Larch Bark.
Botanical Origin—Pinus Larix L.—[see p. 609].
History—The bark of the larch has long been known to possess astringent properties; hence it has been used in tanning. Gerarde,[2288] who wrote near the close of the 16th century, likened it to that of the pine, which he described to be of a binding nature; but there is no evidence that it was an officinal drug.
About the year 1858 larch bark was recommended by Dr. Frizell of Dublin, and afterwards by other physicians, as a stimulating astringent and expectorant. In consequence of the favourable effects which have resulted from its use it has been included in the Additions to the Pharmacopœia of 1867.
Description—The bark that we have seen is in flattish pieces or large quills, externally reddish-brown. In those taken from older wood there is a large amount of an exfoliating corky coat, displaying as it is removed bright rosy tints, while the liber is of a different texture, slightly fibrous and whitish. The inner surface is smooth and of a pinkish-brown, or pale yellow. The bark breaks with a short fracture, exhaling an agreeable balsamic terebinthinous odour; it has a well-marked astringent taste. For medicinal use the inner bark is to be preferred.
Microscopic Structure—A transverse section exhibits resin-ducts, but far less numerous than in the bark of many allied trees. The medullary rays are not very distinct. Throughout the middle layer of the bark large isolated thick-walled cells of very irregular shape are scattered.
Chemical Composition—Larch bark has been examined by Stenhouse,[2289] who finds it to contain a considerable amount of a peculiar tannin, yielding olive-green precipitates with salts of iron. The same chemist also discovered[2290] in larch bark an interesting crystallizable substance called Larixin or Larixinic Acid, which has the composition C₁₀H₁₀O₅. It may be obtained by digesting the bark in water in 80° C. and evaporating the infusion to a syrupy consistence. From this, by still further cautious heating in a retort, the larixin may be distilled, during which operation some of it crystallizes on the inner surface of the receiver, the remainder being dissolved in the distilled liquor. From the latter it may be obtained in crystals by evaporation. The substance forms colourless crystals, sometimes as much as an inch long; it volatilizes even at 93° C., and melts at 153°. It requires about 88 parts of water for solution at 15° C., but more freely dissolves in boiling water or in alcohol. From ether, in which it is but sparingly soluble, it separates in brilliant crystals. The solutions have a bitterish astringent taste and a slightly acid reaction, and assume a purple hue on addition of ferric chloride. When a solution of baryta is added to a concentrated solution of larixin, the latter being in excess, a bulky gelatinous precipitate falls; it is readily soluble in boiling water and is deposited again on cooling. Stenhouse failed to obtain it either from the bark of Pinus Abies L., or from that of P. silvestris L.
Uses—Larch bark, chiefly in the form of tincture, has been prescribed to check profuse expectoration in cases of chronic bronchitis; it has also been found useful in arresting internal hæmorrhage.
TEREBINTHINA CANADENSIS.
Balsamum Canadense; Canada Balsam, Canadian Turpentine; F. Térébenthine ou Baume de Canada; G. Canada Balsam.