History—The employment of this root among the Seneca Indians as a remedy for the bite of the rattlesnake attracted the notice of Tennent, a Scotch physician in Virginia; and from the good effects he witnessed he concluded that it might be administered with advantage in pleurisy and peripneumonia. The result of numerous trials made in the years 1734 and 1735 proved the utility of the drug in these complaints, and Tennent communicated his observations to the celebrated Dr. Mead of London in the form of an epistle, afterwards published together with an engraving of the plant, then called the Seneca Rattlesnake Root.[325] Tennent’s practice was to administer the root in powder or as a strong decoction, or more often infused in wine. The new drug was favourably received in Europe, and its virtues discussed in numerous theses and dissertations, one written in 1749 being by Linnæus.[326]
Description—Senega root is developed at its upper end into a knotty crown, in old roots as much as an inch in diameter, from which spring the numerous wiry aerial stems, beset at the base with scaly rudimentary leaves often of a purplish hue. Below the crown is a simple tap-root ²/₁₀ to ³/₁₀ of an inch thick, of contorted or somewhat spiral form, which usually soon divides into 2 or 3 spreading branches and smaller filiform rootlets.
The bark is light yellowish-grey, translucent, horny, shrivelled, knotted and partially annulated. Very frequently a keel-shaped ridge occurs, running like a shrunken sinew through the principal root; it has no connexion with the wood, but originates in a one-sided development of the liber-tissue. The bark encloses a pure, white woody column about as thick as itself. After the root has been macerated in water the bark is easily peeled off, and the peculiar structure of the wood can then be studied. The latter immediately below the crown is a cylindrical cord, cleft however by numerous, fine, longitudinal fissures. Lower down these fissures increase in an irregular manner, causing a very abnormal development of the wood. Transverse sections of a root therefore differ greatly, the circular wood portion being either penetrated by clefts or wide notches, or one-half or even more is altogether wanting, the space where wood should exist being in each case filled up by uniform parenchymatous tissue.
Senega root has a short brittle fracture, a peculiar rancid odour, and a very acrid and sourish taste. When handled it disperses in irritating dust.
Microscopic Structure—The woody part is built up of dotted vessels surrounded by short porous ligneous cells; the medullary rays consist of one or two rows of the usual small cells. There is no pith in the centre of the root. The clefts and notches are filled up with an uniform tissue passing into the primary cortical tissue without a distinct liber; the large cells of this tissue are spirally striated. In the keel-shaped rider the proper liber rays may be distinguished from the medullary rays. The former are made up of a soft tissue, hence the cortical part of the root breaks short together with the wood.
Neither starch granules nor crystals of oxalate of calcium are present in this root; the chief contents of its tissue are albuminoid granules and drops of fatty oil.
Chemical Composition—The substance to which the drug owes its irritating taste was distinguished by the name of Senegin by Gehlen as early as 1804, and is probably the same as the Polygalic Acid of Quevenne (1836) and of Procter (1859). Christophsohn (1874) extracted it by means of boiling water, evaporated the solution and exhausted the residue with boiling alcohol (0·853 sp. gr.). The liquid after a day or two, deposits the crude senegin, which is to be washed with alcohol (0·813 sp. gr.), and again dissolved in water, from which it is precipitated by a large excess of hydrate of baryum. The barytic compound, dissolved in water, is decomposed by carbonic acid, by which carbonate of baryum is separated, senegin remaining in solution. It is lastly to be precipitated by alcohol. It is amorphous, insoluble in ether and in cold water; it forms with boiling water a frothing solution. Like saponin, to which it is very closely allied, it excites violent sneezing.
Dilute inorganic acids added to a warm solution of senegin throw down a flocculent jelly of Sapogenin, the liquid retaining in solution uncrystallizable sugar. Alkalis give rise to the same decomposition; but it is difficult to split up the senegin completely, and hence the formulas given for this process are doubtful. Even the formula of senegin itself is not definitely settled. According to Christophsohn, the root yields about 2 per cent. of this substance; according to earlier authorities, who doubtless had it less pure, a much larger proportion. From Schneider’s investigations (1875) it would appear that the rootlets are richest in senegin.
Senega root contains a little volatile oil, traces of resin, also gum, salts of malic acid, yellow colouring matter, and sugar (7 per cent. according to Rebling, 1855). The Virginic Acid said by Quevenne to be contained in it, and the bitter substance Isolusin mentioned by Peschier, are doubtful bodies.
Uses—Senega is prescribed as a stimulating expectorant and diuretic, useful in pneumonia, asthma and rheumatism. It is much esteemed in America.