. Petersburg, whither it is brought, through Siberia and European Russia, from the borders of Tartary. Before being permitted to enter the Russian dominions, it undergoes a rigid inspection, on which account, as well as from its inherent qualities, it is superior to the Chinese, and commands a much higher price in the market. The pieces are more irregular in shape than the former variety, and are distinguished by their somewhat angular surface, caused apparently by the successive cuts of a knife, in paring off the cortical portion. They are destitute of the small perforating passage of the Chinese variety, but generally have a larger hole, penetrating only to the centre, and clearly intended for the purpose of inspection. The pieces are lighter and less compact than the best Chinese, have usually a cleaner and brighter surface, and, when broken, exhibit a livelier hue, though of the same reddish-gray colour. The powder is of a fine bright yellow, without the reddish-brown tinge of the Chinese; and the odour is more aromatic.
General Properties. Rhubarb, in the state of powder, is bright-yellow, or brownish-yellow, with a somewhat aromatic odour, and a bitter, astringent, and peculiar taste. When pieces of the root are chewed, they have a gritty feel under the teeth, and stain the saliva of a bright yellow. it yields its sensible properties of colour, taste, and smell, as well as its medical virtues, to water and alcohol. By heat odorous yellow fumes are given off, and the colouring property and bitterness are diminished, without an equal diminution of the astringency. The virtues of rhubarb are also diminished by long boiling.
Chief Constituents. Besides other less interesting ingredients, rhubarb contains tannic and gallic acids, gum, starch, pectin, resinous matter, one or more colouring principles, and a large proportion of oxalate of lime, to which it owes its white veins, and its grittiness under the teeth. Many attempts have been made to isolate its active purgative principle, but hitherto with imperfect success; for the various substances for which this title has been claimed, under the names of caphopicrite, rhabar-barin, rhabarbaric acid, rheumin, and rhein, have been shown to be more or less complex. There can be little doubt, I think, that the bitter and cathartic principle is the same, for rhubarb is purgative in proportion to its bitterness; and as little doubt that it is driven off at an elevated temperature; for the purgative property is diminished by much or long continued heat, while yellow and odorous fumes escape. The astringency of rhubarb is owing to its tannic acid.