History—The astringent properties of all parts of the oak[2226] were well known to Dioscorides, who recommends a decoction of the inner bark in colic, dysentery and spitting of blood. Yet oak bark seems at no time to have been held in great esteem as a medicine, probably on account of its commonness; and it is now almost superseded by other astringents. For tanning leather it has always been largely employed.
Description—For medicinal use the bark of the younger stems or branches is collected in the early spring. It varies somewhat in appearance according to the age of the wood from which it has been taken: that usually supplied to English druggists is in channelled pieces of variable length and a tenth of an inch or less in thickness, smooth, of a shining silvery grey, variegated with brown, dotted over with little scars. The inner surface is light rusty-brown, longitudinally striated. The fracture is tough and fibrous. A transverse section shows a thin, greenish cork-layer, within which is the brown parenchyme, marked with numerous rows of translucent colourless spots. The smell of dry oak bark is very faint; but when the bark is moistened the odour of tan becomes evident. The taste is astringent and in old barks slightly bitter.
Microscopic Structure—The outer layer of young oak bark consists of small flat cork-cells; the middle layer of larger thick-walled cells slightly extended in a tangential direction, and containing brown grains and chlorophyll. This tissue passes gradually into the softer narrower parenchyme of the inner bark, which is irregularly traversed by narrow medullary rays. It exhibits moreover a ring, but slightly interrupted, of thick-walled cells (sclerenchyme) and isolated shining bundles of liber-fibres.
Groups of crystals of calcium oxalate are frequent in the middle and inner bark, but the chief constituents of the cells are brown granules of colouring matter and tannin. As the thickness of the bark increases the liber is pushed more to the outside, the middle cortical layer being partly thrown off by secondary cork-formation (rhytidoma, see pp. [354] and [538]). Hence the younger barks, which alone are medicinal, are widely different from the older in structure and appearance.
Chemical Composition—The most interesting constituent is a peculiar kind of tannin. Stenhouse pointed out in 1843 that the tannic acid of oak bark is not identical with that of nutgalls; and such many years afterwards was proved to be the case.
The first-named substance, now called Querci-tannic Acid, yields by destructive distillation pyrocatechin, and according to Johanson (1875) very little pyrogallol. By boiling it with dilute sulphuric acid querci-tannic acid is split up into a red derivative and sugar. A solution of gelatine is precipitated by querci-tannic acid as well as by gallo-tannic acid; yet the compound formed with the latter is very liable to putrefaction, whereas the tannin of oak bark, which is accompanied by a large amount of extractive matter, furnishes a stable compound, and is capable of forming good leather.
As querci-tannic acid has not yet been isolated in a pure state, the exact estimation of the strength of the tanning principle in oak bark has not been accomplished, although it is important from an economic as well as from a scientific point of view. The method of Neubauer (1873) depends upon the amount of permanganate of potassium decomposable by the extract of a given weight of oak bark. Neubauer found in the bark of young stems, as grown for tanning purposes, from 7 to 10 per cent. of querci-tannic acid, soluble in cold water.
Braconnot (1849) extracted from the seeds of the oaks under notice a crystallized sugar, which was shown in 1851 by Dessaignes to be a peculiar substance, which he termed Quercite. Prunier proved (1877-1878) that it agrees with the formula C₆H₇(OH)₅ + 4 OH₂, and is closely allied to kinic acid, C₆H₇(OH)₄COOH ([see page 363]). Quercite gives off water at 100°, melts at 225° C., and again losing water yields a crystallized anhydride. In the oak bark extremely small quantities of quercite appear also to be present, as pointed out by Johanson.
A colourless, crystallizable, bitter substance, soluble in water, but not in absolute alcohol or ether, was extracted from oak bark in 1843 by Gerber, and named Quercin. It requires further examination: Eckert (1864) could not detect its existence in young oak bark.
Uses—Occasionally employed as an astringent, chiefly for external application.