This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Fig. 189. - Chinese Rhubarb (Shensi flat). Transverse section, showing the remains of the bark (as a narrow exterior layer), the cambium (as a dark line), the wood (with a more or less distinctly radiate structure), and an irregular circle of starspots (abnormal fibro-vascular bundles). (Planchon and Collin).
The structure of this drug is complex, but it must be remembered that it is a very fleshy rhizome, the internodes of which are so closely approximated as to be almost suppressed.
The drug possesses a characteristic odour and bitter, astringent taste; when chewed it is very gritty between the teeth, a character due to the calcium oxalate, which occurs in considerable quantity in large cluster-crystals.
Fig. 190. - Chinese Rhubarb (Shensi flat). Flat (inner) surface of a plano-convex piece, showing irregularly distributed star-spots. (Planchon and Collin).
The student should particularly observe
(a) The firm, compact, texture,
(b) The outer surface, seldom much wrinkled,
(c) The continuous ring of star-spots on the transverse section,
(d) The network of white lines on the outer surface (not shown when the peeling has been carried too deep).
Rhubarb exerts an astringent and also a laxative action.
The chief astringent constituent is gallic acid, in addition to which a small quantity of tannin and (?) catechin is present.
The chief laxative constituent is, according to Tutin and Clewer (1911), an amorphous non-glucosidic resinous mixture. By hydrolysis with dilute sulphuric acid this yields gallic acid, cinnamic acid, rhein, emodin, aloe-emodin, emodin monomethyl ether, chrysophanic acid, and a trihydroxydihydroanthracene. The gallic and cinnamic acids are probably present as esters.
In addition to the resin the drug contains rhein, emodin, aloe-emodin, emodin monomethyl ether and chrysophanic acid in the free state and also in the form of glucosides. These substances are all derivatives of oxymethylanthraquinone and are accompanied in the drug by rheinolic acid.
Further constituents are starch, fat, dextrose, levulose, pectin, calcium oxalate, etc.
Chrysophanic acid, CH3.C14H502(OH)2, is a dihydroxymethylanthraquinone, but the positions of the groups are not known with certainty.
Emodin, CH3.C]4H402(OH)3, is hydroxy-chrysophanic acid and is therefore a trihydroxymethylanthraquinone.
Emodin monomethyl ether,
CH3.C14H402(OH)2O.CH3, is the corresponding methoxychrysophanic acid; it has been found in Rumex Ecklonianus, Meissner, and may be prepared from emodin.
Aloe-emodin, C14H502(OH)2.CH1.OH, is the primary alcohol (hydroxymethyl-dihydroxyanthraquinone) corresponding to chrysophanic acid; as its name indicates, it is found in aloes.
Rhein, C14H502(OH)2.C02H, is the corresponding carboxylic acid, and may be obtained by the oxidation of aloe-emodin.
The total quantity of oxymethylanthraquinones, free and combined, present in rhubarb has been estimated to vary from 2 to 4 per cent. Only chrysophanic acid and aloe-emodin are laxative. The mixed glucosides of the oxymethylanthraquinones crystallise readily together; this crystalline mixture, termed by Gilson rheopurgarin, is, however, destitute of purgative action. The suggestion that has been made that the comparative medicinal value of rhubarb may be determined by estimating the total oxymethylanthraquinones present would appear, therefore, to be unacceptable.
According to Gilson, the gallic acid is present as a glucoside (glucogallin), and gallic acid, cinnamic acid, and rheosmin as another glucoside (tetrarin), but this is improbable.
The amount of calcium oxalate contained in rhubarb varies considerably, averaging about 7.3 per cent. (Fluckiger). The ash is subject to a similar variation; it has been found as low as 3.5, and as high as 43.27; usually good Chinese rhubarb yields from 7 to 13 per cent. (B.P. not more than 15). English rha-pontic rhubarb yields usually less than Chinese. It occurs in very large cluster crystals, and is the cause of the gritty taste of the drug.
Fig. 191. - Chinese Rhubarb (Canton round), showing granular fracture. Natural size.
In small doses rhubarb is a bitter stomachic and intestinal astringent; in larger doses it causes purgation, which is followed by an astringent effect due to the tannoid constituents. It is given in cases of indigestion with diarrhoea and as a mild laxative.
Fig. 192. - Chinese Rhubarb (flat High-dried). Slightly reduced.
Three varieties of Chinese rhubarb are recognised on the market, viz. Shensi, Canton, and High-dried, each of which may occur in ' flats ' or ' rounds.'
Shensi Rhubarb is characterised by its very compact nature, bright yellow coat, distinct whitish reticulations, and 'nutmeg' fracture (fig. 189): the odour is agreeable and free from marked empyreuma, the taste slightly bitter. That which exhibits a bright pink fracture is preferred.
Canton Rhubarb may be distinguished from Shensi by the fracture, which is more or less uniformly granular, exhibiting no marked marbling, and by the odour and taste which are more distinctly empyreumatic and disagreeable. It is more fibrous in its nature and less compact, the coat is not so bright, and the whitish reticulations are less marked. Formerly the pieces bore a deep triangular nick, but this is now more often wanting. It is less esteemed than Shensi rhubarb.
Fig. 193. - English Rhubarb (R. officinale), showing the star-spots on the radial section. Natural size.
Fig. 194. - English Rhubarb (R. rhapon-ticum). Rhizome. Slightly reduced.
High-dried Rhubarb may resemble either Shensi or Canton in the fractured surface, but the coat is much duller and rougher, and the odour and taste more empyreumatic than Canton. The rounds are often much shrunken, and frequently exhibit the remains of the bud, dark patches being often visible on the outer surface. The flats show distinct severe paring, and are often so hard as to spring when broken with the hammer; they are usually of better quality than the rounds.
In England two species of Rheum, viz. R. officinale and R. rhaponticum, Linne, are cultivated (Oxfordshire, Bedfordshire,
&c). The rhizomes are dried and sold separately from the roots. Those of R. officinale resemble the Chinese drug, but, being more spongy, shrink and wrinkle as they dry, and are softer to cut; the white reticulations are commonly absent, the white and red lines being parallel to one another; the star-spots are also fewer and more scattered. The roots are readily distinguished by their long cylindrical shape and distinctly and entirely radiate transverse section. The constituents of this variety of English rhubarb are similar to those of the Chinese.
Fig. 195. - English Rhubarb (R. rhaponticum). A, transverse section of rhizome; B, transverse section of root. Natural size.
The rhizomes of R. rhaponticum are also much shrunken and usually pinkish in colour; they may be readily distinguished by the transverse section which exhibits a diffuse circle of isolated star-spots. Rhapontic rhubarb contains no emodin, aloe-emodin, or rhein; its most characteristic constituent is a crystalline glucoside, rhapontiein (rhapontin, ponticin; not an anthraquinone derivative), the presence of which can be demonstrated by the following test:
Percolate 10 gm. of the powdered drug with 60 per cent. alcohol; collect 25 c.c. of the percolate, evaporate at 80° to 7 gm., shake vigorously while still warm with 10 c.c. of ether, pour off the ethereal solution into a small flask, cork and set aside; needle-shaped crystals of rhaponticin will separate within 24 hours. This test serves to identify the drug derived from R. rhaponticun, and will detect it in a mixture of 1 part of rhapontic rhubarb with 3 of Chinese but in this case crystals do not separate for a few days.
Rhapontic rhubarb also contains chrysophanic acid, a crystalline substance chrysopontin, C16H1605, and a glucoside yielding by hydrolysis chrysorhapontin, C16H1604. According to Hesse the drug contains, in addition to rhaponticin and chrysophanic acid, anhydrorhapontigenin, rhabarberone, chrysarone, gluco-chrysarone, gallic acid and rhapontic acid.
This variety has latterly been imported from China; it closely resembles English rhapontic but is usually darker, often hollow in the centre, the section exhibiting alternating paler and darker concentric rings and a yellow rather than pink colour; it yields rhaponticin by the test described above.
Many species of Rumex contain oxymethylanthra-quinone derivatives and have been used as substitutes for rhubarb, e.g., R. alpinus, Linne, R. obtusifolius, Linne, etc.
R. hymenosepalus, Torrens, yields canaigre root, used in America for tanning (30 per cent. of tannin).